U.S. patent application number 09/828177 was filed with the patent office on 2001-08-30 for enhancement of flow rates through porous media.
This patent application is currently assigned to PE-TECH Inc.. Invention is credited to Davidson, Brett Charles, Dusseault, Maurice Bernard, Geilikman, Mikhail Boris, Hayes, Kirby Warren, Spanos, Thomas James Timothy.
Application Number | 20010017206 09/828177 |
Document ID | / |
Family ID | 10809752 |
Filed Date | 2001-08-30 |
United States Patent
Application |
20010017206 |
Kind Code |
A1 |
Davidson, Brett Charles ; et
al. |
August 30, 2001 |
Enhancement of flow rates through porous media
Abstract
For extracting a liquid (such as oil) from a porous medium, the
liquid is subjected to pulses that propagate through the liquid
flowing through the pores of the medium. The pulses cause momentary
surges in the velocity of the liquid, which keeps the pores open.
The pulses can be generated in the production well, or in a
separate excitation well. If the pulses travel with the liquid, the
velocity of travel of the liquid through the pores can be
increased. The solid matrix is kept stationary, and the pulses move
through the liquid. The pulses in the liquid can be generated
directly in the liquid, or indirectly in the liquid via a localized
area of the solid matrix.
Inventors: |
Davidson, Brett Charles;
(Cambridge, CA) ; Dusseault, Maurice Bernard;
(Conestogo, CA) ; Geilikman, Mikhail Boris;
(Waterloo, CA) ; Hayes, Kirby Warren;
(Lloydminster, CA) ; Spanos, Thomas James Timothy;
(Edmonton, CA) |
Correspondence
Address: |
Anthony ASQUITH
173 Westvale Drive
Waterloo
ON
N2T 1B7
CA
|
Assignee: |
PE-TECH Inc.
Cambridge
CA
|
Family ID: |
10809752 |
Appl. No.: |
09/828177 |
Filed: |
April 9, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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|
09828177 |
Apr 9, 2001 |
|
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|
09046762 |
Mar 24, 1998 |
|
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6241019 |
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Current U.S.
Class: |
166/249 ;
166/268 |
Current CPC
Class: |
E21B 28/00 20130101;
B01J 8/02 20130101; E21B 43/16 20130101; E21B 43/003 20130101; B01J
2208/00548 20130101; B01J 2208/00539 20130101 |
Class at
Publication: |
166/249 ;
166/268 |
International
Class: |
E21B 043/00 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 24, 1997 |
GB |
9706044.6 |
Claims
1. A procedure for increasing the permeability of the
ground-material around a borehole in the ground, wherein the
procedure includes: first ensuring that the pores of the said
ground-material are saturated with liquid, and the liquid is under
a head of pressure, and is coherent with liquid inside the
borehole; then applying perturbations to the ground-material, and
continuing to apply same for a substantial period of time; wherein,
in respect of each perturbation: the nature of the perturbation, as
applied, is such as to create, in respect of each perturbation, a
corresponding porosity-pulse in the ground-material; the
porosity-pulse comprises a momentary physical deformation of the
ground-material, and the said physical deformation is manifested as
an increase in the porosity of the ground-material; the
perturbation is strong enough that the said physical deformation,
being the increase in porosity, is substantial; the perturbation is
weak enough that the said physical deformation, being the increase
in porosity, can be accommodated within the elastic limit of the
ground-material, being not so strong as to produce any irreversible
residual deformation of the ground-material.
2. Procedure of claim 1, which includes, in respect of each
perturbation, applying the perturbation in the form of a pulse of
increased pressure applied directly to the liquid in the borehole,
in such manner that the pulse of increased pressure propagates out
from the borehole into the ground-material, and the strength of the
pulse of increased pressure is such as to give rise to a
corresponding pulse of increased porosity of the ground-material,
and the pulse of increased porosity propagates, through the ground
material, away from the borehole.
3. Procedure of claim 2, wherein the nature of the ground-material,
and the nature of the applied pulse of increased pressure, are such
that the pulse of increased porosity comprises a gradual increase
followed by a gradual decrease in the porosity of the
ground-material; and the procedure includes applying the pulses of
increased pressure at such frequency that a new pulse of increased
porosity is started applied before the previous pressure pulse has
quite died away.
4. Procedure of claim 1, wherein: the pulse of increased porosity
of the ground-material has a characteristic speed of propagation
through the ground-material, the speed being a property of the
elastic constant of the ground-material as modified by the presence
of the liquid saturating the pores thereof; the pulse of increased
pressure in the ground-material has a characteristic speed of
propagation through the ground-material, and the pressure-period is
a property of the elastic constant of the liquid saturating the
pores of the ground-material; the said propagation speeds are
substantially independent of the amplitudes of the pulses, so long
as the amplitudes are below a limit characteristic of the elastic
properties of the ground-material; the procedure includes applying
the perturbation in such manner that the amplitudes of the pulses
consequent upon the applied perturbation are below the said
limit.
5. Procedure of claim 4, wherein: the ground-material in which the
procedure is carried out is ground-material in which the
propagation speed of the pulse of increased porosity is similar to
the propagation speed of the pulse of increased pressure; whereby
the pulse of increased porosity and the pulse of increased pressure
reinforce each other as they propagate through the ground
material.
6. Procedure of claim 2, wherein: the nature of the ground-material
is such that each pulse of increased pressure applied to the liquid
in the borehole gives rise to a gradual increase followed by a
gradual decrease in the pressure of the liquid in the
ground-material; the procedure includes applying the pulses of
increased pressure to the liquid in the borehole at such frequency
that the pressure of the liquid in the ground-material starts to
rise, due to a newly-applied pulse, before the pressure of the
liquid in the ground-material, due to the previously-applied pulse,
has quite died away.
7. Procedure of claim 1, including: providing, in the borehole, a
body of a solid material; in respect of each perturbation, applying
the perturbation by applying a mechanical impact to the body of
solid material, in such manner that the perturbation is transmitted
to the ground-material, and is such as to give rise to a
corresponding pulse of increased porosity of the ground-material,
and the pulse of increased porosity propagates, through the ground
material, away from the borehole.
Description
GENERAL DESCRIPTION OF THE INVENTION
[0001] This invention relates to the dynamic enhancement of fluid
flow rates in a porous medium, using pressure and strain pulsing.
The invention relates to devices and methods designed to explicitly
enhance the flow rate of fluids (liquids or gases) and mixtures of
fluids and solids (e.g. oil and sand particles) in porous media by
means of application of pressure pulsing or strain pulsing to the
region of flow. The pressure pulsing is applied to the liquid phase
of a porous medium through periodic cycling of liquid volumes by
mechanical, hydraulic, or pneumatic devices at one or more points.
Strain pulsing can similarly be applied through mechanical or
electromechanical excitation. The two processes are intimately
linked in that a pressure pulse generates a strain pulse, and
vice-versa. Dynamic enhancement of fluid flow rate can be applied
to the following technologies:
[0002] Flow of liquids or liquid-solid mixtures to wellbores in
petroleum or water extraction processes from porous media.
[0003] Flow of liquids or liquid-solid mixtures in porous media to
wells, sumps or other pressure sinks during cleaning of
contaminated shallow aquifers comprised of sand, gravel, or
fractured rock.
[0004] Flow of liquids or liquid-solid mixtures in contained or
natural porous media beds used for chemical engineering reaction
processes, filtration, refining, cleaning, or other processes where
liquids or liquid-solid mixtures are flowing from one point to
another under the effect of a pressure or gravity-induced
gradient.
