The present invention relates to a method for measuring the wettability of rock samples by measuring the relaxation time by low-field nuclear magnetic resonance.
The method finds applications notably for analysis of rocks taken from an underground formation containing or likely to contain hydrocarbons.
Knowledge of various parameters, and notably of the wettability of the rocks, is also useful notably for enhanced recovery of a formation, by injection of a fluid under pressure, and when the fluid (liquid or gas) best suited for effluents displacement is to be determined by means of preliminary tests.
The invention also finds applications in civil engineering for formation hydrology in order to evaluate the degree of pollution of formations for example, or in the building trade for testing building materials notably in order to determine waterproofing treatments for example.
There are well-known methods for determining the wettability of rocks to the water and oil that they may contain, comprising carrying out rock drainage cycles, i.e. displacement of fluids intended to decrease the water saturation, followed by imbibition, this term relating to a displacement of the fluids allowing to increase the water saturation (Sw) of the rock. The capillary pressure Pc at one point is defined as the difference at equilibrium between the pressure P(oil) of the oil and the pressure P(water) of the water. This parameter makes sense only if the two fluids are in the continuous phase in the porous medium. For a water wet medium, only the positive values make sense. On the other hand, when the medium has a mixed wettability, the fluids can remain in the continuous phase for the positive as well as for the negative capillary pressures (Pc).
For an application of this type, a complete capillary pressure measuring cycle generally comprises (
There are various types of devices allowing the curves of
In a device referred to as <<porous plate>> device, notably described in patent U.S. Pat. No. 4,506,542, the porous rock sample containing two fluids in the continuous phase is placed in an elongate cell ended at its two opposite ends by capillary barriers permeable to a first fluid. This first fluid is injected under pressure through the first membrane and the pressure difference between the injection pressure and the pressure of the fluid discharged at the other end is measured. The pressures of the two fluids and the capillary pressure Pc are constant all along the sample, and the saturation is assumed to be uniform.
It is also known to carry out progressive-speed centrifugation by means of centrifugation devices such as those described for example in patents FR-2,772,477 (U.S. Pat. No. 6,185,985) or FR-2,763,690, or in patents EP-603,040 (U.S. Pat. No. 5,463,894) or FR-2,798,734 filed by the applicant.
The sample initially saturated with a first liquid (brine for example) is placed (
During the re-imbibition stage, the velocity is decreased so as to study the return of the initial fluid into the sample. The local saturations measured with this type of device are calculated by an inversion programme from the total amount of water expelled from the sample.
According to another method, referred to as <<dynamic>> method, a sample is placed in an elongate cell ended at its two ends by water-permeable membranes. At a first end, oil under pressure is directly injected into the enclosure. Water is also injected, but this injection is carried out through the membrane and at a lower pressure. At the opposite end, the oil is directly discharged whereas the water flows out through the terminal membrane. Adjustment of the oil and water injection rates allows the capillary pressure to be the same at the inlet and at the outlet of the enclosure, which leads to a uniform saturation that can be deduced from the fluids balance. The capillary pressure is obtained for example by measuring the difference between the pressure of the oil and of the water at the enclosure outlet. Such a method is notably described by Brown H. W. in <<Capillary Pressure Investigations>>, Petroleum Transaction AIME, vol. 192, 1951. Examples of implementation are for example described in patents EP-729,022 (U.S. Pat. No. 5,698,772) or EP-A-974,839 (U.S. Pat. No. 6,229,312) filed by the applicant.
A method referred to as semi-dynamic method is also known, wherein a rock sample imbibed with a first fluid is confined in a closed cell, another fluid under pressure is injected at a first end of the enclosure and the opposite end is swept by a low-pressure fluid circulated by pumping means which carries the drained fluid outside. The device comprises means for measuring the pressure and the saturation of the sample, the amount of fluid discharged and the electric resistivity of the sample. This method is implemented for example in patent FR-2,708,742 (U.S. Pat. No. 5,679,885) filed by the applicant.
These drainage and imbibition curves being established, it is well-known to calculate the wettability indices WI from the surface areas Ad and Ai delimited by the positive and negative capillary pressure curves, as shown in
The method for measuring the wettability of a porous rock sample in the presence of water and oil according to the invention comprises determining the water wet pore surface and the oil wet pore surface when the sample is saturated with water and oil, and calculating the wettability index by combination of the values obtained for said surfaces.
Determination of the water wet pore surface and of the oil wet pore surface when the sample is saturated with water and oil is obtained for example by means of measurements of relaxation times of the sample placed in a nuclear magnetic resonance device.
The wettability index is for example determined by the relation:
where SMw is the water wet pore surface and SMo is the oil wet pore surface when the porous medium is saturated with water and oil.
According to an implementation mode, the wettability index is determined by the following operations:
According to a preferred implementation mode, the relaxation times of stages a) to c) are determined after subjecting the sample to centrifugation.
According to a preferred implementation mode, the relaxation times of stage d) are determined after forced displacement of the fluids in the sample placed in a containment cell.
An oil whose intrinsic relaxation time (TB) is as great as possible and as close as possible to that of water, dodecane for example, is preferably selected.
