U.S. patent application number 13/827639 was filed with the patent office on 2014-05-08 for optimizing enhanced oil recovery by the use of oil tracers.
This patent application is currently assigned to Glori Energy Inc.. The applicant listed for this patent is GLORI ENERGY INC.. Invention is credited to Thomas Ishoey, Michael Raymond Pavia, Egil Sunde, Terje Torsvik.
Application Number | 20140124196 13/827639 |
Document ID | / |
Family ID | 49328067 |
Filed Date | 2014-05-08 |
United States Patent
Application |
20140124196 |
Kind Code |
A1 |
Sunde; Egil ; et
al. |
May 8, 2014 |
OPTIMIZING ENHANCED OIL RECOVERY BY THE USE OF OIL TRACERS
Abstract
A method of optimizing recovery of oil from a formation that
includes injecting an oil tracer into an injection well in the
formation and applying a tertiary oil recovery process for
recovering the oil from the formation. A production well is
monitored to detect the oil tracer in oil from the production wells
within particular times. The method also includes modifying the
tertiary oil recovery process based on the detection of the oil
tracer in oil from the production well.
Inventors: |
Sunde; Egil; (Sandnes,
NO) ; Pavia; Michael Raymond; (Durham, NC) ;
Ishoey; Thomas; (Houston, TX) ; Torsvik; Terje;
(Boenes, NO) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
GLORI ENERGY INC. |
Houston |
TX |
US |
|
|
Assignee: |
Glori Energy Inc.
Houston
TX
|
Family ID: |
49328067 |
Appl. No.: |
13/827639 |
Filed: |
March 14, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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61623946 |
Apr 13, 2012 |
|
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Current U.S.
Class: |
166/250.12 |
Current CPC
Class: |
E21B 47/00 20130101;
E21B 47/11 20200501; E21B 43/16 20130101 |
Class at
Publication: |
166/250.12 |
International
Class: |
E21B 43/16 20060101
E21B043/16; E21B 47/00 20060101 E21B047/00 |
Claims
1. A method of optimizing recovery of oil from a formation
comprising a production well and an injection well; said method
comprising: water flooding said formation to a residual oil
saturation between said injection well and said production well;
after said residual oil saturation, injecting an oil tracer into
said injection well in said formation; applying a tertiary oil
recovery process for recovering said oil from said formation;
monitoring said production well to detect said oil tracer, said
monitoring being conducted within a period that starts at the time
the oil tracer is injected and ends 30 days after the oil tracer is
injected or the period starts when the tertiary oil recovery
process is started and ends 30 days after the oil recovery process
is started; and modifying said tertiary oil recovery process based
on a detection of said oil tracer in oil recovered from said
production well during said period.
2. The method of claim 1 wherein said applying of said tertiary oil
recovery process is started before said injection of said oil
tracer.
3. The method of claim 1 wherein said tertiary oil recovery process
includes a selection from the list consisting of: microbial
enhanced oil recovery, surfactant injection, polymer injection,
water injection, natural gas reinjection, air injection, air
injection, carbon dioxide injection, other gas injection, pressure
pulses and combinations thereof.
4. The method of claim 1 wherein said monitoring comprises
determining the quantity of tracer detected as a function of
time.
5. The method of claim 1 wherein said modification comprises a
selection from the list consisting of: changing a type or amount of
surfactant used in a surfactant flooding process, changing amount
of gas used in a gas injection process, changing a type or amount
of nutrients used in a microbial enhanced oil recovery process.
6. The method of claim 1 wherein said period is further limited to
start at the time the oil tracer is injected and end 7 days after
the oil tracer is injected or to start when the tertiary oil
recovery process is started and end 7 days after the oil recovery
process is started.
7. The method of claim 1 wherein said residual oil saturation is
determined by detecting water breakthrough in said production
well.
8. The method of claim 1 wherein the oil tracer is an oil soluble
tracer.
9. The method of claim 1 wherein the oil tracer is a partitioning
tracer.
