FR3111706A1 - Method for determining the pore volume of a sample of porous medium - Google Patents

Abstract

The invention relates to a method for determining at least the pore volume of a sample of a porous medium, implemented by means of at least one device for measuring an average saturation of the pore volume of the sample and of two fluids which can be distinguished by means of measuring the average saturation. The method comprises at least the following steps: (i) the device is calibrated for measuring the average saturation for the two fluids; (ii) A plurality of successive injections of predetermined volumes of the second fluid, previously saturated with the first fluid, are carried out and the average saturation of the porous volume with second fluid is measured for each of these volumes; (iii) at least the pore volume of the sample is determined by determining the slope of a curve representative of the change in the volume of second fluid as a function of the average saturation of the pore volume in second fluid. Figure 4 to be published. .

Description

Method for determining the pore volume of a sample of porous medium

The present invention generally relates to the field of the characterization of a porous medium, in particular the characterization of the porous volume of a porous medium, and in particular the characterization of the porosity of a porous medium.

The porous medium according to the invention can be a sample of rock from an underground formation: in this case, the method according to the invention can find its application in the field of exploration and exploitation of oil or geological storage sites for gases, such as CO ₂ or methane. The invention is in particular advantageously applied in the context of enhanced oil recovery (EOR: “Enhanced Oil Recovery”). But the porous medium according to the invention can also relate to a catalyst, a polymer, concrete, or even filter membranes.

Generally speaking, the porosity of a porous medium refers to its fraction of void volume (i.e. pore volume) over its total volume. It is a characteristic of a porous medium, expressed as a percentage or as a fraction. To calculate the porosity of a porous medium, it is therefore necessary to know the dimensions of the sample and the pore volume.

We can, however, distinguish "total" porosity (or even "intrinsic" porosity) from "useful" porosity. The total porosity corresponds to the ratio of the volume of voids in the sample of porous medium to the total volume of the sample. The total porosity thus corresponds to the volume of all the pores of a porous medium, whether they are connected or isolated. The useful porosity corresponds to the ratio of the void volume accessible to fluids (i.e. able to contribute to the flow of these fluids) in the sample of porous medium to the total volume of the sample. The useful porosity therefore only takes account of the pores connected to each other. The estimation of the connected porous volume and/or of the useful porosity is particularly important in applications in the field of the exploitation of hydrocarbons from a geological reservoir because these properties contribute to estimating the volume of hydrocarbons in place in the geological reservoir. .

Several methods are known for measuring porosity.

Document WO 2013/154854 A1 is known in particular, which relates to a method for determining the porosity of a rock sample, by multiplying a measurement of the pore volume of the sample by the bulk density which itself results from the measurement of a mass of the sample.

The method described in the document EP 0871045 A1 is also known, which is based on the quantification by an NMR measurement of the quantity of water included in a rock sample, when this sample.

The method described in the document (WO 2011/089367 A1) which determines the porosity from the interpretation of the transient states in a sample subjected to a flow of a fluid is also known.

These methods have at least one of the following drawbacks, which can distort or complicate the measurement of porosity:

- the measurement of porosity cannot be carried out under real conditions of pressure, temperature and confinement, which can be very different from atmospheric conditions in the case of an EOR application. However, these real conditions can significantly change the porosity of a rock. This concerns at least the methods described in the documents WO 2011/089367 A1, WO 2013/154854 A1 and EP 0871045 A1

- the 'total' porosity is measured, and not the 'useful' flow porosity. All empty spaces (or spaces that contain fluid) are not necessarily connected to each other, and therefore do not contribute to the flow of a fluid that one would seek to inject into the rock. This concerns at least the methods described in the documents WO 2011/089367 A1 and EP 0871045 A1.

- the fluids injected into the rock are limited to gas, and do not allow the use of fluids conventionally present in an oil field. This concerns the methods described in the documents WO 2011/089367 A1, WO 2013/154854 A1 and EP 0871045 A1.