LIST OF THE DRAWINGS
[0005] FIG. 1 is a section of a porous medium;
[0006] FIG. 2 is a diagram of an apparatus for demonstrating
dynamic enhancement of flow rate through the medium of FIG. 1;
[0007] FIG. 3 is a graph of a fluid flow rate enhancement, without
entrained solids;
[0008] FIG. 4 is a graph of a fluid flow rate enhancement, with
entrained solids;
[0009] FIG. 5 is a graph showing pressure pulse transmission
through the porous specimen;
[0010] FIG. 6: Strain Pulse Flow Enhancement Apparatus
[0011] FIG. 7a is a plan view of a field implementation for oil
production;
[0012] FIG. 7b is a section on line Y-Y of FIG. 7a;
[0013] FIG. 8 is an implementation of flow enhancement in
horizontal wells;
[0014] FIG. 9 is a section of a pressure pulsing device;
[0015] FIG. 10a is a section of a well having a strain-pulsing
device;
[0016] FIG. 10b is a section of a strain-pulsing device in a
well;
[0017] FIG. 10c is a cross-section of a portion of the device of
FIG. 10a;
[0018] FIG. 11 is a section of a vibrational enhancement device
located at the ground surface;
[0019] FIG. 12 is a section through a reaction bed of granular
material;
[0020] FIG. 13 is a section of an installation for creating pulses,
producing oil, and monitoring the production;
[0021] FIG. 14a is a graph of the velocity of liquid passing
through a pore in a porous medium, with pulses;
[0022] FIG. 14b is a corresponding graph to FIG. 14a, when the
pulses are at a different frequency.
1 DEFINITIONS
[0023] In the context of this specification, a porous medium is a
natural or man-made material comprising a solid matrix and an
interconnected pore (or fracture) system within the matrix. The
pores are open to each other and can contain a fluid, and fluid
pressure can be transmitted and fluid flow can take place through
the pores. Examples of natural materials include gravels, sands and
clays; sandstones, limestones and other sedimentary rocks; and
fractured rocks including fractured sedimentary rocks which have
both fractures and pores through which fluids may flow. Examples of
man-made porous media include filtration beds of natural or
artificial granular materials or manufactured solid porous
materials, as well as beds of catalysts used to accelerate
reactions between fluid phases or fluid-solid phases during
refining, chemical synthesis, or other processes. Structures such
as tailings dikes, dams, fluid recharge or filtration beds, and so
on, can be regarded as porous media.
[0024] The porosity of a porous medium is the ratio of the volume
of open space in the pores to the total volume of the medium
Systems of practical interest in the present context have
porosities that lie in the range 5% to 60%.
[0025] The porosity (pore, fractures, and channels) is filled with
fluids, which may be gases or liquids or a combination of the two.
Liquids can be oil, water (with dissolved constituents), or
man-made liquids such as gasoline, chlorinated bi-phenyls,
polymers, and non-aqueous phase liquids deliberately or
accidentally introduced into the porous medium. Gases may be
natural hydrocarbons, air, carbon dioxide, or man-made gaseous
products introduced deliberately or accidentally into the porous
medium.
[0026] All porous media are characterized by a permeability.
Permeability is an average measure of the geometry of the pores,
pore throats, and other properties which describes the flow rate of
fluids through the medium under the effect of a pressure gradient
or a gravity force induced because of differences in density among
fluid phases or solid-fluid phases.
[0027] Pressure pulsing is a deliberate variation of the fluid
pressure in the porous medium through the injection of fluid,
withdrawal of fluid, or a combination of alternating periods of
injection and withdrawal. The pressure pulsing may be regular or
irregular (periodic or aperiodic), continuous or episodic, and it
may be applied at the point of withdrawal or at other points in the
region of the porous medium affected by the flow process.
[0028] Strain pulsing is a deliberate variation of the strain at a
point or local region in the porous medium by applying changes in
strain through a device which vibrates, oscillates, or which
expands and contracts in volume. The strain pulsing may be regular
or irregular, continuous or episodic, and it may be applied at the
point of withdrawal or at other points in the region of the porous
medium affected by the flow process.
[0029] Dilational and shear pulses are the two basic types of
excitation. In a dilational pulse, the perturbation is isotropic
(equal in all directions) at the point of application, and may be
termed a volumetric pulse. Pressure pulsing is dominantly a
dilational perturbation. The dilational perturbation moves out in
all directions approximately equally and is subject to scattering
phenomena. In a shear pulse, a relative lateral excitation is
applied so that the energy imparted to the porous medium is
dominated by shear motion, such as occurs when slip occurs along a
plane. Shear perturbation is highly anisotropic, and the
distribution of energy depends on the orientation of the perturbing
source. Shear perturbations can therefore in principle be focused
so that more energy propagates in one direction than in another.
Strain pulsing can be anisotropic or isotropic, depending on the
nature of the excitation source.
[0030] Flow takes place in a porous medium through generating a
pressure gradient in the mobile (moveable) phases by creating
spatial differences in fluid pressures. Reducing or increasing the
pressure at a number of points may generate this by the withdrawal
or injection of fluids. It may also be generated through the force
of gravity acting upon fluids of different density, such as oil,
formation water, gas or air, injected non-aqueous phase liquids and
other fluids. In a system where the solid particles are partly free
to move, density differences between solids and fluids may also
lead to gravity-induced flow.
[0031] In a porous medium containing two or more non-miscible
fluids (oil and water for example), the wetting phase is that gas
or liquid which, because of surface tension and wettability
effects, is in contact with the majority of the solid material. It
forms the pendular fluid contacts between grains in a granular
porous medium, and coats the walls of flow channels (FIG. 1). The
non-wetting phase is that gas or liquid which lies in the
interstices and channels and is separated from the solid material
by a film of the wetting phase fluid. In FIG. 1, the mineral grains
1 are coated with a wetting liquid 2, while a non-wetting liquid 3
occupies the rest of the pore space. The pore throat dimension 4,
averaged through the medium, is important in dictating the velocity
at which liquid can pass through the pores 5 of the medium.
[0032] The non-wetting phase 3 might be continuous or
discontinuous. If it is continuous, then an interconnected and
uninterrupted path of that liquid exists in the medium. If it is
discontinuous, the non-wetting phase may exist as isolated droplets
or regions, which are nowhere in direct physical contact with other
regions of the same phase.
2 EVIDENCE OF DYNAMIC ENHANCEMENT OF FLUID FLOW
[0033] There exist in the public literature observations of
increased flow rates in oil wells and water wells during and after
dynamic excitation from earthquakes or other events which can
create sufficient strain in the medium to affect the porosity, and
the through-flow velocity of the liquid, even in a minuscule
manner.
[0034] In the systems as described herein, periodic or irregular
pressure pulsing in a flowing system under a pressure gradient
increases the flow rate of the mobile phase toward the extraction
point.
[0035] Field observations confirm that a porosity perturbation
applied to a petroleum well enhances flow to the well for some time
thereafter by increasing the mobility of the fluid phase. In the
case of a petroleum well producing fluid and sand, a general
increase in the mobility of the complex solid-liquid-gas flowing
phase takes place. The perturbation in these cases may also be a
single sharp pressure pulse applied at the production well.
[0036] Theoretical developments and field observations show that
fluid flow rate to a producing water well or petroleum well is
enhanced if the liquid-flow-borne solids are allowed to enter the
wellbore in an unimpeded manner. This is analogous to a porosity
diffusion process in that a porosity change occurs as the solid
phase is produced along with the liquids. This porosity change
slowly propagates out from the production point into the porous
medium through a diffusive mechanism, and is accompanied by changes
in the pressure and pressure gradient with time and location around
the wellbore. In the oil industry, the process of allowing the sand
to flow unimpeded is called cold production, cold flow, or sand
production.
[0037] In general, the flow enhancement accompanying any porosity
diffusion process takes place in a system with a pressure gradient,
and the processes preferentially increase flow rates of the mobile,
non-wetting phase if more than one fluid is present as a continuous
phase.
[0038] One feature of the invention lies in the recognition that
dynamic excitation through application of a pressure pulse, a
strain pulse, or a series of pulses anywhere in the flowing porous
medium can enhance the flow rate. Fluid rate enhancement occurs at
the exit points of a given system (wellbore, reaction bed, and
pipeline), that are also the points of low pressure in the medium.