10. A method of optimizing recovery of oil from a formation
comprising a production well and an injection well; said method
comprising: water flooding said formation until water breakthrough
occurs at said production well; after said water breakthrough,
injecting an oil tracer into said injection well in said formation;
applying a tertiary oil recovery process for recovering said oil
from said formation; monitoring said production well to detect said
oil tracer, said monitoring being conducted within a period that
starts at the time the oil tracer is injected and ends 30 days
after the oil tracer is injected or the period starts when the
tertiary oil recovery process is started and ends 30 days after the
oil recovery process is started; and modifying said tertiary oil
recovery process based on a detection of said oil tracer in oil
recovered from said production well during said period.
11. The method of claim 10 wherein said applying of said tertiary
oil recovery process is started before said injection of said oil
tracer.
12. The method of claim 10 wherein said tertiary oil recovery
process includes a selection from the list consisting of: microbial
enhanced oil recovery, surfactant injection, polymer injection,
water injection, natural gas reinjection, air injection, air
injection, carbon dioxide injection, other gas injection, pressure
pulses and combinations thereof.
13. The method of claim 10 wherein said monitoring comprises
determining the quantity of tracer detected as a function of
time.
14. The method of claim 10 wherein said modification comprises a
selection from the list consisting of: changing a type or amount of
surfactant used in a surfactant flooding process, changing amount
of gas used in a gas injection process, changing a type or amount
of nutrients used in a microbial enhanced oil recovery process.
15. The method of claim 10 wherein said period is further limited
to start at the time the oil tracer is injected and end 7 days
after the oil tracer is injected or to start when the tertiary oil
recovery process is started and end 7 days after the oil recovery
process is started.
16. The method of claim 10 wherein the oil tracer is an oil soluble
tracer.
17. The method of claim 10 wherein the oil tracer is a partitioning
tracer.
18. A method of optimizing recovery of oil from a formation
comprising a production well and an injection well; said method
comprising: water flooding said formation to a residual oil
saturation between said injection well and said production well;
after said residual oil saturation, injecting an oil tracer into
said injection well in said formation; applying a tertiary oil
recovery process for recovering said oil from said formation;
monitoring said production well to detect said oil tracer, said
monitoring being conducted within a period that starts at the time
the oil tracer is injected and ends 30 days after the oil tracer is
injected; and modifying said tertiary oil recovery process based on
a detection of said oil tracer in oil recovered from said
production well during said period.
19. The method of claim 18 wherein said applying of said tertiary
oil recovery process is started before said injection of said oil
tracer.
20. The method of claim 18 wherein said tertiary oil recovery
process includes a selection from the list consisting of: microbial
enhanced oil recovery, surfactant injection, polymer injection,
water injection, natural gas reinjection, air injection, air
injection, carbon dioxide injection, other gas injection, pressure
pulses and combinations thereof.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The present application claims priority to co-pending U.S.
Provisional Patent Application No. 61/623,946, entitled
"IDENTIFICATION OF OPTIMAL INJECTOR/PRODUCER WELL CONFIGURATION FOR
ENHANCED OIL RECOVERY BY THE USE OF OIL TRACERS," filed Apr. 13,
2012, the disclosure of which is hereby incorporated herein by
reference.
BACKGROUND OF THE INVENTION
[0002] Crude oil remains an important energy source. Crude oil
producers typically produce oil by drilling wells into underground
oil reservoirs in a formation. For some wells, the natural pressure
of the oil is sufficient to bring the oil to the surface. This is
known as primary recovery. Over time, as oil is recovered by
primary recovery for these wells, the natural pressure drops and
becomes insufficient to bring the oil to the surface. When this
happens, a large amount of crude oil may still be left in the
formation. Consequently, various secondary and tertiary recovery
processes may be employed to recover more oil. Secondary and
tertiary recovery processes may include: pumping, water injection,
natural gas reinjection, air injection, carbon dioxide injection or
injection of some other gas into the reservoir.