The characteristic relaxation times are for example those corrresponding to either the saturation curves maxima, or to mean values of said curves.
Experience shows that the measurement of INMR obtained by means of the method is as sensitive but that it requires much less time and that it is applicable to a large number of samples.
Other features and advantages of the method and of the device according to the invention will be clear from reading the description hereafter of an embodiment given by way of non limitative example, with reference to the accompanying drawings wherein:
a and 2b respectively show the distribution of relaxation times T2 for a water wet rock saturated with water and oil, and a representation of the phase distribution (matrix in hatched lines, water in light grey and oil in darker grey),
a and 3b respectively show the distribution of relaxation times T2 for a rock of intermediate wettability saturated with water and oil (same central part as in
It may be reminded that the NMR analysis technique essentially consists in applying to an object to be tested a first static magnetic polarization field Bo intended to align the initially randomly oriented hydrogen protons nuclei in the direction of the field, then a second impulsive magnetic field oscillating at the Larmor frequency, perpendicular to the first one, created by coils excited by a control signal to carry out a nuclear magnetic resonance experiment. When this impulsive field stops, the return of the nuclei to their initial state or relaxation generates electromagnetic signals (echoes) which are detected and analyzed. The presence of physical parameters of the object is determined from the amplitude characteristics of these signals.
We propose defining a new wettability index constructed by combining values of the water wet pore surface SMw and of the oil wet pore surface SMo when the porous medium is saturated with water and oil. The index can be calculated for example by the relation as follows:
or by the relation:
Quantities SMw and SMo are obtained for example by measuring the dominant relaxation time in a low-field nuclear magnetic resonance experiment.
In fact, in such an experiment, the fundamental relation connecting the longitudinal T1 or transverse T2 relaxation time to surface S and to the volume of this pore is as follows:
where T1B,2B is the relaxation time of the fluid outside the porous medium. Basically, this relation comes from the fact that the molecules in the pore undergo diffusion motions and interact with the porous wall during the measuring time (the typical relaxation time is of the order of 100 ms). The surface interactions are designated by coefficient ρ1,2 referred to as surface relaxivity. Besides, we have disregarded a gradient term that is important when measurements are performed at a high magnetic field (>0.1 T). Relation 2 is strictly valid for a pore saturated with a single fluid. In general, natural porous media have a pore size and therefore ratio S/V distribution. A relaxation times distribution is thus generally observed, but this does not modify the method described here.
When two fluids are present in a pore within the porous medium, the same physical diffusion mechanism is valid, but the distribution of the two fluids in relation to the pore surface is of crucial importance. For example, when the medium is water wet, the water is at the surface and interacts therewith, whereas the oil is inside the pore and does not interact with the surface. When the distribution of the relaxation times is measured for such a system, the result of
We thus generalize relation (2) to a two-phase water-oil system. For the water, we have:
and for the oil:
We can thus determine quantities SMw and SMo by measuring relaxation times T1 or T2 in the porous medium, relaxation times T1B or T2B of the fluids outside the porous medium, liquid volumes Vo or Vw present in the porous medium The interaction constants ρ can be determined separately, but we will see that only the ratio of these constants is necessary.
If we introduce relations 3 and 4 in base relation 1, we obtain:
The most suitable saturations for measuring quantities SMw and SMo thus remain to be determined. For reasons linked with the calculation of the relaxation time distributions, we select the irreducible water saturation (Swi,
From the distribution of the relaxation times at the different saturations (see the example of
In order to determine the surface relaxivity ratio Cρ, we use the dominant relaxation times Tw100 and To100 respectively at the two saturations Sw=100% and So=100%. The formula used is as follows:
Implementation
For implementing the method, we use for example an NMR measuring device conventionally comprising (
Practical Implementation Example
The wettability index is obtained by carrying out for example the following succession of stages with a reservoir sample whose wettability is intermediate:
1. The sample is 100% saturated with reservoir brine and placed in the NMR measuring device (as shown in
2. It is then placed in the centrifugation device (as schematized in
3. After being placed again in the centrifugation device, the sample is centrifuged at maximum speed until the residual oil saturation Sor is reached; it is then transferred into the NMR device again to measure relaxation times T1, T2 and to deduce Tw;
4. The sample is thereafter placed in a containment cell such as those described in the aforementioned patents and a water and oil miscible solvent, then oil (dodecane for example) is injected until the 100% oil saturation is reached. This point being reached, relaxation times T1, T2 are again measured in the NMR device and the value of To100 required for determination of Cρ is deduced.
We thus know all the parameters for calculation of wettability coefficient INMR.
Comparison with Index IUSBM
Comparison between wettability index IUSBM and the new index INMR shows a good correlation between these two quantities (
We have described an implementation example where a NMR type relaxometry method is used to measure the surface and the volume of the pores. This method is however not limitative. Any other analysis method can be used, notably cryomicroscopy.
Number | Date | Country | Kind |
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02/11283 | Sep 2002 | FR | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/FR03/02544 | 8/18/2003 | WO | 10/3/2005 |