Furthermore, we have recognised that the fluid flow enhancement can
be theoretically predicted and analysed, measured in the
laboratory, and physically explained.
[0039] In addition to the porosity diffusion effect and the
enhancement in flow rate that it generates, dynamic excitation has
several other beneficial effects on production performance of
wells. The dynamic excitation may be induced as a pressure pulse or
a strain pulse, generated by a pulsating or vibrating source.
Excitation may be periodic or aperiodic, continuous or episodic,
and applied in the stratum or at the surface, provided that
sufficient porosity diffusion amplitude is transmitted to the
region of interest.
[0040] The permeability of a conventional producing well can be
impaired by the migration and consequent accumulation in the
near-wellbore environment of fine-grained solid particles, which
can pass through the pore throat constrictions in the porous
medium. When, as described herein, the porous medium is being
dynamically excited the tendency for these particles to bridge and
block porosity is substantially reduced, thus allowing the well to
maintain flowing conditions with a minimum of impairment.
[0041] Particularly in viscous heavy oils but also in some
conventional oils, certain liquids (asphaltenes in general) can be
precipitated as small size solid particles when the liquid
encounters the lower pressures near the wellbore. These particles
can accumulate in the pore throats, impairing the permeability of
the system and reducing the flow rate to the producing well.
Dynamic excitation, as described herein, provides cyclic strain
energy aimed at mitigating the tendency for blockages of these
precipitants, maintaining the well in a superior flowing
condition.
[0042] Finally, under conditions where the granular particles of
the porous medium are allowed to flow along with the fluids (as in
sand production), the flowing particles may bridge together near
the wellbore, forming a stable sand arch, and stop the solids flow.
This condition leads to a massive deterioration in the fluid
productivity of the well. Dynamic excitation, as described herein,
provides a perturbation energy, which tends to destabilize these
arches because of the small cyclic strains induced at the contacts
between sand grains.
3 EXPERIMENTAL VERIFICATION
[0043] FIG. 2 shows an experimental set-up 20 to demonstrate the
physical principle of dynamic enhancement of fluid flow. The
cylindrical device 23 contains a dense sand pack 24, which is under
an applied stress of 1.5 MPa. The sand pack is flushed through with
paraffin oil (or any other wetting phase) to coat the grains as a
continuous wetting phase. Then, glycerin (or other non-miscible
liquid) is allowed to flow through the sand and form a continuous
non-wetting phase that is immiscible with the wetting phase. The
fluid exit port 25 allows production under the action of a pressure
gradient maintained constant by keeping a reservoir 26 of the
mobile non-wetting phase liquid 27 at an elevation higher than the
device 23. Exit port 25 has a screen 28 between the port 25 and the
sand pack 24 for experiments where the sand is not allowed to flow;
however, for experiments where the sand is permitted to flow, the
screen is removed.
[0044] The flow experiment is allowed to reach a condition of
steady-state exit port flow rate Q. Once this condition is reached,
a dynamic perturbation is applied to the system by one of two
methods: a small strain pulse is applied through a transducer
embedded in the sand 24; or, a periodic pulse is applied to the
upstream part of the device by perturbing the flexible flow lines
manually or automatically (at point 29). The varying excitation is
indicated by the symbol in the circle. Pressure transducers (P1,
P2, and P3) are electronic devices designed to monitor any changes
in pressure in the system induced by the dynamic excitation. The
sand pack 24 is maintained in compression by hydraulic pistons
35.
[0045] The strain pulse is applied through a small acoustic
transducer linked to an oscilloscope and signal generator 30. The
acoustic transducer (not shown in FIG. 2) is embedded in the sand
24 during the assembly of the experiment. It has a diameter of 15
mm and is encased in latex to seal it from the fluid and to provide
good coupling with the sand pack. Being of such small size with
respect to the cell, it does not impede the flow of liquids through
the experimental apparatus. The frequency of the sonic pulse was
varied from 10 Hz to 60 Hz during the excitation period in the
experiments. The period of excitation is indicated in FIG. 3 as
pulsing-started 32 to pulsing-stopped 34. In between periods of
excitation, no pulsing takes place, but flow is allowed to
continue; this is necessary to evaluate flow enhancement through
contrasting periods of excitation and periods of no excitation, in
the same apparatus without other changes on the pressure head or
flow properties.
[0046] The pressure pulsing is applied by manually squeezing the
upstream flexible tube 29 connecting the fluid reservoir 26 to the
top of the flow apparatus. This manual squeezing is applied at a
frequency of 0.5 to 2 Hz continuously during the excitation
period.
[0047] FIG. 3 demonstrates quantitatively the change in the flow
rate from the experimental device. The lower line 36 is the steady
flow at a hydraulic head of 0.25 meters (the top of the fluid in
the reservoir was maintained at an elevation of 0.25 meters above
the entry port). This line 36 is to demonstrate that without
pressure or strain pulsing, no flow enhancement takes place. The
upper line 37 is the demonstration of enhancement. In this case,
the fluid reservoir was maintained 0.5 meters above the fluid
entrance port, and one may note that the slope of the non-pulsed
portions of the line 38 is almost exactly twice the slope of the
lower line 36. This is in accordance with the conventional view of
flow through porous media: a doubling of the hydraulic head without
pulsing leads to a doubling of the flow rate.
[0048] The slope of the upper line without pressure pulsing or
strain pulsing (38) is approx 2.67 cm3/min. With pressure pulsing
or strain pulsing, the flow rate increases (37) to approx 5.7
cm3/min, an enhancement factor of about 2.15. Various experiments
conducted with different excitation frequencies and excitation
times showed flow rate enhancement factors of from 1.5 to 2.2,
demonstrating that the porosity diffusion process increases the
flow rate of the mobile phase under conditions of continuous
pressure or strain excitation.
[0049] This enhancement is also observed in a set of experiments
where the sand is allowed to move from the exit port (screen is
removed). Experiments where sand was allowed to exit are intended
to simulate the behavior of wells producing heavy oil or other
liquids by the process of sand production, discussed below in more
detail. Results similar to those shown in FIG. 3 are obtained if
the sand in the specimen is allowed to exit. Flow rate enhancement
ratios of 2.0 to 2.5 are typically obtained. Typical results are
shown in FIG. 4. The only difference in experimental set-up between
this figure and the previous one is that now the sand is allowed to
flow out with the fluids at the exit port.
[0050] In the sand+liquid flow experiments (screen 28 removed), it
was observed that after some time the sand spontaneously stops
exiting because of the formation of a stable sand arch behind the
exit port 25. This blockage causes the fluid exit rate to drop to a
negligible value, <0.2 cm3/min, indicating that the sand grain
arch is impeding the flow of liquids. The pulsing and the strain
perturbations overcame this blockage. The results therefore
indicate that not only is there a basic flow rate enhancement, but
also that the natural tendency of sand to create blockages can be
overcome by pressure or strain pulsing, and if such blockages
exist, they can be de-stabilized by pulsing. Clearly, this has
substantial positive implications on maintaining free fluid and
sand flow to a well producing sand and liquids.
[0051] FIG. 5 shows the pressure response from the three pressure
transducers in FIG. 2 (P1, P2, P3) when the device is subjected to
a series of continuous pressure pulses applied by manually
squeezing the inflow hose at point 29. As mentioned earlier, the
actual magnitude of this pressure pulse is less than 0.2 kPa, and
it has no effect on the average pressure head applied to the sand
pack. With continued pressure pulsing, however, the actual fluid
pressure in the specimen begins to rise (the curves swing upward);
this is the effect of the porosity diffusion process being built up
through the continuous excitation. When the pulsing is stopped, the
pressure enhancement begins to decay slowly back to its original
values, but the flow rate at the exit port drops to its initial
values within 2-5 seconds. This suggests that fluid flow
enhancement requires continuous excitation. FIG. 5 shows that the
pressure build up is less the farther away from the excitation
source because the pressure build up attenuates as the porosity
diffusion wave is transmitted through the system.