[0003] Current models used to apply these secondary and tertiary
recovery processes employ relative permeability, wettability and
capillary forces as the key variables. These models are based on
the prevailing theory in the art that the oil exists in the
reservoir primarily as droplets after water flooding. According to
the traditional models, when a continuous oil phase in a water-wet
rock is displaced by water, the continuity of oil is broken at a
given residual oil saturation where the residual oil is located as
droplets within the 3-dimensional pore network. See e.g. Amyx, J.
W, Bass, D. M. and Whiting R. L. 1960. Petroleum Reservoir
Engineering, Physical Properties. McGraw-Hill, New York. ISBN
07-001600-3.
[0004] In the petroleum industry, tracers are used to track fluid
flow, for example, in a formation. Water soluble, oil soluble and
partitioning tracers are known to be used in tracking fluid flow.
An oil soluble tracer is soluble in oil but not in water. A
partitioning tracer has affinity for both oil and water. In other
words, partitioning tracers are soluble in both oil and water. In
an oil/water system, the partitioning tracer is in equilibrium
between the oil phase and the water phase. The equilibrium constant
of the partitioning tracer determines how much of the partitioning
tracer is in the water phase as compared to the oil phase. The
phrase "oil tracer," as used herein, means oil soluble tracer or
partitioning tracer. The prevailing theory as to how the residual
oil exists in a reservoir has influenced how tracers have been used
to monitor fluid flow. For example, for partitioning tracers,
though it is soluble in oil and water, its movement is usually
determined by measuring tracer concentration in the water. Further,
prevailing theory suggests that tracers will take a long time (e.g.
months or years) to reach a production well after being injected
into an injection well. Thus, current methods start monitoring the
production wells long after the injection of the tracer and only
after breakthrough of water tracer. Further, the current methods
seek to identify a pulse (sharp increase and decline in
concentration) of tracer in the production well.
BRIEF SUMMARY OF THE INVENTION
[0005] One aspect of arriving at the present disclosure involved a
new theory that the oil in the reservoir exists primarily as long
continuous strands as opposed to the prevailing theory in the art
that the oil exists in the reservoir primarily as droplets after
water flooding. According to the new theory, long strands of oil
extend from an injector well to a producer well. Further to this
theory, embodiments of the invention include using an oil tracer to
identify when oil is delivered to a production well consistent with
the existence of strands between an injection well and a production
well. When such a phenomenon is identified, oil recovery methods
that are applied to the formation may be optimized so as to
increase oil recovery. The oil tracer can be an oil soluble tracer
or a partial oil soluble tracer (partitioning tracer).
[0006] Embodiments of the invention include a method of recovering
oil from a formation that comprises a production well and an
injection well. The formation, having been water flooded to a
residual oil saturation in all the formation or at least in part of
the formation between the injection well and the production well,
wherein water breakthrough has occurred in the production well. The
method includes injecting an oil tracer into the injection well in
the formation and applying a tertiary oil recovery process for
recovering the oil from the formation. The method also includes
monitoring oil recovered from the production well to detect oil
tracer. The monitoring being conducted at least within a period
that starts at the time the oil tracer is injected and ends 30 days
after the oil tracer is injected or the period starts when the
tertiary oil recovery process is started and ends 30 days after the
oil recovery process is started.
[0007] The foregoing has outlined rather broadly the features and
technical advantages of the present invention in order that the
detailed description of the invention that follows may be better
understood. Additional features and advantages of the invention
will be described hereinafter which form the subject of the claims
of the invention. It should be appreciated by those skilled in the
art that the conception and specific embodiment disclosed may be
readily utilized as a basis for modifying or designing other
structures for carrying out the same purposes of the present
invention. It should also be realized by those skilled in the art
that such equivalent constructions do not depart from the spirit
and scope of the invention as set forth in the appended claims. The
novel features which are believed to be characteristic of the
invention, both as to its organization and method of operation,
together with further objects and advantages will be better
understood from the following description when considered in
connection with the accompanying figures. It is to be expressly
understood, however, that each of the figures is provided for the
purpose of illustration and description only and is not intended as
a definition of the limits of the present invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] For a more complete understanding of the present invention,
reference is now made to the following descriptions taken in
conjunction with the accompanying drawing, in which:
[0009] FIG. 1 shows a diagram of a system for implementing methods
according to embodiments of the invention; and
[0010] FIG. 2 shows a flow chart illustrating steps according to
embodiments of the invention.
