U.S. patent application number 09/883387 was filed with the patent office on 2002-03-14 for method of evaluating physical parameters of an underground reservoir from fock cuttings taken therefrom.
Invention is credited to Egermann, Patrick, Lenormand, Roland.
Application Number | 20020029615 09/883387 |
Document ID | / |
Family ID | 8851598 |
Filed Date | 2002-03-14 |
United States Patent
Application |
20020029615 |
Kind Code |
A1 |
Lenormand, Roland ; et
al. |
March 14, 2002 |
Method of evaluating physical parameters of an underground
reservoir from fock cuttings taken therefrom
Abstract
System of evaluating physical parameters such as the absolute
permeability of porous rocks of a zone of an underground reservoir,
from fragments taken from this zone, such as rock cuttings carried
along by the drilling mud. Rock fragments (F) are immersed in a
viscous fluid contained in a vessel (1). Pumping means (2, 3) first
inject into vessel (1) a fluid under a pressure that increases with
time, up to a determined pressure threshold, so as to compress the
gas trapped in the pores of the rock. This injection stage is
followed by a relaxation stage with injection stop. The pressure
variation measured by detectors (7, 8) during these two successive
stages is recorded by a computer (9). The evolution of the pressure
during the injection process being modelled from initial values
selected for the physical parameters of the fragments, the computer
adjusts them iteratively so as to best get the modelled pressure
curve to coincide with the pressure curve really measured.
Application: petrophysical measurement.
Inventors: |
Lenormand, Roland;
(Rueil-Malmaison, FR) ; Egermann, Patrick;
(Rueil-Malmaison, FR) |
Correspondence
Address: |
ANTONELLI TERRY STOUT AND KRAUS
SUITE 1800
1300 NORTH SEVENTEENTH STREET
ARLINGTON
VA
22209
|
Family ID: |
8851598 |
Appl. No.: |
09/883387 |
Filed: |
June 19, 2001 |
Current U.S.
Class: |
73/38 |
Current CPC
Class: |
G01N 15/08 20130101 |
Class at
Publication: |
73/38 |
International
Class: |
G01N 015/08 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 23, 2000 |
FR |
00/08059 |
Claims
1) A method of evaluating physical parameters such as the absolute
permeability of porous rocks of a zone of an underground reservoir,
from fragments taken from this zone, characterized in that it
comprises: immersing fragments (F) in a viscous fluid contained in
a containment vessel (1), a stage of injection, into the vessel, of
this fluid under a pressure that increases with time, up to a
determined pressure threshold, so as to compress the gas trapped in
the pores of the rock, a relaxation stage with injection stop,
measuring the evolution of the pressure in vessel (1) during the
two injection and relaxation stages, modelling the evolution of the
pressure during the injection and relaxation process, from initial
values selected for the physical parameters of fragments (F), and a
stage of iterative adjustment of the values of the physical
parameters of the rock fragments so that the modelled evolution is
best adjusted to the measured evolution of the pressure in the
vessel.
2) A method as claimed in claim 1, characterized in that the
containment vessel is filled with cuttings invaded by drilling
fluids.
3) A method as claimed in claim 1, characterized in that the
containment vessel is filled with cuttings that have been
previously cleaned.
4) A device for evaluating physical parameters such as the absolute
permeability of porous rocks of a zone of an underground reservoir,
from rock fragments taken from this zone, characterized in that it
comprises: a containment vessel (1) for porous rock fragments (F),
means (2, 3) for injecting a viscous fluid into vessel (1) so as to
first fill the vessel containing the rock fragments and then to
perform a cycle comprising a stage of injection of the fluid into
the vessel under a pressure that increases with time, up to a
determined pressure threshold (P.sub.M), so as to compress the gas
trapped in the pores of the rock, and a relaxation stage with
injection stop, means (7, 8) for measuring the evolution of the
pressure in vessel (1) during the injection and relaxation stages,
and a processing system (9) for modelling the evolution of the
pressure during the injection and relaxation process, from initial
values selected for the physical parameters of the rock fragments,
and for iteratively adjusting the values to be given to these
physical parameters so that the modelled pressure evolution is best
adjusted to the measured pressure evolution in the vessel.
5) A device as claimed in claim 4, characterized in that the
injection means comprise a pump (2) injecting water at a constant
flow rate into a surge tank (3) filled with high-viscosity oil.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a method and to a device
for evaluating the absolute permeability of a zone of an
underground hydrocarbon reservoir from rock samples taken from this
zone, such as cuttings obtained during well drilling
operations.
[0002] The current petroleum context leads operators to taking an
interest in new zones (deep offshore) and in new types of
reservoirs (marginal structures close to existing surface
installations). Considering the drilling costs linked with the
difficult environment of these new discoveries or with the limited
size of certain structures, operators can no longer allow
themselves to drill complementary appraisal wells without taking
the risk of compromising the economic viability of the project. The
development strategy set before starting production is therefore
less strict so as to allow <<real time>> adaptation to
the nature of the information collected as a result of production
well drilling, which is referred to as appraisal development.