[0052] FIG. 6 shows details of the experimental set up where a
small-embedded acoustic transducer A1 (or several small
transducers) is providing dynamic excitation. This excitation is of
extremely small amplitude, yet it has the same effect as the
pressure pulsing: it alters slightly the pressure in the fluid
phase, and also changes the stresses between the grains, which
builds up the pressure in a way similar to FIG. 5. This also is a
porosity diffusion process because the acoustic excitation is a
small-amplitude strain wave, which leads to small perturbations in
the porosity of the porous medium. Experimental data show that this
process also leads to a fluid flow rate enhancement of the same
order of magnitude as the pressure pulsing, and the enhancement
effect can also be predicted and analysed theoretically.
4 THE PHYSICAL EFFECT IN COLD PRODUCTION WELLS
[0053] The proposed technology has wide applicability to a number
of conditions and cases. However, we believe that it has particular
value in the petroleum industry. Therefore, we describe in detail
one production process, Cold Production (CP), which will be
substantially aided by the application of dynamic pressure or
strain pulsing. This detailed presentation is in no way meant to
exclude any of the other possible production practices for
conventional oil, heavy oil, or other fluids present in porous
media. This example was chosen because it has two major aspects of
the beneficial effect of dynamic excitation through pressure or
strain pulsing: the effect of increasing basic flow rate, and the
effect of breaking down the stable sand arches that form and tend
to block oil flow.
[0054] 4.1 Cold Production Mechanisms
[0055] It is best to have a clear understanding of the production
mechanisms involved in the oil rate enhancement observed during
Cold Production (CP) in order to understand how pressure or strain
pulsing can enhance flow rates and prevent blockages through the
formation of sand arches.
[0056] First, movement of the solid matrix (sand) directly
increases the velocity of the fluid (oil+water+gas). Thus, sand
movement increases flow velocity, enhancing production. This can be
seen in FIG. 4, where the initial slope of the flow line 39 when
solids and fluids are both allowed to flow is greater than for the
case of no solids 37, even under the same hydraulic head.
[0057] Second, sand extraction creates a more permeable zone around
the wellbore through dilation of the sand matrix from an average of
perhaps 30% porosity to a porosity of 35-38% porosity. This zone
grows in mean radius as more sand is produced (some wells produce
in excess of 1200 cu m of sand in their lifetime). If the growth of
this zone is stopped or impeded by sand blockages, flow rates will
be lower. If stable sand arches form near the well perforations,
the flow rates may drop to a small fraction of their values when
the sand is free to flow. If these sand arches are continuously
destabilized by dynamic excitation so that they cannot form in a
stable manner, oil flow is not only more continuous, but it occurs
at a greater rate.
[0058] Third, dissolved gas (mainly CH4) in the heavy oil exsolves
gradually in response to a pressure drop. Bubble nucleation and gas
exsolution is retarded in time because of low gas diffusivity in
viscous oil. The gas also tends to remain as a separate bubble
phase during flow toward the wellbore, and bubbles expand as the
pressure drops toward the production site, giving an internal drive
mechanism referred to as foamy-flow. It is believed that the foamy
flow mechanism aids solids extraction and enhances fluid flow rate.
The high viscosity of the oil retards gas exsolution during flow,
and bubble mobility in the pores and throats is retarded by
interfacial tensions. This alters permeability and enhances
development of small-scale tensile stresses, which help destabilize
the sand.
[0059] Fourth, asphaltene precipitation and pore throat blocking by
clays or fine-grained minerals are reduced during CP because of
continuous solids movement, which liberates pore-blocking
materials. Regular pulsing of pressure or strain will greatly
reduce the frequency of pore throat blockages, which may arise.
[0060] Oil production in CP wells can, exceptionally, be as high as
20-25 cu.m/day, although 4-10 cu.m/day is more typical. After
prolonged CP, done conventionally, rates as low as 1 or 2 cu.m/day
can be accepted providing that initial rates were sufficient (e.g.
>5 cu.m/day) for a long enough period (e.g. 2 years) to warrant
well drilling and field development. However, the systematic
application of pressure or strain pulsing is expected to extend the
productive life of a well, and will also increase the production
rate of the well on a daily basis.
[0061] CP mechanisms depend on continued sand movement, which
allows foamy oil mechanism to operate efficiently, and which allows
continued growth of a disturbed, dilated, partly liquefied region
around the well.
[0062] 4.2 When Cold Production Stops
[0063] Some wells in Alberta have produced oil and sand stably for
over 11 years, with sand flow being successfully re-established
after workovers, or even during production. However, some wells are
extremely difficult to maintain on stable sand production.
Generally, a failure to sustain solids flow is directly related to
a major drop in oil production. Therefore, re-establishment of
sanding would have positive economic consequences in increased oil
rates or prolonged production periods. This re-establishment can be
a consequence of a continuous destabilizing of the formation,
unblocking perforations, or otherwise destroying any stable
structures, which may have been generated in the sand. Dynamic
excitation, as described herein, is aimed at achieving these
goals.
[0064] Stable sand structures are desirable, for good CP. These
include: stable perforation sand arches which greatly retard fluid
flow into the well; re-compaction of the sand in the near-wellbore
environment; collapse and blockage of flow channels within the
strata; or perhaps generation of some form of natural gravel-pack
created by a natural settling around the wellbore of the coarser
grains in the formation. Changes of fluid saturation leading to
increases in capillary cohesion have been suggested as a common
blocking mechanism. This idea suggests that gas evolution leads to
increasing gas saturation near the wellbore until a continuous gas
phase exists, with an apparent cohesion increase in the sand.
[0065] Little is known in detail about the actual blocking
mechanisms because of difficulties in exploring the wellbore region
and difficulties in laboratory simulation, and therefore there is
some difficulty over a method of evaluation and implementing
ameliorative measures. What methods are used have been arrived at
empirically and developed through practice. To our knowledge, no
one uses pressure or strain pulsing of a continuous nature during
continued production.
[0066] Workovers have been used to perturb the formation and
re-establish sand ingress. The conventional methods used vary from
surge and swab operations to much more aggressive approaches such
as Chemfrac (TM), involving igniting a rocket propellant charge to
blow materials out of the perforations, as well as to shock the
formation and perturb the sand. Considering the rise time and the
fluid velocity, this method is probably the most effective in
unlocking perforations plugged with sand and small gravel
particles. However, none of these methods are continuous in nature
during the production of the well.
[0067] Mechanical sand bailers on wirelines are conventionally used
to clean the well of sand before replacing a worn pump. The bailer
is dropped repeatedly into the sand until filled, and then
withdrawn at a relatively rapid rate. This has a vibrational effect
on the near wellbore area, and a swabbing effect during withdrawal.
Often, after bailing, sand has flowed back into the well through
the perforations, and cases have been reported of six to eight days
of bailing, removing as much as 1-3 cu.m of sand; that is, 10-15
times the amount that was in the wellbore in the first place.
Bailing is relatively successful in re-establishing sanding, but
extensive periods of bailing are clearly to be avoided if better
alternatives exist.
[0068] Injection of various chemical formulations to break
capillary effects is relatively common, as is injection of several
cubic meters of heated oil. These methods are thought to break any
apparent (capillary) cohesion in the sand, and the outward flow is
thought to reopen some perforations that may have become
blocked.
[0069] Thus, although many conventional wells produce sand freely,
blockages occur. The method of strain or pressure pulsing, as
described herein, through the process of porosity diffusion, can
provide long-term continuous production at an enhanced flow rate by
activating the ambient stress field dynamically. This process can
destroy small-scale stable sand arches and keep pore throats open.