The method according to the invention aims to overcome these drawbacks. In particular, the method according to the invention makes it possible to measure at least the porous volume accessible to a fluid of a porous medium, and for example to deduce the useful porosity therefrom. In addition, the method according to the invention can be implemented by means of a measurement system that can reproduce pressure, temperature, and confinement conditions) corresponding to the in situ conditions, for example the pressure, temperature, and confinement conditions. containment of a geological reservoir several kilometers deep. The method according to the invention can also advantageously be implemented using fluids of interest for the intended application, such as brines and/or oils involved in the field of enhanced hydrocarbon recovery.

The invention relates to a method for determining at least one property relating to the pore volume of a sample of a porous medium, said method being implemented by means of at least one device for measuring an average saturation said pore volume of said sample, by means of a first fluid and a second fluid distinguishable by said device for measuring an average saturation of said pore volume of said sample when said first and second fluids are injected sequentially into said sample . The method according to the invention comprises at least the following steps:

A) Said device is calibrated for a measurement of an average saturation of said porous volume of said sample, at least by completely saturating said sample with said first fluid, then with said second fluid, and by measuring a response variation of said means for measuring said average saturation for said sample fully saturated with said first fluid and for said sample fully saturated with said second fluid;

B) A plurality of successive injections into said sample of predetermined volumes of said second fluid are carried out, said sample being, prior to each of said successive injections, completely saturated in said first fluid, and measurement is made, for each of said predetermined volumes in said second fluid, said average saturation of said pore volume in said second fluid by means of said device for measuring an average saturation of said pore volume of said calibrated sample;

C) from said average saturation measurements of said pore volume in said second fluid carried out for each of said predetermined volumes of said second fluid injected into said sample, at least said pore volume of said sample is determined by determining a slope of a curve representative of the evolution of said volume in said second fluid as a function of said average saturation of said porous volume in said second fluid.

According to an implementation of the invention, said device for measuring an average saturation of said porous volume of said sample can comprise means for measuring an intensity of a signal having passed through said sample.

According to one implementation of the invention, said calibration may consist in measuring a first difference between an average intensity of said signal having passed through said sample completely saturated by said first fluid and an average intensity of said signal having passed through said sample completely saturated by said second fluid.

According to one implementation of the invention, said average saturation of said porous volume with said second fluid can be determined by measuring a second difference between an average intensity of said signal having passed through said sample completely saturated by said first fluid and an average intensity of said signal having passed through said sample comprising said predetermined volume of said second fluid, said second average intensity difference being normalized by said first average intensity difference.

According to an implementation of the invention, said means for measuring an intensity of a signal having passed through said sample may comprise means for X-ray radiography, said means for X-ray radiography comprising at least one source of X-rays and an X-ray detector.

According to one implementation of the invention, said means for measuring an intensity of a signal having passed through said sample may comprise a means for measuring by neutron rays.

According to an implementation of the invention, said first fluid can be a brine and said second fluid can be an oil.

According to one implementation of the invention, said first and second fluids can be brines with salinities at least 50 g/L apart.

According to one implementation of the invention, said plurality of successive injections may comprise at least 5 successive injections, preferably 10 successive injections.

According to an implementation of the invention, in step C), it is also possible to determine a porosity of said sample of said porous medium, from said porous volume and dimensions of said sample of said porous medium.

Other characteristics and advantages of the method according to the invention will appear on reading the following description of non-limiting examples of embodiments, with reference to the appended figures and described below.

List of Figures

FIG. 1 illustrates a non-limiting example of a measurement system for implementing the method according to the invention.

Figure 2 shows, for an example of application, the evolution of the average saturation in a brine measured over time in a sample of a porous medium, and the evolution of the volume of this brine injected into this sample at course of time.

FIG. 3 presents, for the application example of FIG. 2, a curve representative of the evolution of the volume of brine injected into the sample during stage 1, as a function of the average saturation measured.

Figure 4 presents, for the application example of figure 2, measurement points and a curve representative of the evolution of the volume of brine injected into the sample during step 2, as a function of the average saturation measured.

Figure 5 presents, for the application example of Figure 2, X-ray radiographs as well as saturation profiles for 4 examples of predetermined volumes of brine injected into the sample.