[0011] FIGS. 3A-3B shows a diagram of a system for implementing
methods according to embodiments of the invention; and
[0012] FIG. 4 shows a flow chart illustrating steps according to
embodiments of the invention.
[0013] FIG. 5 shows a protocol set up for preparation of core
experiments; and
[0014] FIGS. 6A-6D shows results of production of water and oil
tracer from core flooding experiments.
DETAILED DESCRIPTION OF THE INVENTION
[0015] FIG. 1 shows a diagram of a system for implementing methods
according to embodiments of the invention. System 10 includes an
injection well 100 and a production well 101 in oil bearing
formation 105. Oil 102 resides in oil-bearing formation 105.
Oil-bearing formation 105 may be any type of geological formation
and may be situated under overburden 104. Although formation 105 is
shown as being onshore in FIG. 1, it should be appreciated that
formation 105 may be located onshore or offshore. According to the
new theory, previously mentioned, oil 102 primarily exists as
continuous phase strands 102-1 to 102-n within formation 105. The
strands are of various lengths and may extend from injection well
100 to production well 101 as shown. In addition, the strands are
3-dimensional in nature and may cross link to other strands
throughout formation 105. See E. Sunde, B.-L. Lillebo, T. Torsvik,
SPE 154138, Towards a New Theory for Improved Oil Recovery from
Sandstone Reservoirs (attached hereto as Appendix A), the
disclosure of which is incorporated herein by reference in its
entirety.
[0016] According to the new theory, oil 102 is trapped within
formation 105, not as distinct droplets, but as strands (e.g.
strands 102-1 to 102-n) in portions of formation 105's network of
pores small enough to put up resistance to the surrounding drag and
pressure drop of surrounding water flow and keeps the oil from
flowing to the production well 101. Oil 102 is continuous and
present throughout the pore networks between injection well 100 and
production well 101. Between the pore networks, there may be other
parts of formation 105 where water flow has almost completely
cleared out the oil.
[0017] In a three-dimensional system, the oil will self organize
according to the sum of pressures acting on it and the available
pore network, thereby also redistributing some of its surrounding
film of water. This and the fact that oil and water will seek the
greatest possible separation to minimize friction, will leave the
residual oil in continuous oil strands occupying pore spaces in all
three dimensions. However, the general orientation of the oil
strands will be parallel to the direction of flow due to the effect
of shear forces.
[0018] With regard to FIG. 1, if formation 105 is at a residual oil
saturation, it is expected that injector well 100 is connected to
producer well 101 by oil strands 102-1-102-n. However, it will not
be known if there are several or just a few oil strands providing
the connection. That is, it will not be known whether there are
sufficient strands between injection well 100 and producer well 101
to produce oil in sufficient quantities, when a recovery method
such as water flooding is applied. Moreover, in a formation where
there are a plurality of injection wells and a plurality of
production wells it may be unknown if and to what extent these
wells are interconnected by strands in accordance with the strand
theory.
[0019] Identifying whether injector well 100 is connected to
producer well 101 by oil strands 102-1-102-n may involve the use of
an oil tracer. Once the connectivity by oil strands 102-1-102-n is
identified between injector well 100 and producer well 101, the oil
recovery process can be tailored towards increasing production from
production well 101.