Blocking materials such as asphaltenes and clay particles are much
less likely to plug pore throats under conditions of dynamic
pressure or strain excitation.
5 FIELD CONFIGURATIONS
[0070] FIGS. 7 and 7a show an example of how dynamic enhancement
through pressure pulsing can be implemented in the field. A
pressure pulsing system is installed in the central well 40 of a
porous stratum containing oil and water. Perforations in the steel
casing 42 of the well 43 allow full and unhindered pressure
communication between the liquid in the wellbore and the liquids in
the pores and fractures of the porous medium. The well is
completely liquid-filled between the pulsing device and the
perforations, and is maintained in that condition.
[0071] A number of adjacent wells (H1, H2, H3, and H4) are
producing fluids and therefore have a well pressure that is less
than the excitation well 40. In other words, the pressure gradient
in the porous medium is directed by the induced pressure
differences so that fluid flow is toward the producing wells. FIG.
7b shows a typical pressure decline curve between the excitation
well and the producing wells. The distance d between the well 40
and the producing wells 43 is dictated by the physical properties
of the medium (compressibility, permeability, fluid viscosity,
porosity, thickness, fluid saturation), and must be determined
through calculations and field experience for individual cases. The
pattern shown, or any other suitable pattern of producing wells and
excitation wells, may be repeated to give the necessary spatial
coverage of a producing field.
[0072] In the field, the amplitude, frequency, and waveform of the
dynamic excitation can be varied to find the optimum values
required to maximize the dynamic enhancement effect. Because porous
media have certain characteristic frequencies at which energy
dissipation is minimal, analysis, laboratory experimentation, and
empirical field optimization methods (based on outflow rates at the
producing wells and other monitoring approaches, discussed below)
might be required to find the best set of operating parameters
which maximize the dynamic flow rate enhancement. Monitoring
approaches for optimizations are discussed later.
[0073] FIG. 8 shows another possible configuration for
implementation of pressure or strain pulsing to enhance fluid flow
to wells. For illustration purposes, suppose that a vertical well
45 is completed with a number of short-radius laterals 46, each of
which is considered a horizontal well. Fluid is to be withdrawn
through the well 45 with the horizontal drains. A number of
excitation wells 47 are emplaced above the horizontal laterals, and
pressure pulsing or strain pulsing is applied in these wells
through excitation devices 48.
[0074] In both cases pulsing can be generated either through a
downhole or a surface pressure pulsing which can be activated by
mechanical, hydraulic or pneumatic means.
6 PRESSURE AND STRAIN PULSE DEVICES FOR OIL EXPLOITATION
[0075] FIG. 9 shows one example of a pressure pulsing device that
causes a periodic pressure excitation at a controllable frequency
and amplitude. The pressure pulsing can be varied in frequency
(number of pulses over a time interval), in amplitude (magnitude of
the pressure pulse), and in waveform (the shape of the pressure
pulse). The pulsing is governed from the surface through an
appropriately designed electronic or mechanical control system. The
major elements of the diagram are:
[0076] a) A wellbore 50, having a casing 52, embedded in cement 53,
perforated into the target formation 54.
[0077] b) A piston pump barrel 56 which, when mechanically
actuated, generates a pressure pulse.
[0078] c) A one-way valve 57 to allow entry of fluid into the zone
below the piston pump on the upstroke of the piston.
[0079] d) An actuating device, in this drawing represented as a rod
58 to surface within the production tubing 59 that is isolated from
the casing annulus with a packer 60. This driving mechanism can be
varied in frequency and stroke length (volume).
[0080] The driving mechanism for the piston pump 56 in FIG. 9 is a
surface-driven reciprocal or rotary mechanical drive that creates
an up-and-down motion of the piston 56. Alternatively, the driving
mechanism can be an electromechanical device above the piston pump
driven by electrical power. Alternatively, a surface pressure
impulse can be applied through the tubing. In this case, the piston
pump may be replaced by a flutter valve top-hole or bottom-hole
assembly which opens and closes to create pressure surges which
enter the formation 54 through the perforations, but does not
affect the annulus pressure because of the packer 60.
[0081] The piston 56 may contain the one-way valve 57 to allow
intake of fluid on the upstroke, and expelling the incremental
fluid on the down stroke, generating the pressure pulse.
Alternatively, the fluid valve 57 can be closed, and a periodic
pressure impulse generated with a closed system.
[0082] As shown in FIGS. 10a, 10b, and 10c, a single well 62 is
producing fluids through perforations in the steel casing 63
because the pressure in the well is maintained at a value lower
than the fluid pressure in the far-field, generating a pressure
gradient which drives fluids (or a fluid-solid mixture) to the
wellbore 62. The examples show both an inclined well and a vertical
well integrated with progressive cavity pump system for purposes of
illustration only. Operational descriptions will focus on a
rotating elliptical mass, but it is understood that the principles
apply to other pulse-like sources of strain energy.
[0083] FIG. 10a shows a typical down-hole assembly for the
application of a periodic mechanical strain to the casing in the
producing formation, and the cemented casing serves as a rigid
coupling system that transmits the periodic straining to the
formation. The major elements of the diagram are:
[0084] a) A cased 63 cemented wellbore perforated into the target
formation 64 with tubing assembly and other peripheral devices.
[0085] b) A fluid pump 65 to withdraw fluids and sand from the
wellbore 62.
[0086] c) Housings and devices that couple the fluid pump 65 to the
tubing and if desired to the well casing 63, through a rigid packer
(not shown).
[0087] d) A system of rods 67 connecting the fluid pump to the
drive mechanism.
[0088] e) A drive mechanism to give rotary action to the fluid pump
and eccentric mass 68.
[0089] f) An eccentric mass 68 which is mechanically linked to the
fluid pump 65 (FIG. 10c).
[0090] Installed in the wellbore is a mechanical or
electromechanical device that applies vibrational energy to the
casing through rotation of an eccentric mass or through volumetric
straining. The device is fixed to the exterior casing 63 through
conventional means, using a packer with steel contacting pads
(slips) or other means whereby the vibrational energy is
efficiently transmitted to the steel casing with a minimum of
energy losses. A schematic cross-section of a rotating elliptical
mass is given in FIG. 10c. The central square hole is stabbed by a
square rod on the bottom of the power rods 67, which are rotated
from the surface. As the rods rotate and thereby also activate the
fluid pump 65, the eccentric mass is rotated at the same angular
velocity, or else the velocity may be less or greater if a
mechanical gearing device is included.
[0091] The rotation of the eccentric mass 68 creates an imbalance
of force, which causes the casing 63 to apply a rotational strain
to the surrounding porous medium through which the casing
penetrates. The rotational strain generates an outward moving
porosity diffusion wave that perturbs the liquid in the porous
medium, causing an accompanying pressure pulse in the liquid. The
energy thereby applied to the liquid (and entrained mobile solids)
in the porous medium leads to an enhancement of liquid flow into
the wellbore, irrespective of the direction of propagation of the
porosity perturbation. Furthermore, the strain energy thereby
applied reduces or eliminates tendencies for the material pore
throats or fractures to become blocked by fine-grained particles,
precipitants, or through the formation of stable granular arches.
The fluid produced is removed from the wellbore through the pump
65, which in this example sits above the elliptical rotating mass,
but the order of the devices may be altered. Both the pump and the
rotating mass may be mechanically driven, electrically driven, or
one may be mechanical and the other electrical.
[0092] FIG. 10b shows a typical down-hole assembly for the
application of a periodic mechanical strain to the casing in the
producing formation, and the cemented casing serves as a rigid
coupling system that transmits the periodic straining to the liquid
in the formation. The major elements of the diagram are:
[0093] a) A cased cemented wellbore 69 perforated into the target
formation 70 with tubing assembly 72 and other peripheral
devices.
[0094] b) A progressive cavity (PC) pump 73 to withdraw fluids and
sand from the wellbore 69.