[0003] Petrophysical measurements play a key part in the appraisal
of the quality of a reservoir. However, the delays linked with this
type of measurement are often very long and thus incompatible with
the reactivity required for the success of such appraisal
developments. New, faster and less expensive evaluation means are
therefore sought as a decision-making support.
[0004] The cuttings carried along by the mud have been subjected to
in-situ examinations for a long time. They are carried out by the
teams in charge of mud logging operations and they are essentially
intended to complete the description of the geologic layers crossed
through during drilling, which is performed from logs.
BACKGROUND OF THE INVENTION
[0005] Work has already been done to evaluate petrophysical
properties from cuttings. Acoustic properties relative to S and P
waves have been measured for example. Various parameters have also
been studied, such as the hardness and the deformation of rock
fragments, or the porosity and the permeability thereof.
[0006] According to a first known permeability measurement method,
the rock fragment is previously coated with resin. A thin slice is
cut from the coated rock and placed in a measuring cell. It
comprises means for injecting a fluid under pressure at a
controlled flow rate and means for measuring the pressure drop
created by the sample. Since the resin is impermeable, the absolute
permeability is deduced from Darcy's equation by taking into
account the real surface area occupied by the cuttings.
[0007] This method is for example described by:
[0008] Santarelli F. J. et al; <<Formation evaluation from
logging on cuttings>>, SPERE, June 1998, or
[0009] Marsala A. F. et al; <<Transient Method Implemented
under Unsteady State Conditions for Low and Very Low Permeability
Measurements on Cuttings>>; SPE/ISRM No.47202, Trondheim,
Jul. 8-10, 1998.
[0010] This type of measurement can only be obtained in the
laboratory after long cuttings conditioning operations.
[0011] Another method is based on an NMR (Nuclear Magnetic
Resonance) measurement that is performed directly on the cuttings
after previous washing followed by brine saturation. This type of
measurement gives a directly exploitable porosity value.
Permeability K is determined by means of correlations of the same
nature as those used within the scope of NMR logging.
[0012] An illustration of this method can be found in the following
document:
[0013] Nigh E. et al; P-K.TM.: <<Wellsite Determination of
Porosity and Permeability Using Drilling Cuttings>>, CWLS
Journal, Vol.13, No.1, December 1984.
SUMMARY OF THE INVENTION
[0014] The object of the method according to the invention is to
evaluate physical parameters such as the absolute permeability of
porous rocks of an underground reservoir zone from rock fragments
(cuttings for example) taken from this zone.
[0015] The method comprises:
[0016] immersing the fragments in a viscous fluid contained in a
containment vessel,
[0017] a stage of injection, into the vessel, of the viscous fluid
under a pressure that increases with time, up to a determined
pressure threshold, so as to compress the gas trapped in the pores
of the rock,
[0018] a relaxation stage after injection stop,
[0019] measuring the evolution of the pressure in the vessel during
the two injection and relaxation stages,
[0020] modelling the evolution of the pressure during the injection
and relaxation process, from initial values selected for the
physical parameters of the fragments, and
[0021] a stage of iterative adjustment of the physical parameter
values of the rock fragments so that the modelled evolution is best
adjusted to the measured pressure evolution in the vessel.
[0022] According to the circumstances, the containment vessel can
be filled with cuttings invaded by drilling fluids or previously
cleaned.
[0023] The device according to the invention allows to evaluate
physical parameters such as the absolute permeability of porous
rocks of an underground reservoir zone, from rock fragments taken
from this zone. It essentially comprises:
[0024] a containment vessel for porous rock fragments,
[0025] means for injecting a viscous fluid into the vessel in order
first to fill the vessel containing the rock fragments, and to
perform a cycle comprising a stage of injection, into the vessel,
of fluid under a pressure that increases with time (preferably at a
constant flow rate to facilitate measurement of the volume of fluid
injected), up to a determined pressure threshold, then to compress
the gas trapped in the pores of the rock, and a relaxation stage
after injection stop,
[0026] means for measuring the evolution of the pressure in the
vessel during the two injection and relaxation stages, and
[0027] a processing system for modelling the evolution of the
pressure during the injection and relaxation process, from initial
values selected for the physical parameters of the rock fragments,
and for iteratively adjusting the values to be given to these
physical parameters so that the modelled pressure evolution is best
adjusted to the measured pressure evolution in the vessel.
[0028] The injection means comprise for example a pump injecting
water at a constant flow rate into a surge tank filled with a
high-viscosity oil communicating with the containment vessel
through valves.