The invention relates to a method for determining at least one property relating to the pore volume of a sample of a porous medium. By property relating to the porous volume, we mean the porous volume itself, but it can also include the porosity (and in this case the useful porosity), the permeability, the density of the porous structure (in other words the porous matrix) etc

The porous medium can be a rock from an underground formation (such as a rock from an oil reservoir for example), or else a porous material of the polymer type, concrete, a filtering membrane or any material having a volume porous. By sample of porous medium is meant a piece of the porous medium. In the case of a rock from an underground formation, the sample may have been taken by coring.

The present invention is described below in a non-limiting manner in the context of the field of the petrophysical characterization of a rock originating from an underground formation, in particular in the field of the exploitation of hydrocarbons from an underground formation, and in particular enhanced oil recovery from a subterranean formulation

The method according to the invention is implemented by means of at least one device for measuring an average saturation of the pore volume of the sample. Saturation means the fraction of the pore volume occupied by a fluid. Mean saturation means an average of the saturation of a fluid in the whole of the pore volume of the sample considered, or even an average of the saturation of a fluid at any point of the sample.

According to a main variant of the invention, the device for measuring an average saturation of the pore volume of the sample can be a means for measuring an intensity of a signal having passed through the sample considered. According to an implementation of this main variant of the invention, the device for measuring an average saturation of the pore volume of the sample can comprise means for X-ray radiography, and/or means for measuring by X-rays neutron. It should be noted that according to the invention, an average intensity of a signal having passed through the sample considered is exploited. By average intensity is meant an average of the intensity measured by the means for measuring the intensity for the entire volume of the sample. For example, a means of measurement by 2D X-ray radiography leads to a 2D image of the intensity of the signal having propagated in the sample. The average intensity is then the average of the local intensities measured at any point on the surface of the 2D image resulting from the 2D X-ray radiography.

But the device for measuring an average saturation of the pore volume of the sample can be any means of measuring the saturation, not based on the measurement of an intensity of a signal having passed through the sample, such as means of measurement by NMR (Nuclear Magnetic Resonance).

The method according to the invention is implemented at least by means of a first fluid and a second fluid, these two fluids having to be distinguishable by the device for measuring the average saturation when the first and second fluids are injected. sequentially in the sample. In other words, the two fluids are chosen so that they can be distinguished by the device for measuring the average saturation when they are both present in the sample, after having been injected sequentially. For example, when the device for measuring the average saturation corresponds to an X-ray radiography means, the two fluids must have an average atomic number that is sufficiently far apart, in particular far apart by at least 3%, preferably 6%. According to one implementation of the invention, the couple of fluids can be a brine and an oil, in particular typical of those encountered in a geological reservoir of hydrocarbons. This makes it possible to get closer to the in situ conditions, and thus to estimate a property relating to the realistic porous volume. Alternatively, the pair of fluids can be two brines of different salinities, that is to say having salinities far apart, preferably at least 50 g/L, very preferably at least 100 g/L.

According to a particular but non-limiting implementation of the main variant of the invention, the device for measuring an average saturation of the pore volume comprises X-ray radiography means. Conventionally, X-ray radiography means X-ray comprises an X-ray source, and an X-ray detector. Typically, the signal intensity measured by the X-ray radiography means varies log-linearly with saturation. Thus implemented, the process exploits an X-ray attenuation contrast between two fluids injected into the porous medium sample studied. Advantageously, the porous medium sample can be positioned so that the X-ray source, the porous medium sample and the X-ray detector are substantially aligned. Advantageously, the X-ray radiography means are capable of performing a plurality of radiographs, for example at regular intervals during the implementation of the method according to the invention. The regular interval can be between 0.1 and 5 seconds, and can be equal to 1 second, for example. Thus, the production of X-rays at regular intervals makes it possible to monitor regularly and in real time the evolution of the fluids injected into the sample.

Advantageously, the method according to the invention can be implemented by means of a measuring system comprising, in addition to the device for measuring the average saturation of the pore volume of the sample, at least one cell to contain the sample of porous medium, and means for injecting fluid into the sample placed in the cell. Advantageously, the cell makes it possible to apply pressure, temperature and confinement conditions which are close to in situ conditions, for example the pressure, temperature and confinement conditions of a geological reservoir located at a depth of several kilometers. An example of such a measurement system is described in document WO 2017/129312 A1.