[0020] FIG. 2 shows a flow chart illustrating steps according to
embodiments of the invention. Method 20 may include, at step 201,
waterflooding formation 105 to a residual oil saturation. In some
instances, this may be evident because water breakthrough has
occurred at a production well. Formation 105 may have other
producer wells in addition to producer well 101. Step 202 involves
injecting an oil tracer via injection well 100. The oil tracer may
include tritium labeled tetradecane (37 MBq, tetradecane [1,2-3H]4,
pentacosane (C.sub.25H.sub.52); hexacosane (C.sub.26H.sub.54),
heptacosane (C.sub.27H.sub.56), octacosane (C.sub.28H.sub.58),
nonacosane (C.sub.29H.sub.60), triacontane (C.sub.30H.sub.62),
hentriacontane (C.sub.31H.sub.64), dotriacontane
(C.sub.32H.sub.66), tritriacontane (C.sub.33H.sub.68),
tetratriacontane (C.sub.34H.sub.70) and the like and combinations
thereof. Some of the above oil tracers are disclosed in U.S. Pat.
No. 6,331,436 entitled "Tracers For Heavy Oil." At step 203, a
tertiary oil recovery process is applied to recover oil from
formation 105. It should be noted that step 202--injection of the
oil tracer--may be done at the same time, before or after step
203--tertiary oil recovery process--is started. The tertiary oil
recovery process may include microbial enhanced oil recovery,
surfactant injection, polymer injection, water injection, natural
gas reinjection, air injection, carbon dioxide injection, other gas
injection, pressure pulses (water hammers) and combinations
thereof.
[0021] Produced oil from production well 101 is monitored for the
presence of the oil tracer at step 204. Consistent with the strand
theory, it is assumed that once tertiary recovery step 203 has
started, oil in the vicinity of injection well 100 and consequently
the injected oil tracer may rapidly be produced at production well
101. Consequently, the monitoring is conducted at least within a
period that starts at the time the oil tracer is injected and ends
30 days after the oil tracer is injected. Alternatively, the period
starts when the tertiary oil recovery process is started and ends
30 days after the tertiary oil recovery process has started.
Embodiments of the invention may use a period fewer than the 30
days described above. The actual length of time will depend on the
type of tertiary oil recovery process. For example, for gaseous oil
recovery processes, it may be desirable that the period of
monitoring starts at the time the oil tracer is injected and ends 7
days after the oil tracer is injected. Alternatively, the period
may start when the tertiary oil recovery process is started and
ends 7 days after the oil recovery process has started. The
equivalent period for a surfactant flooding tertiary oil recovery
process is about 30 days. It should be noted that although a period
is identified herein during which monitoring should be conducted to
identify the existence of oil strands between an injection well and
a production well, in embodiments of the invention, monitoring can
continue beyond this period to gather other information and to
further optimize the tertiary oil recovery process.
[0022] The monitoring includes determining whether the oil tracer
is present in oil 102 produced from production well 101. Various
methods for making this determination may be used. For example,
atomic absorption, atomic emission, plasma emission spectrometry,
flame atomic absorption, X-ray fluorescence, mass spectrometry, and
neutron activation. Some of the above methods are disclosed in U.S.
Pat. No. 4,755,469 entitled "Oil Tracing Method." See also U.S.
Pat. No. 6,016,191 entitled "Apparatus and Tool Using Tracers and
Singles Point Optical Probes For Measuring Characteristics of Fluid
Flow in a Hydrocarbon Well and Methods of Processing Resulting
Signals." In embodiments of the invention, the monitoring process
seeks to identify a consistent level of oil tracer in the oil
produced as opposed to identifying a tracer pulse as is done for
other tracer methods. See e.g. FIG. 6A (showing a pulse of water
tracer). As such, embodiments of the invention monitor the quantity
of oil tracer in the oil produced from production well 101 as a
function of time.
[0023] Based on whether the tracer is identified and at what
concentrations, an analysis can be made as to whether and to what
extent injection well 100 is connected to production well 101 by
strands 102-1 to 102-n. Form this, the tertiary oil recovery
process may be modified to enhance the oil recovery process.