[0095] c) Housings and devices that couple the stator of the PC
pump 73 to the tubing 72 and if desired to the well casing, through
a rigid packer, not shown.
[0096] d) A system of rods 74 connecting the PC pump to a drive
mechanism 75.
[0097] e) A drive mechanism 75 to give rotary action to the PC pump
and eccentric mass 76.
[0098] f) An eccentric mass 76 which is mechanically linked to the
PC pump.
[0099] The driving mechanism for the PC pump 73 in FIG. 10b is a
surface-driven rotary mechanical drive that creates a variable
frequency rotation of the rods 74, rotor, and the eccentric mass
76. Alternatively, the driving mechanism for the bottom-hole
assembly can be an electro-mechanical device located above or below
the rotor, and driven by electrical power.
[0100] The device that applies a large rotational strain to the
casing, is an eccentric mass driver 76, which is rigidly coupled to
the rotor of the pump 73. The strain is a circular impulse
triggered by rotation of a mass that is located off the centre of
rotation of the PC pump assembly, and it may be located above or
below the rotor. To transmit the strains effective to the well
casing, it is necessary that the rotating eccentric mass be rigidly
coupled to the casing. This is achieved through a packer seating
assembly (not shown) either below or above the PC pump, but close
to the eccentric mass, so that the rotary impulse is efficiently
transmitted.
[0101] FIG. 11 shows an approach to transmit periodic mechanical
energy down the tubing assembly in a cased well through application
of mechanical excitation at the surface. These strains are
transmitted to the bottom of the well, where they may be converted
to a pressure pulse, or mechanically linked to the casing to
transmit mechanical strains to the liquid in the formation. The
major elements of the diagram are:
[0102] (a) A cased cemented wellbore 78 perforated into the target
formation (not shown).
[0103] (b) A tubing and rod assembly.
[0104] (c) A drive head 79 that rotates the rod to provide motive
power to the bottom-hole fluid pump (not shown) which may be a
progressive cavity pump or a reciprocating pump.
[0105] (d) A packer device 80 to allow the polished section of the
tubing 82 to undergo a periodic vertical movement independent of
the casing or the rods.
[0106] (e) A driving mechanism 83 of variable frequency and stroke
that imparts a vertical periodic motion to the tubing 82, separate
and distinct from the pump drive-head 79.
[0107] (f) A set of reaction springs 84 and a flange 85 on the
wellhead to act in unison with the tubing drive mechanism 83 to
give the periodic vertical movement.
[0108] (g) Housings and devices that isolate yet allow the movement
of the tubing and rods to allow production from the well while
tubing excitation is active.
[0109] The example shown in FIG. 11 is a rotating motor actuating
the tubing through an eccentric cam, with counter-stroke reaction
provided by a set of springs. A variety of other driving mechanisms
can be used, including a direct mechanical linkage of a
reciprocating device to the tubing (perhaps eliminating the
springs).
7 PRESSURE OR STRAIN PULSING IN A REACTION BED
[0110] A reaction bed (FIG. 12) of granular or porous material 86
is used to foster chemical interaction by introducing two fluid
species (liquid-liquid or liquid-gas). The pore-and-throat
structure similar to that in FIG. 1 of the porous medium helps
break up the two fluids into intermingled phases with a large
surface contact area, which accelerates the reaction process. The
solid phase may, for example, be an inert material such as silica
particles, or it may be a bed of particles of catalyst or of
ceramic particles coated by a catalyst. In the case of a catalyst,
the use of a porous bed gives a high surface contact area between
the catalyst and the reacting phases. The flow through the system
is achieved either through downward gravitational flow, or through
a difference in the fluid pressure between the input and exit
ports. The flow in this case may be in any direction, but always in
the direction of the induced pressure gradient. In the example
shown, flow is from top to bottom. To increase the efficiency of
the process, the flow rate of the fluids through the reaction bed
should be maximized.
[0111] Fluid rate flow is accomplished through the application of
pressure pulses on the reaction bed by pulsing the pressure in the
liquid inflow lines (S1, S2) or exit lines (S3), or by applying
pressure pulses through a port (T) or ports in liquid (pressure)
communication with the permeants. Alternatively, vibrational strain
energy can be applied either externally or internally (U1-U4)
through the use of mechanical devices or electro-mechanical
transducers. The symbol inside the small circles indicates that
pulsating pressure or strain is being applied at these points.
[0112] In these cases, porosity diffusion processes and the coupled
pressure-strain responses create the necessary flow enhancement
effect.
8 PRESSURE OR STRAIN PULSING TO FACILITATE AQUIFER REMEDIATION
[0113] We give the example of cleaning of a potable water aquifer
that has been contaminated by a non-wetting phase, which has
permeated the pores and exists as a continuous liquid phase. Using
strategies, which, for example, may be of similar configurations to
those in FIGS. 7 and 8, and devices presented in FIGS. 8 to 10b,
pump-out wells are configured to give the best areal coverage of
the contaminated water reservoir. Furthermore, excitation leading
to fluid flow enhancement through porosity diffusion effects at
these shallow depths can be implemented as well at the surface,
through the use of harmonic oscillators, for example (not
shown).
[0114] The aquifer clean-up proceeds by continued pumping and can
also be enhanced by the input of water or other suitable liquid or
solid/liquid mixtures at the excitation wells, or at other wells
installed specifically for this purpose. The key aspects in this
case are the continued excitation, the continued provision of a
source of liquid to account for the voidage generated by pumping
the wells, and the maintenance of a pressure gradient in the
aquifer that maintains flow to the low pressure production
(clean-out) wells.
9 MONITORING AND OPTIMIZATION IN THE FIELD
[0115] Periodic straining or pulsing can enhance the flow rate in a
porous medium. The excitation gives rise to dynamic porosity
diffusion effects. Optimization of the excitation process involves
determining the most effective frequency, amplitude, and waveform
to be applied. Control of the excitation is applied through a
controller and a power source, with an oscilloscope or other
read-out device to examine the characteristics of the
excitation.
[0116] In order to optimize the process, it is necessary to monitor
both the excitation effects and the production rate. This is
achieved through monitoring production rates using flow meters or
tank gauges, and through monitoring the transmission of the
excitation within the reservoir. The important excitation factors
to monitor are the nature of the excitation and the nature of the
waves transmitted through the reservoir, and these data are
collected at a data acquisition system connected to a computer. The
parameters of importance in the reservoir are the pressure and the
wave trains. The pressure is monitored at a number of points
through pressure ports in observation wells and excitation wells,
and the wave train is monitored using geophones, accelerometers, or
other suitable devices placed in observation wells, excitation
wells, or behind the casing in production wells.
[0117] In order to optimize the process, the data streams are taken
to a central computer where the data are plotted and correlated.
Then, the parameters are optimized to allow maximization of the
production rate, subject of course to the limitations of the
equipment used for the excitation.
10 CRITERIA FOR SITE SELECTION
[0118] The preferred framework for field implementation of dynamic
enhancement is outlined below. It is designed to answer a number of
basic requirements to facilitate proper site selection, which
should increase the probability of successful implementation and
oil recovery.
[0119] 10.1 Reservoir Porosity
[0120] The effect of vibrational enhancement is relative to the
current parameters, which make economic recoverability viable. For
example, porosity simply determines the amount of oil in the
reservoir. It does not, in theory, play a direct role in the
effectiveness of the process until large porosity values are
obtained. It is suggested that for maximum effectiveness the bounds
of porosity range from 18% to 35%. At porosity levels above 35% the
effects dynamic enhancement becomes less cumulative, diminishing
with further increases. Below about 18% (i.e oil shale),
enhancement by pulsing would not be expected to occur.