Advantageously, the measurement system further comprises means for processing and analyzing the measurements, for example by computer using a microprocessor, to determine at least one property relating to the pore volume of the medium sample porous. Preferably, the fluids of the process according to the invention are injected by means of fluid injection means configured for a sequential injection of fluids into the cell automatically. This makes it possible to accelerate the implementation of steps 1) and 2) of the method according to the invention described below, and thus to obtain properties relating to the pore volume of a sample of porous medium more quickly.

FIG. 1 illustrates, in a schematic and non-limiting manner, an embodiment of a measurement system that can be implemented to carry out the steps of the method according to the invention. The measurement system 1 comprises a cell 3 which contains a sample of porous medium (not shown), a means for measuring an average saturation 4 of the pore volume of the sample, means 2 for injecting fluid into the cell 3. In addition, the measurement system according to this embodiment comprises means for processing and analyzing the measurements in the form of a computer 1. According to this embodiment, the computer 1 can also control the means for injecting fluid 2. The measuring system according to this embodiment further comprises a means of measuring the volume flow rates 5 and a pipe 6 allowing the line between the fluid injection means 2 and the cell 3 to be purged.

The method according to the invention comprises at least the following steps:

1) Calibration of the device for the measurement of the average saturation

2) Measurement of the average saturation for several predetermined volumes of the second fluid

3) Determination of at least the pore volume of the sample

The steps of the method according to the invention are detailed below.

1) Calibration of the device for the measurement of the average saturation

During this step, the device is calibrated for measuring the average saturation of the pore volume of the sample, at least by completely (i.e. totally) saturating the sample with the first fluid then with the second fluid, and by measuring a variation in response of the device for measuring the average saturation as a function of the fluid completely saturating the sample.

In other words, during this step, the means for measuring the average saturation is calibrated from the variation in response of the means for measuring the average saturation for two states of total saturation in each of the two fluids involved in the commissioning. implementation of the method according to the invention.

According to the main variant of the invention according to which the device for measuring the average saturation of the pore volume of a sample is a means for measuring an intensity of a signal having passed through the sample, the device is calibrated for measuring the average saturation by measuring a difference in the average intensity (or even a variation in average intensity) between the signal having passed through the sample totally saturated by the first fluid and the signal having passed through the sample totally saturated by the second fluid. This difference is, in the rest of the process, called "calibration intensity difference" and is used for the purpose of normalizing the measurements from the device for measuring the average saturation, as will be described in step 2) .

According to one embodiment of the main variant of the invention according to which the method is implemented by means of X-ray radiography means, it is possible to measure a difference in intensity (or even a variation in intensity) average between the signal having passed through the sample saturated completely by the first fluid and by the second fluid, expressed in a proper unit by means of measurement by X-ray radiography. For example, the difference in average intensity between two brines of salinities different can be for example 100 gray level points.

According to an implementation according to which the two fluids are miscible (case of two brines for example), it suffices to inject each of the fluids one after the other, sequentially.

According to an implementation according to which the two fluids (called main) are not miscible (case of a brine and an oil for example), one can use one or more intermediate fluids allowing successive miscibility of the two main fluids, between each injection of the main fluids. According to an implementation according to which the first fluid is a brine and the second fluid is an oil, one can for example use as intermediate fluid isopropyl alcohol, miscible with the brine when in contact with the brine, and miscible with certain oils, when in contact with them.

Advantageously, to achieve total saturation in each of the two (main) fluids, each fluid is injected with large volumes, for example at least 5 times the total volume Vt of the sample for each fluid. This injection of each of the fluids until total saturation of the sample can advantageously be carried out continuously over time.

According to an advantageous embodiment, during this step, the average saturation of the pore volume with second fluid is measured at several instants of the injection phase of the second fluid (preferably in a regular manner over time), by means of the device for measuring an average saturation of the porous volume. At this stage of the step, the device for measuring an average saturation of the porous volume is not yet calibrated, but the measurements can be calibrated a posteriori. For example, according to the main variant of the invention according to which the device for measuring the average saturation of the porous volume of a sample is a means for measuring an intensity of a signal having passed through the sample, normalizes the average saturation measurements by the calibration intensity difference as defined above.