Modifying the tertiary oil recovery process may involve changing
any parameter in the tertiary oil recovery process that affects oil
production. For example, changing the type and/or amount of
surfactant injected during surfactant flooding, changing the type
of gas used in gas injection, changing the amount and/or type of
nutrients used in a microbial enhanced oil recovery process and the
like.
[0024] FIGS. 3A to 3B show diagrams of a system for implementing
methods according to embodiments of the invention. System 30 shows
oil formation 305, which has a plurality of injection wells and
productions wells. Formation 305 has been water flooded to a
residual oil saturation around the injection wells. In some
instances, this may be evident because water breakthrough has
occurred at a production well. Formation 305 is shown under
overburden 304 and has drilled in it injection wells 300-A and
300-B and production wells 301-A to 301-C. In the configuration of
system 30, it is desirable to know how oil production from
production wells would be affected by treatment of the injection
wells. To answer this question, one or more oil tracers may be used
as described below.
[0025] FIG. 4 shows a flow chart illustrating steps according to
embodiments of the invention. Method 40 may include, at step 401,
water flooding formation 305 to a residual oil saturation. Step
402, involves the application of a tertiary oil recovery process,
examples of which are described above. At step 403, a first oil
tracer may be injected into, for example, injection well 300-A.
Step 404 involves monitoring one or more of production wells
301-A-301-C to see if oil produced from these one or more
production wells contain the first oil tracer. As described above,
the monitoring may be conducted at least within a period that
starts at the time the oil tracer is injected and ends 7 or 30 days
after the oil tracer is injected. Alternatively the period starts
one month after the tertiary oil recovery process is started and
ends 7 or 30 days after the oil recovery process has started. The
tertiary oil recovery process may then be modified at step 405
based on the detection of oil tracer in oil from production wells
301-A-301-C during the monitoring period. Modification in
embodiments may include changing any parameter in a tertiary oil
recovery process that affects oil production. The changes may be
based on the amount of oil tracer detected over time during the
monitoring period. For example, changing the type or amount, or
both of surfactant injected during surfactant flooding, changing
the amount of gas used in gas injection, changing the amount or
type, or both of nutrients used in a microbial enhanced oil
recovery process and the like.
[0026] Subsequent to the modification at step 405, a second tracer
may be injected in injection well 300-A, at step 406. At step 407,
the one or more production wells 301-A-301-C are monitored for the
second oil tracer (and if desired the first oil tracer). Monitoring
may be done for periods as described in step 404. Based on the
results of the monitoring steps 404 and 407, a determination may be
made whether the modification at step 405 achieved positive results
in terms of oil recovery from formation 305. Further, the
production wells that provide the most improved results may be
identified. At step 408, a determination may be made whether the
tertiary oil recovery process has been improved to the extent where
it is considered optimized. If it is determined that the tertiary
recovery process has not been optimized, then steps 405 to 408 may
be repeated. If it is determined that the tertiary recovery process
has been optimized, then the method may proceed to oil recovery at
step 409.
[0027] Embodiments of the invention may also include the injection
of a second, third and fourth oil tracer in, for example, injection
well 300-B in a similar way as described above with injection well
300-A. The tertiary oil recovery process is then applied via
injection well 300-B. This may be done at the same time, before or
after the injection of first oil tracer as described above for
injection well 300-A. As such, each of production wells 301-A to
300-C may be monitored for the first, second, third and fourth oil
tracers. In this way, the interrelationships between the various
wells can be determined.
[0028] Based on the information learned from the method described
above with respect to the interconnection of the wells, changes may
be made to aspects of the tertiary recovery process being used for
oil recovery. For example, changing the type or amount, or both of
surfactant injected during surfactant flooding, changing amount of
gas used in gas injection, changing the amount or type, or both of
nutrients used in a microbial enhanced oil recovery process and the
like.