[0029] The method is satisfactory for rocks of very different
permeabilities ranging from some millidarcy to several hundred
millidarcy. Considering the limited surface area occupied by the
implementation device and the speed with which the measurements and
the adjustment between the theoretical data and the experimental
data can be performed, the method lends itself particularly well to
field conditions. It is thus quite possible to envisage measurement
and interpretation directly on the site within a very short time,
therefore with no possible comparison with those required to obtain
equivalent results by means of laboratory methods. This opens up
interesting possibilities as regards characterization since this
new source of information can be put to good use as a support for
interpretation of electric logs and to fine down evaluation of a
well in terms of production potential.
BRIEF DESCRIPTION OF THE DRAWINGS
[0030] Other features and advantages of the method and of the
device according to the invention will be clear from reading the
description hereafter of non limitative examples, with reference to
the accompanying drawings wherein:
[0031] FIG. 1 diagrammatically shows the device,
[0032] FIG. 2 diagrammatically shows the structure of a porous rock
cutting or particle wherein the effects of the injection of a
high-viscosity fluid such as oil are modelled,
[0033] FIGS. 3a to 3c diagrammatically show the variation curves of
the pressure prevailing in the vessel of the device of FIG. 1,
during the injection and relaxation stages, for four different
rocks,
[0034] FIG. 4 shows the accordance between the permeabilities
obtained for four rock particles, by means of a conventional core
testing method and by the method according to the invention,
and
[0035] FIGS. 5a to 5d show, for the previous four rocks, the
accuracy that can be obtained when adjusting the modelled pressure
curves in relation to the experimental curves.
DETAILED DESCRIPTION
[0036] As mentioned above, determination of physical parameters of
rocks such as the absolute permeability thereof, for example,
essentially comprises three stages:
[0037] I) a stage of acquisition of experimental measurements of
the pressure variations from cuttings, leading to experimental
curves,
[0038] II) a stage of modelling the physical phenomena that appear
in the cuttings during this operating cycle, for arbitrary values
of the physical parameters sought (permeability K) involved in the
model, allowing to establish similar theoretical curves, and
[0039] III) an adjustment stage where the values to be given to the
physical parameters involved in the model are determined so as to
obtain best adjustment of the experimental curves and of the
theoretical curves.
[0040] I) Measurement acquisition
[0041] The first stage is carried out by means of a device as
diagrammatically shown in FIG. 1. It comprises a containment vessel
1 in which the cuttings are initially introduced. A constant
delivery rate water pump 2 communicates through a line 3 with the
base of a surge tank 4 containing a high-viscosity oil. The
opposite end of surge tank 4 communicates with a first end of
containment vessel 1. A series of valves V1 to V4 allows selective
communication of vessel 1 with surge drum 3 containing the oil and
with a drain line 4, and isolation thereof. The opposite end of
containment vessel 1 communicates via an isolating valve V5 with a
separator 6. Two manometers 7, 8 are respectively connected to the
opposite ends of vessel 1. The pressure variations measured by
manometers 7, 8 are acquired by a computer 9.
[0042] The vessel is first filled with cuttings. The latter can be
cuttings that are immediately available on the site, i.e. invaded
by drilling mud and gas released by decompression.
[0043] It is also possible to use cuttings available after
cleaning, from which all the fluids have been previously drawn
away. In cases where containment vessel 1 is filled with cleaned
cuttings, helium coming from a bottle 5 is injected so as to expel
the air contained in the vessel.
[0044] Vessel 1 is then filled with a high-viscosity oil. The oil
occupies the free space between the cuttings and it also enters the
rock by spontaneous imbibition. A degassing process occurs, whose
intensity and duration depends on the nature of the rock (mainly
the porosity thereof). This degassing process only affects part of
the gas. A certain residual volume remains trapped in the cuttings
in form of disconnected clusters.
[0045] An oil injection is then performed (at a constant injection
rate, for example, so as to readily measure the amount of oil that
has entered the pores of the rock) with a gradual pressure rise
stage (part C1 of the pressure curve) as the residual gas trapped
in the pores is compressed. When the pressure reaches a determined
threshold P.sub.M, oil injection is stopped. A stabilization occurs
then. The fluids tend to rebalance in the cuttings and a slow
return to pressure equilibrium (part C2 of the pressure curve) is
observed.
[0046] FIGS. 3a to 3d show examples of evolution of the pressure
signal observed for cuttings of four different rocks with a flow
rate of 480 cc/h. Whatever the rock considered, the same general
pressure evolution is observed. A progressive increase is noticed
during the injection stage as the residual gas is compressed. The
time required to increase the pressure by 5 bars ranges, according
to rocks, from 15 to 40 seconds depending on the initial volume of
trapped gas. As soon as injection is stopped, the pressure
decreases. Although this decrease is significant for rocks 1 and 2,
it remains more moderate for rocks 3 and 4. A gradual stabilization
of the signal can be observed in the long run.