2) Measurement of the average saturation for several predetermined volumes of the second fluid

During this step, a plurality of successive injections of predetermined volumes of the second fluid are carried out in the sample, and, for each of the predetermined volumes injected into the sample, the average saturation of the porous volume with second fluid is measured, by means of the device for a measurement of an average saturation of the calibrated pore volume. According to the invention, the sample is, prior to each of the successive injections, completely saturated with the first fluid. In other words, controlled volumes of the second fluid are injected into the porous structure of the sample considered, and the average saturation is measured for each of these volumes.

According to the invention, at least two successive injections of two distinct volumes of the second fluid are carried out in the sample, preferably at least five successive injections and very preferably ten successive injections. Indeed, as will be described in step 3), these measurements are intended to determine a slope of a curve representative of the evolution of the volume of second fluid injected as a function of the average saturation measured. Consequently, the greater the number of measurement points, the greater the freedom from potential measurement errors for certain measurement points.

According to the main variant of the invention according to which the device for measuring the average saturation of the pore volume of a sample is a means for measuring an intensity of a signal having passed through the sample, it is possible to determine the average saturation of the pore volume with second fluid by measuring a difference (or even a variation) between the average intensity of the signal having passed through the sample completely saturated by the first fluid and the average intensity of the signal having passed through the sample comprising a predetermined volume of the second fluid, this intensity difference being normalized by the calibration intensity difference as determined in step 1). In other words, we determine the average saturation pore volume by a predetermined volume of second fluid according to a formula of the type:

(1)

Or is a difference between the average intensity of the signal having passed through the sample completely saturated with the first fluid and an average intensity of the signal having passed through the sample comprising a predetermined volume of the second fluid and is the calibration intensity difference determined in step 1) described above.

According to an implementation of the main variant of the invention according to which a means of X-ray radiography is used to measure the average saturation of the pore volume of the sample, one can apply the equation (1) above with corresponding to the average intensity difference measured by the X-ray radiography means measured in step 1) between a state of the sample totally saturated with fluid 2 and a state of the sample totally saturated with fluid 1 , And the average intensity difference measured by the X-ray radiography means in step 2) between a state of the sample totally saturated with fluid 1 and a state of the sample saturated with a predetermined volume of fluid 2.

According to one implementation of the invention, the maximum volume of the second fluid injected into the sample is predetermined so that the entire volume of the second fluid injected is contained in the porous structure. In other words, the maximum volume of the second fluid injected into the sample is predetermined so that the second fluid does not pierce (i.e. so that it does not come out of the sample) when injected into the sample, as this would lead to a skewed measurement of the average saturation for the injected volume. One can advantageously use a means of measurement by X-ray radiography as means of measurement of the average saturation for this purpose, because this means of measurement of the saturation allows a local measurement (2D images) which can be used to monitor that the second fluid does not break through during injection, as will be shown in the application example below.

According to an implementation according to which one has an a priori estimate of the pore volume of the sample (for example if it has been previously determined by another process or if values are given in the literature for this sample or for geological analogues) of the sample considered, the maximum volume of the second fluid injected into the sample is less than 0.8 times the pore volume of the sample a priori. Since the a priori value of the pore volume may be marred by error, this makes it possible to avoid a breakthrough of the second fluid if the a priori value of the pore volume is underestimated by less than 20%.

According to the advantageous embodiment according to which measurements of the average second fluid saturation were carried out during step 1) as described above, an a priori value of the pore volume of the sample can be estimated at from these measurements of the average saturation carried out during the continuous injection of the second fluid, measurements from which it is possible to deduce an a priori estimate of the porous volume by determining the slope of a straight line representative of the evolution of the volume of second fluid injected according to the average saturation. This advantageous embodiment will be illustrated in the application example below. It should be noted that the pore volume determined by this embodiment may be marred by error (it is therefore indeed an a priori estimate). Indeed, as the second fluid is injected continuously and always at the same rate according to this embodiment, if an error is made on the injection rate, the estimation of the volume is distorted, and consequently the calculation of the volume also porous. It should be noted that the average saturation measurements carried out during step 2 allow a more reliable determination of the pore volume according to step 3), because these measurements are carried out independently of each other, for a succession of volumes of second fluid injected into the sample previously completely saturated with the first fluid, which reduces the overall uncertainty in the estimation of a volume-dependent slope. Furthermore, the advantage of injecting a plurality of predetermined volumes of second fluid, after complete saturation with first fluid before each injection of the second fluid, makes it possible to use different flow rates and injection times, and thus to reduce the uncertainty on each measurement point. Indeed, it is thus possible, for example, to reduce the flow rate (i.e. increase the injection time) for the injection of smaller volumes, and conversely increase the flow rate (i.e. reduce the injection time) for the injection of larger volumes. volumes.