[0121] 10.2 Minimum and Maximum Porous Media Thickness
[0122] An aim of dynamic enhancement through application of
pressure or strain pulses, as described herein, is to propagate a
slow moving wave in three-dimensional space. This may be in an oil
reservoir or in a system comprised of a natural or artificial
porous media. For optimum operation, the preferred constraints on
propagation of a continuous or episodic pressure or strains in the
systems described previously are as follows:
[0123] a) For oil reservoirs and aquifers a minimum thickness of
3.0 meters to a maximum of 50.0 meters.
[0124] b) For contained reaction beds, a minimum thickness of 0.20
meters to a maximum of 1.0 meters.
[0125] 10.3 Caprock
[0126] Caprock, the geomaterial that overlies an oil reservoir or
aquifer, serves two purposes. First, it prevents the pressure or
strain pulse from upward propagation beyond the parameters outlined
in Section 10.2, and it prevents upward flow of fluid. A caprock
may consist of shale, dolostone, salt (or other evaporites), very
dense clays, tight limestones, and so on. The key element for a
caprock in the case of pressure or strain pulse propagation is that
it be of extremely low permeability (e.g salt), or have very low
permeability (e.g shales, dolostone, and very dense clays). It is
important to note that the propagation of the pressure or strain
pulse propagates through the liquid in the porous medium. It is the
elastic properties of the matrix and the mobility and
compressibility conditions of the fluid, which will ultimately
determine the viability of the process. If the matrix is weak, or
brittle, the matrix might tend to crack and consolidate under the
action of pulses that have enough energy to create the dynamic
enhancement of liquid flow rate as described herein. In that case,
the invention would be contra-indicated. The caprock conditions are
of a secondary nature but are listed here for completeness.
[0127] 10.4 Permeability
[0128] The ratio of viscosity to permeability defines the mobility
of the oil in a reservoir, a contaminant in an aquifer, of fluid in
a reaction bed. The range of permeability for aquifers and bed
reactors preferably should be on the order of 1000 sq.cm (gravel)
to 0.01 sq.cm (silt). For light oil and heavy oil reservoirs the
dynamic enhancement process is viable at a range from 10.sup.-11
cm2 to 10.sup.-13 cm2.
[0129] 10.5 Viscosity
[0130] The magnitude of the diffusion constant and the scale of the
interaction determine the speed of the pressure or strain pulse.
The diffusion constant is directly proportional to permeability
divided by viscosity. From our calculations of the speed of
propagation of a pressure or strain pulse without the advantage of
large tectonic stresses in the earth or large hydraulically induced
stresses (i.e. bed reactors) we place the cutoff at 30 API gravity.
When the earth's tectonic stresses can be used as an energy source
both grain slippage and fluid flow will effect the propagation
speed of the pressure or strain pulse. In this case, and in the
case of high hydraulic stresses, the cutoff to oils can be as low
as 10 API gravity.
11 ESTIMATION OF ENHANCED FLUID PRODUCTION FROM A PULSE SERIES
[0131] It has sometimes been observed, after an earthquake, that
the flow rate of liquid through a porous medium has been
significantly improved, at least for a time. This has led to
techniques and proposals for subjecting the porous structure to
artificial seismic perturbations. However, the technique of
applying pulses to the liquid in the porous medium is quite
different from the technique of imparting seismic perturbations to
the medium itself, being much less disruptive (and less costly).
Besides, although seismic operations might open up the pores, it
might happen instead that the medium consolidates and closes the
pores; the system as described herein is aimed rather at pulsing
the liquid (and any grains that might be entrained in the liquid)
relative to the solid matrix, not at pulsing or shaking the solid
matrix itself.
[0132] A quantitative estimation of the cumulative enhancement of
fluid production, which is observed in porous media subjected to a
periodic impulse, depends on the geometric disposition between the
pulse generator and the production wellbore. Such a quantitative
estimate can be achieved for an arbitrary geometry through
numerical calculations based on the pressure pulse and a porosity
diffusion model for earthquake sources or explosive perturbations.
Those perturbations produce irreversible changes in porous media,
i.e. fracture, dilatancy and compaction. Any impulse-triggered
decrease of porosity leads to effective compaction, and this can
squeeze an additional amount of fluid from the porous medium. From
a physical point of view this mechanism is clear, and such a
mechanism is known to lead to excess pore pressures and sand
liquefaction during strong earthquakes.
[0133] In contrast to the irreversible compaction arising from
single strong perturbations, the invention is aimed at providing
reversible strains arising from continuous weak perturbations. Each
perturbation (e.g. tapping or short-term cyclic straining) is
assumed to be of an elastic nature which does not produce any
residual, irreversible deformation, but does cause a periodic
perturbation in the porosity of the system through compression and
relaxation.
[0134] In the aftermath of an impact, a porous medium relaxes to
the equilibrium state in a diffusional manner because the
relaxation process involves flow of the viscous saturating fluid
with respect to the porous skeleton. If we apply another
perturbation before the proceeding one fully decays, while
withdrawing the produced fluid through a port, a cumulative,
synergetic effect can be achieved. A quantitative estimation of
this effect for specific cases involving non-symmetric dispositions
of the perturbation source and the wellbore requires extensive
analytical and computer model calculations based on numerical
methods. This approach, however, tends to obscure the physical
logic on which the model is based.
12 FURTHER CONSIDERATIONS
[0135] An aim of the invention is to apply pressure pulses and
strain pulses to a liquid in natural and man-made porous media to
enhance the flow rate of the mobile fluid phases and to diminish
the probability of flow-rate impairment through the internal
bridging of particles. The approach has been verified
theoretically, in the laboratory, and through empirical
observations in field situations in the petroleum industry and for
water wells. A key element is the concept of porosity waves and
attendant pressure pulses, which travel through the medium by
diffusional processes. To our knowledge, this phenomenon has not
been previously identified in such media and considered for the
purposes of fluid flow rate enhancement. Applications are
envisioned particularly but not exclusively for the petroleum
industry and the chemical processing industry. Also, in reservoirs
contaminated by non-aqueous phase, non-wetting liquids,
implementation of pressure pulsing and other means of generating
porosity diffusion enhanced flow is expected to accelerate clean-up
operations, and make them more effective.
[0136] The techniques as described herein should be distinguished
from fluidized bed technology, in which a granular material is
pulsed at such an energy level that the whole solid matrix is in a
state of heaving motion. In the present case, the intention is that
the solid matrix does not move, but rather that the pulses pass
through the liquid while the solid matrix remains substantially
stationary.
[0137] Liquid flowing through a porous medium has a flow rate,
which depends on the impressed pressure differential. Within the
porous medium, the velocity of the liquid, as caused by that
impressed pressure differential, will vary from pore to pore, but
the velocity may be averaged as a volumetric flow rate over the
whole treatment volume.
[0138] Considering a pore P: if the porosity of pore P should
decrease, i.e if the pore should close up, the velocity of liquid
passing through that pore would go down, for a given impressed
pressure differential. The porosity might go down if, for example,
a grain of sand might become snagged in the pore.
[0139] The pressure pulses spread through the liquid, as a
wave-front, with a wave velocity. The wave front velocity (and
magnitude) will not be the same at every pore in the treatment
volume. The velocity of propagation of the wave-front may be
averaged over the treatment volume.
[0140] In a real porous medium, the average velocity of propagation
of the wave-front will be much faster than the average flow-through
velocity of the liquid. Similarly, at each pore, the velocity of
propagation of the wavefront will be much faster than the velocity
of the liquid travelling passing through the pore.
[0141] The pressure pulse, as it passes through a pore, causes a
surge in the liquid present in the pore. As the wavefront passes,
the pressure differential across the pore increases, and so the
through-flow velocity of the liquid in the pore momentarily speeds
up (assuming the wave-front is travelling in the same direction
through the pore as the liquid). Afterwards, the pressure
differential across the pore drops back, as the wavefront passes,
and the liquid in the pore slows down and reverts back to the
background velocity of the liquid through the pore.