[0029] Although methods according to embodiments of the present
invention have been described with reference to the steps of FIG. 2
and FIG. 4, it should be appreciated that operation of the present
invention is not limited to the particular steps or the particular
order of the steps illustrated in FIG. 2 and FIG. 4. Accordingly,
alternative embodiments may provide functionality as described
herein using some or all the steps shown in FIG. 2 and FIG. 4 in a
sequence different than that shown. For example, though the methods
described above discusses the use of oil tracers for identifying
the existence of oil strands between wells, embodiments of the
invention may include methods other than those described above for
identifying the existence of the oil strands. As such, embodiments
of the invention include a method of optimizing recovery of oil
from a formation that has a plurality of production wells and at
least one injection well. The method includes a process for
identifying the existence of oil strands between the at least one
injection well and at least one of the plurality of production
wells. The method also includes implementing a tertiary oil
recovery process for recovering the oil from the formation based on
the identification of oil strands and recovering oil from the
formation as a result of the use of the implemented tertiary oil
recovery process.
[0030] The use of the strand theory to optimize oil recovery as
described herein gives unexpected results when compared with
existing methods. For example, embodiments of the invention could
result in an increase in production in the highest production well
(the well having the largest pressure difference with the injection
well) of a formation even if there are other production wells in
between the highest production well and the injection well.
Laboratory Data and Up Scaling
[0031] The Strand theory has been verified in core flooding
experiments with North Sea crude oil using oil and water soluble
tracers. In this work, labelled oil was placed at the entrance of a
core with residual oil, and flooded with brine. Oil was mobilized
by a pressure pulse.
[0032] In the experiment described below water wet Bentheimer
sandstone core was used. The physical properties of the core are
shown in Table 1. The process of core preparation and flooding is
visualized in FIG. 5. Synthetic sea water containing 20.0 g NaCl,
4.0 g Na.sub.2SO.sub.4, 3.0 g MgCl.sub.2x6H.sub.2O, 0.5 g KCl and
0.15 g CaCl was used as the connate brine. A water soluble tracer;
35S labeled sodium sulphate (37 MBq, 1.0 mCi Sodium [35S] sulphate
dissolved in 10 ml brine), and an oil tracer; tritium labeled
tetradecane (37 MBq, tetradecane [1,2-3H]4, 1 mCi dissolved in 40
ml crude oil) was used. For tracer analysis 50 .mu.l oil sample was
mixed with 1.8 ml heptane and 18 ml Ultima Gold LLT. Water samples
were prepared by mixing 100 .mu.l water with 3 ml Ultima Gold LLT.
All samples were analyzed in a Packard Bell scintillation
counter.
[0033] As shown in FIG. 5, the cylindrical sandstone core was
prepared to resemble a reservoir in the residual situation having
water and oil in representative positions and locating the oil
tracer near the inlet. After filling the core with synthetic sea
water, a volume of 35S-sulphate labelled brine was flooded through
the core. The first water tracer emerged after flooding 345 ml of
water (equivalent to 0.88 pore volumes), the peak was obtained
after flooding of 385 ml (equivalent to 0.99 pore volumes) and
99.5% of the tracer was recovered after flooding 1.5 pore volumes
(FIG. 6A). The core was filled with North Sea crude oil and 40 ml
tritium labelled oil was injected. The flow direction was reversed
during water flooding to reach residual oil concentration, leaving
the labelled oil at the inlet side at the start of the experiment
(FIG. 6B). The oil and water production profiles indicate water wet
sandstone core, as expected from Bentheimer cores with low clay
mineral content.
[0034] In order to initiate production of the residual oil the core
was pressurized by pumping brine into the core with closed outlet
valve to obtain 2 bars overpressure. A pressure pulse was generated
by opening the valve and thereby depressurizing the core. This
resulted in immediate production (within 10 seconds) of 1.0 ml oil.