According to an implementation of the invention, the minimum volume of the second fluid injected into the sample is predetermined according to the precision of the measuring devices used for the implementation of the method according to the invention, for example the uncertainty on the means for measuring the flow rate of the fluids injected into the sample and/or the uncertainty on the means for measuring the average saturation according to the invention.

Advantageously, the predetermined volumes of the second fluid injected into the sample of porous medium during the second step are between 0.05 and 0.8 times of an a priori estimate of the porous volume. In this way, the errors in estimating the average saturation linked to uncertainties in the measurement and to a breakthrough of the second fluid are avoided.

According to one implementation of the invention, the injection of a predetermined volume of the second fluid is controlled using a means of measuring the volume flow.

According to one implementation of the invention, it is possible to inject a predetermined volume of second fluid into the sample of porous medium considered in the following way, by means for example of the measurement system described above and in particular in Figure 1:

- the first fluid is injected into the measuring system to purge the lines of any fluid between the pump of the injection means and the sample;

- the first fluid is injected into the sample so as to completely saturate the pore volume of the sample, by means of the fluid injection means 2;

- By means of fluid injection means 2, the second fluid is injected to purge the lines between the pump and the porous structure, for example by means of conduit 6 in Figure 1;

- A predetermined volume of the second fluid is injected into the sample. This volume can be quantified using the volume flow measurement, by means of the volume flow measuring means 5, and the injection time.

All of these steps can be repeated for each predetermined volume of second fluid to be injected into the sample of porous medium.

3) Determination of at least the pore volume of the sample

During this step, from the measurements carried out in the previous step, at least the pore volume of the sample is determined by determining a slope of a curve representative of the evolution of the predetermined volume of second fluid as a function of the average second fluid saturation measured in the previous step.

In other words, the pore volume is determined of the sample considered according to a formula of the type:

(2)

Or represents the slope of a curve representative of the evolution of the volume predetermined in second fluid according to the average saturation in the second fluid, this curve being produced using the measurements of step 2, calibrated as described in step 1.

According to the main variant of the invention according to which the means for measuring the average saturation of the porous volume of a sample comprises a means for measuring the intensity of a signal having passed through the sample, by combining the equations (1 ) and (2), the pore volume of the sample can be determined using a formula of the type:

[Math 3

(3)

According to an advantageous implementation of the invention, the slope of the curve representative of the evolution of the predetermined volume of second fluid is determined as a function of the average saturation of second fluid by a linear regression, for example implemented by means processing and analysis means such as a computer.

Advantageously, prior to the determination of the slope of the curve representative of the evolution of the predetermined volume of second fluid as a function of the average saturation of second fluid, the measurements carried out in step 2 are pre-selected so as to exclude the values for which, for the highest volumes of second fluid injected, the measurements of the average saturation as a function of the volume of second fluid injected deviate from a straight line passing through the measurements made for the lowest volumes injected. This makes it possible to disregard the saturation measurements carried out in step 2 when the volume of second fluid injected is greater than the pore volume (breakthrough of fluid 2).

Advantageously, from the pore volume , we can determine the porosity of the sample of porous medium according to the formula below:

(4)

Or is the total volume of the sample considered.

According to an implementation of the invention according to which the sample is of cylindrical shape, of length L and of diameter D, it is possible to determine the porosity of the sample of porous medium according to the formula below:

(5)

Such a formula is advantageously used for rock samples, which are generally taken by coring and therefore have a cylindrical shape.

According to one implementation of the invention, it is also possible to determine the permeability K of the sample of porous medium from a correlation law called “ K-ϕ correlation law”. These laws, which are generally determined empirically and are in the form of correspondence tables, make it possible to estimate a permeability value from a porosity value.