[0142] If the wavefront were travelling against the liquid travel
velocity, the pulse would cause the velocity of the liquid in the
pore to drop momentarily, then gradually speed up again to the
background velocity, as the pulse passes. In some cases, the
velocity of the flow of liquid in the pore might even reverse (and
back flush the pore) momentarily.
[0143] It is the sudden changes in the through-velocity of the
liquid in the pore that prevents grains settling in the pore,
whether the pulses cause a momentary speeding up of the liquid in
the pore, or a momentary slowing down (or even reversal) of the
liquid in the pore.
[0144] Thus, the pores are kept open by the surges. The sudden
change in velocity of the liquid dislodges or flushes away grains
that might be snagged in the pores, and prevents grains from
snagging in the pores. It may be noted that an actual reversal of
the flow velocity of the liquid can be especially effective, by
back-flushing the pores clear. By sweeping or flushing the pores
clean, the flow rate of liquid through the treatment medium can be
increased; or, at the least, the rate at which the pores become
clogged can be slowed.
[0145] An even more beneficial ratcheting effect also can be
engineered. The pulses have a specific wave form, which includes a
gradual rise in pressure, followed by a gradual fall in pressure.
See FIG. 5. (The wave form at pore P might not be the same as the
wave form as created by the pulse generating means.) Insofar as
this pressure pulse gives rise to a change in the pressure
differential across the pore, the velocity of the liquid in the
pore undergoes a change that follows a similar waveform.
[0146] If the pulses are infrequent, the next (junior) pulse
reaches the pore P after the earlier (senior) pulse has died away,
and so each pulse of pressure has an independent, i.e
non-cumulative, effect on the through-velocity of the liquid
passing through the pore. This condition is illustrated in FIG.
14a. However, if the pulses are more frequent, the junior pulse
might reach the liquid in the pore before the senior pulse is
finished. That is to say: the senior-surge in the flow rate of the
liquid through the pore is still present when the junior-surge in
the flow rate arrives. The senior-surge in liquid flow rate is
caused by the pressure differential imposed by the senior pulse,
and the junior-surge in liquid flow rate is caused by the pressure
differential imposed by the junior pulse.
[0147] With the arrival of the next pulse after that, the velocity
of the flow of liquid in the pore is given a further incremental
increase, and so on. This condition is shown in FIG. 14b.
[0148] The effect is repeated in all the other pores, and thus the
effect is manifested as an increase in the overall flow rate of the
liquid through the treatment volume of the porous medium. It has
been found that the velocity of the flow of liquid through the
treatment volume can be increased asymptotically to an upper limit
93 (FIG. 14b), which is considerably faster than the background
flow rate 94 arising simply from the differential pressure imposed
on the treatment volume without pulsing. That is to say: the flow
rate is increased by the pulsing as if a larger pressure
differential had been imposed, or as if the porosity had been
increased.
[0149] Thus, not only does the pulsing as described herein tend to
keep the pores clear as the changes in flow velocity flush the
pores, but also the pulsing, if done at the right frequency, can
increase the actual flow rate of the liquid through the treatment
volume.
[0150] The frequency of the pulses should be rapid enough that a
junior pulse arrives at the pore before the senior pulse has died
away. On the other hand, the frequency of the pulses should not be
too rapid. Too high a frequency might set up resonances in the
solid matrix material, and cause the material to undergo an
amplitude of movement that might cause damage. Also, the higher the
frequency, the more it becomes difficult to get enough energy into
each pulse to actually cause a significant pressure surge in the
liquid, per pulse.
[0151] The engineer should carry out tests at the treatment site,
in which the overall through-flow rate is measured for different
frequencies of pulsing. The frequency should be increased (starting
from about 1 Hz) until a frequency is reached beyond which no
further increase in through-flow rate is achieved. Typically, that
happens when the frequency of pulsing is in the range 1 Hz to 10
Hz.
[0152] The magnitude or energy of the pulses is important. If the
energy of the pulses is too high, the solid matrix material can be
damaged. That is to say, the matrix material should not be shaken
so vigorously as to cause some consolidation of the material, which
would thereby lose some porosity and permeability. The energy
should be high enough, though, to make the momentary change in the
velocity of the liquid passing through the pores significant.
[0153] It will be understood that, in many cases, the liquid
flowing through the pores will have some sand or other solid grains
entrained in the flow. The sand grains of course come from the
solid material making up the matrix. The movement of the sand
grains, entrained in the moving liquid, should be distinguished
from consolidation of the matrix, which involves a settling
movement of the matrix material.
[0154] The direction of the pulses is important. In some case, for
example if the pulses are generated actually in the extraction well
(as in FIG. 10b, for example) the wavefront of pulses propagates in
the direction away from the extraction well. In that case, the
change in pressure differential across the pore, due to the pulse,
acts to create a momentary velocity which opposes the velocity of
the liquid through the pore towards the extraction well, due to the
imposed background pressure differential. It might be possible in
that case, by adjusting the frequency of the pulses, actually to
reduce the flow rate of the liquid through the pores, i.e to impose
on the liquid such a cumulative effect upon the velocity or
flow-rate that the pulse-created flow-rate opposes the background
flow-rate. Of course, significantly dropping the flow-rate would
run counter to the aims of the invention, and the engineer should
see to it, when operating a system in which the wave-front velocity
is in the opposite direction to the liquid flow-rate velocity, that
the frequency of pulsing stays out of the range in which flow of
the liquid towards the extraction-well might be seriously
attenuated. The ratcheting of flow velocity as shown in FIG. 14b
only applies when the pulses are travelling in the same direction
as the liquid.
[0155] By correctly setting the pulsing frequency, the pulsing can
be used to prevent clogging of the pores, by flushing the pores and
resisting the possible snagging of grains in the pores, whether the
wave-front velocity is with or against the liquid extraction
velocity.
[0156] One of the dangers of using a separate excitation well to
generate the pulses is the possibility of inadvertently
establishing a preferred pathway through the porous material, from
the excitation well to the extraction well. If that happens, the
well would be finished, in that now the liquid being pulled out of
the extraction well is simply the liquid being fed in at the
excitation well.
[0157] A separate excitation well is useful in that the engineer
will find it easier to create the type of pulses that will make a
significant difference to the flow rate of the liquid if he not
only provides a separate excitation well, so that the direction of
the pulses reinforces the flow-rate of liquid towards the
extraction well, but also if he injects a (small) charge of liquid
into the excitation well with each pulse. Injecting a charge of
liquid at each pulse delays the drop-off or fall-back of flow-rate
velocity after the pulse passes, which makes it easier to achieve
the ratcheting of the pulses that can create a significant
improvement in flow rate.
[0158] However, as mentioned, when using an excitation well, the
engineer must make sure he does not kill the production well. It is
recognised that the pulses can be made to travel considerable
distances through the porous medium; sufficiently far indeed that
the excitation well can be placed far enough away that the danger
of killing the well becomes negligible, and yet the pulses can be
made to penetrate large distances into the porous medium.
[0159] It is emphasised that the pulses are pulses of pressure
passing through the liquid; the pulses do not require the solid
matrix material to move. (Of course, if the liquid pressure
changes, a pedant might argue that the solid matrix must undergo
distortions corresponding to the change in pressure, if only very
slightly. But the invention is concerned with real practical
effects, and the pulses as described herein can, as a matter of
substance, be generated, and can perform the useful function as
described, even if the solid matrix notionally did not move at
all.)
[0160] In the case where the pulses are generated as pressure
pulses, the pulses are generated by creating motion directly in the
liquid; in the case where the pulses are generated as strain
pulses, the pulse is first applied to a local region of the solid
matrix material, and only indirectly thereby to the liquid. In that
case, the solid matrix material undergoes, or might undergo, a
measurable strain in launching the pulse into the liquid. However,
such a strain would be very localised, as to the distance of
penetration of the strain into the porous medium, whereas the pulse
that such strain creates in the liquid would then penetrate much
further into the porous medium, through the liquid.
* * * * *