[0047] II) Modelling
[0048] The object of this modelling process is to obtain an
estimation of permeability K from the pressure measurements.
[0049] The cuttings are considered to be of homogeneous size and of
spherical shape, and the gas is assumed to be perfect. The pressure
drop linked with the viscosity of the gas is disregarded in
relation to that of the oil, considering the difference between the
viscosities thereof. The residual gas trapped in the cuttings after
spontaneous imbibition of the oil takes the form of disconnected
clusters that are homogeneously distributed. The capillary pressure
is also considered to be negligible.
[0050] Considering the spherical shape of the cuttings, one will
reason on the basis of a cap of thickness dr (FIG. 2) and calculate
the evolution of the pressure at the boundary of the rock particle
when a flow of oil q is injected.
[0051] One considers that the total flow rate Q of the injected
fluid is equitably divided among the N rock particles, and that
each one receives flow rate 1 q = Q N .
[0052] The gas law allows to deduce the local gas saturation Sg
from the moment that pressure P: 2 S g = S g0 P 0 P
[0053] is known (P.sub.0 is the pressure of the oil). A material
balance is made on the oil in the cap. The accumulation is equal to
the difference between the inflow and the outflow. We thus deduce
therefrom: 3 div V 0 + S 0 t = 0.
[0054] Since S.sub.0=(1-S.sub.g)=(1-S.sub.g0 P.sub.0/P), we deduce
therefrom that: 4 S 0 t = S 0 P P t = ( S g0 P 0 P 2 ) P t .
[0055] Besides, since 5 V 0 = - K 0
[0056] gr{right arrow over (a)}dP.sub.0 and the capillary pressure
can be considered to be negligible, which gives
P.sub.0=P.sub.gas=P, the previous equation can be written as
follows: 6 - K 0 P + S g0 P 0 P 2 P t = 0.
[0057] It follows therefrom that: 7 P = 0 S g0 K P 0 P 2 P t .
[0058] We thus obtain the conventional form of a diffusion type
equation with, however, a 1/P.sup.2 accumulation factor term that
is due to the compressible nature of the gas.
[0059] In spherical coordinates, the Laplacian is equal to 8 1 r 2
r ( r 2 P r ) .
[0060] Finally, the equation to be solved is written as follows: 9
r ( r 2 P r ) = r 2 P 2 P t with ( 1 ) = 0 S g0 P 0 K ( 2 )
[0061] As it is injected, the oil expels the air in the free space
between the cuttings and it enters the rock by spontaneous
imbibition. Despite certain precautions, a certain volume of gas
may remain outside as a result of the non-regular shape of the
cuttings. This trapped volume (V.sub.gp) has a direct effect on the
general form of the pressure response and it has to be taken into
account in the solution.
[0062] A certain compressibility due to the experimental device
also has to be taken into account. It results from the vessel, from
the lines as well as from the properties of the oil. The equivalent
compressibility observed is of the order of 0.0005 bar.sup.-1.
[0063] Since the oil used is saturated with gas at atmospheric
pressure, dissolution phenomena appear when the pressure increases
during measurement. These aspects are taken into account by
introducing a diffusion parameter expressing the molecule exchanges
at the gas-oil interfaces.
[0064] The diffusion equation is solved by means of the finite
difference method with an explicit pattern and by applying the
boundary conditions in time P(r,0)=P.sub.atm and in space
P(R,t)=P.sub.ext and 10 P r ( 0 , t ) = 0.
[0065] The convergence test on P.sub.ext is based on a comparison
between the saturation in gas remaining in the rock particle and
the value obtained by volume balance from the amount of oil
injected.
[0066] Solution of the diffusion equation during the relaxation
period is identical. Only the test condition changes since the
injection stop leads to maintaining the volume of gas in the rock
particle.
[0067] III) Adjustment of the model to the experimental results
[0068] The model is implemented in a calculator such as computer 9
(see FIG. 1) in form of a software and included in an iterative
optimization loop. The model is <<run>> with a priori
values selected for permeability K, factors .PHI. and S.sub.g0
involved in relation 2 by their product, the resulting simulated
pressure curve is compared with the experimental curve and, by
successive iterations where the previous values are changed in the
model, those allowing best adjustment of the theoretical curve and
of the experimental curve are found.
[0069] FIGS. 5a to 5d show the accordance that is rapidly obtained,
by successive iterations, between the theoretical curve and the
experimental curve for the previous four rock fragments. As can
also be seen in FIG. 4, the results obtained by applying the method
are quite comparable, for the four rocks, with those obtained in
the laboratory after long conditioning times using conventional
methods.
[0070] Modelling of the physical phenomena that occur during the
experiments . . . This modelling process is programmed within a
code, which allows to adjust the experiments by trial and error,
and thus to deduce the corresponding value of K.
* * * * *