According to an implementation of the invention, it is also possible to determine the density of the matrix of the sample of porous medium (i.e. the solid part of the porous structure), using the dry mass of the sample m _s , according to a formula of the type:

( 6)

Examples

The advantages of the method according to the invention are presented below in an application example.

For this example of application, the sample of porous medium considered is a cylindrical sample of Bentheimer rock. Its dimensions, measured with a caliper, are as follows: length L = 2.010 cm and diameter D = 0.980 cm. It can therefore be deduced that the sample has a total volume of 1.52cc with an uncertainty of +/- 0.01cc, applying the formula:

[Math 7] (7)

For this application example, the two fluids used are two brines: a brine having a salinity of 10 g/L (brine A) and a brine having a salinity of 200 g/L (brine B).

The implementation of the method according to the invention is carried out by means of the measurement system presented in FIG. 1, the device for measuring the average saturation in the pore volume of the sample corresponding to an X-ray radiography means, an X-ray being taken every 10 seconds.

Furthermore, for the implementation of step 2 of the method according to the invention, the predetermined volumes of brine B are injected at a flow rate of 0.05 cc/min, controlled by the volume flow measurement means 5 in FIG. .

The curve with a solid line in figure 2 represents the evolution of the mean saturation S in brine B in the sample over time T, during the application of steps 1 (t = 0 min to t = 50 min) and 2 ( t=50 min to t=250 min) of the process according to the invention. The dotted curve in FIG. 2 represents the evolution of the volume V of brine B in the sample over time T during these same stages.

Furthermore, the measurements were carried out under the following conditions: confinement pressure of 30 bars, pore pressure of 10 bars, and temperature of 23°C.

The implementation of steps 1 to 3 of the method according to the invention is detailed below.

Step 1 :

In this step, a large volume of brine B is injected into the sample initially saturated with brine A, until its complete saturation. Complete saturation is identified by the stabilization over time of the measurement of the average intensity of the signal having passed through the sample. The difference between the average intensity measured when the sample is completely saturated with brine B and the average intensity measured when the sample is completely saturated with brine A is 100 gray levels. This mean intensity difference is used for the normalization of all the intensity measurements carried out during the implementation of the method. This translates to the plateau P at an average saturation value of 1 in Figure 2.

Furthermore, during this step, a first estimate of the porous volume can be made according to the advantageous embodiment described above. More precisely, the slope of a curve representing the evolution of the volume of second fluid injected as a function of the average second fluid saturation measured during this first step, that is to say from t=0 min to at plus t = 50 min. FIG. 3 represents such a curve, for volumes of second fluid injected between t=0 and approximately 12 min (beyond the sample is completely saturated). It can be observed in FIG. 3 that the curve (in solid line) representing the injected volume V varies linearly (the linear part of the curve is marked in thick line) as a function of the average saturation S for volumes up to about 0.65cc. More precisely, the slope of the corresponding line (in dotted lines) is measured at 0.285 (+/- 0.062), thus making it possible to calculate an estimate of the pore volume Vp = 0.285cc (+/- 0.062cc). This estimate is only approximate but makes it possible to predetermine the volumes of brine B which will be injected in the second stage. It is in fact not necessary to inject volumes of brine B greater than 0.285cc.

2nd step :

A series of seven predetermined volumes of brine B are injected, the sample being completely saturated beforehand with brine A. The intensity measurements taken over time T are normalized by the value determined in step 1. The average saturation for each predetermined volume of brine B is given by plateaus P1 to P7 of curve S in figure 2. The average saturation values measured for each of the seven volumes V1 to V7 of brine B injected are also given in table 1. This table also presents an estimate of the uncertainties (more precisely expanded uncertainties, such that the 95% confidence interval IC95 is equal to twice the standard deviation (i.e. C95=2* δ ), determined by taking an error relative standard on the flow measurement δ _flow = 0.001 cc/min, on the time measurement δ _time = 10 s and on the saturation measurement δ _sat = 0.01 (i.e. 1 saturation point).