The oil was collected and the process of pressurizing and
depressurizing was repeated twice, this time resulting in
production of 0.2 and 0.1 ml oil. Following this, the injection
pump rate was set to 0.1 ml/min and produced oil and water was
collected in 6 ml fractions, one fraction per hour. Tritiated
tetradecane was detected in all oil samples. The first oil fraction
contained 233 Bq (total) of tritiated tetradecane tracer; the next
two contained 40 and 30 Bq respectively. The 4th oil fraction
contained 4133 Bq and was produced after flooding with 31 ml water
(FIG. 6C). In FIG. 6D oil production and the production of oil and
water tracers are plotted together.
TABLE-US-00001 TABLE 1 Properties of sandstone core and crude oil L
(cm) 88.6 D (cm) 5.0 PV (cm.sup.3) 392 Swi 0.27 Soi 0.73 Sor (%) at
start pressure pulse 39.5 Kabs (mD) 1463 Kw 78.13 Flowrate,
ml/minute 0.10 Crude oil density, g/cm.sup.3 at 15.degree. C. 0.834
Crude oil viscosity, cSt at 15.degree. C. 2.85 PV; core pore
volume, Swi; initial water saturation, Soi; initial oil saturation,
Sor; % residual oil, Kabs; absolute permeability, Kw; effective
water permeability at pressure pulse.
[0035] Results from the experiment described above show that the
first oil produced contained oil tracer, in agreement with the
continuous oil Strand theory, while the water tracer pulse moved as
a piston through the core. In this experiment, the oil was produced
within 10 seconds after opening the pressure valve. This means that
tracer labelled oil must have moved at least 76.3 cm in 10 seconds,
an oil tracer displacement of 460 cm/minute. In comparison the
water tracer moved 88.6 cm in 3500 minutes, at a rate of 0.03
cm/minute. This shows that oil can move rapidly, over a long
distance, independent of the water flooding rate. Hence, it has
been observed the oil tracer to be transported from the entrance to
the outlet of the core at a rate 10000 times faster than the
water.
[0036] This laboratory experiment was designed to answer the
question: How is residual oil transported in the reservoir? This
question was also asked by Jones (Jones, S. C. 1985. Some Surprises
in the Transport of Miscible Fluids in the Presence of a Second
Immiscible Phase. SPE J. Paper SPE-12125 pp. 101-112.). Jones
matched his observations with the current idea that residual oil
resides in discontinuous oil droplets that are transported with
water. He therefore expected that the first oil to be produced
would be the oil closest to the outlet and the last oil produced
would be the first oil mobilized at the injection end of the core.
However, Jones unexpectedly found that the first oil mobilized by a
surfactant slug was produced simultaneously with the oil closest to
the outlet. In the alternative oil Strand theory described here,
the oil strands can be produced sequentially, squeezed out by water
flowing perpendicular to the oil strand. This would allow the oil
located near the water injector to be produced together with the
oil closest to the producer, in agreement with Jones findings.
[0037] The pressure pulse has been mathematically modeled and
verified over one pore throat (Skaelaaen, I. 2010. Mathematical
Modelling of Microbial Induced Processes in Oil Reservoirs. PhD
thesis, University of Bergen, Bergen, Norway (2010)). The
laboratory results are a scaling up from a pore length of approx.
20 microns to 0.76 meters, equivalent to approx. 4*104. A further
scaling up to reservoir scale is only two more orders of
magnitude.
[0038] Although the present invention and its advantages have been
described in detail, it should be understood that various changes,
substitutions and alterations can be made herein without departing
from the spirit and scope of the invention as defined by the
appended claims. Moreover, the scope of the present application is
not intended to be limited to the particular embodiments of the
process, machine, manufacture, composition of matter, means,
methods and steps described in the specification. As one of
ordinary skill in the art will readily appreciate from the
disclosure of the present invention, processes, machines,
manufacture, compositions of matter, means, methods, or steps,
presently existing or later to be developed that perform
substantially the same function or achieve substantially the same
result as the corresponding embodiments described herein may be
utilized according to the present invention. Accordingly, the
appended claims are intended to include within their scope such
processes, machines, manufacture, compositions of matter, means,
methods, or steps.
* * * * *