Saturation
measured (frac.) Volume
injected (cc) Uncertainties 2*δ _Sat (frac.) 2*δ _flight (CC) V1 0.050 0.050 0.020 0.01679 V2 0.195 0.102 0.020 0.01714 V3 0.348 0.153 0.020 0.01771 V4 0.523 0.204 0.020 0.01849 V5 0.677 0.255 0.020 0.01944 V6 0.800 0.307 0.020 0.02054 V7 0.885 0.348 0.020 0.02177

Step 3:

Figure 4 presents in the form of a diamond the measurement points of table 1, that is to say the values of the average saturation measured for each of the seven volumes V1 to V7 of brine B, as well as the error bars on the volume measurement and on the average saturation measurement. A linearity can be observed between the measured saturations and injected volumes, except for the points corresponding to volumes V6 and V7, which seem to deviate from the linear trend.

Figure 5 shows the radiographs R1, R3, R5, and R6 taken respectively for the volumes V1, V3, V5 and V6 injected into the sample (which is, for this representation, arranged vertically, with a brine B injected from below of the sample). Brine B is here represented by the black color and brine A by the white color. The intermediate colors correspond to a mixture between these two miscible fluids. This figure 5 also presents the saturation profiles S1, S3, S5 and S6 along the sample, interpreted from the images obtained from the X-ray radiography, the distance D = 0 designating the outlet of the sample and the distance D = 1 designating the entrance.

These representations make it possible to observe that from volume V6, the volume of brine B injected is too large compared to the pore volume, and pierces at the exit of the sample. The breakthrough is marked by a saturation greater than 0 at the sample outlet (indicated by the arrow in Figure 5). This analysis makes it possible to exclude the measurements relating to the volumes V6 and V7 for the determination of the slope of the curve representative of the evolution of the volumes injected according to the average saturation measured. We can also see that the volumes V6 and V7 are greater than the a priori estimate of the porous volume (0.285 cc) carried out in step 1, which confirms that it is preferable not to take these measurement points into account. .

The dotted curve C in FIG. 4 presents a straight line obtained by a linear regression applied to the measurement points corresponding to the volumes V1 to V5. As the intensity differences measured in step 2 are normalized by the intensity difference measured in step 1, the slope of the line directly gives the pore volume of the sample, by applying equation (2 ). In this case, the pore volume is equal to 0.323 cc.

From equation (4), the porosity is determined: ϕ = 0.323 / 1.52 = 21.3%.

The uncertainties measured for each measurement point can then be used to evaluate the uncertainty on the linear regression. The objective is to statistically determine all the linear regression lines passing through the measured data, including their uncertainties. To do this, synthetic data series are generated, in which the measured points (injected volumes and measured saturations) vary around their value according to their previously calculated uncertainty and presented in table 1. A normal distribution law is used to distribute these uncertainties statistically. A linear regression (slope and y-intercept) is calculated for each synthetic series obtained. The statistical distribution of the slopes thus determined is characterized by a standard error δ _slope = 0.019, and therefore a standard error on the pore volume δ _Vporous = δ _slope = 0.019cc. The expanded uncertainty on the porosity is calculated by propagation at δ _porosity = 0.025 which leads to a porosity ϕ = 21.3% +/- 2.5%.

Thus, the method according to the invention makes it possible to measure at least the porous volume accessible to a fluid of a sample of a porous medium, and thus to be able to deduce therefrom in particular the useful porosity. In addition, the method according to the invention can be advantageously implemented by means of a measurement system capable of reproducing pressure, temperature, and confinement conditions) corresponding to in situ conditions, for example the pressure, temperature , and containment of a geological reservoir located several kilometers deep. The method according to the invention can also advantageously be implemented using fluids of interest for the intended field of application, such as brines and/or oils involved in the field of enhanced hydrocarbon recovery.

Claims (10)

Method for determining at least one property relating to the pore volume of a sample of a porous medium, said method being implemented by means of at least one device for measuring an average saturation of said pore volume of said sample, at the means of a first fluid and a second fluid distinguishable by said device for measuring an average saturation of said pore volume of said sample when said first and second fluids are injected sequentially into said sample, characterized in that one carries out at least the following steps:
A) Said device is calibrated for a measurement of an average saturation of said porous volume of said sample, at least by completely saturating said sample with said first fluid, then with said second fluid, and by measuring a response variation of said means for measuring said average saturation for said sample fully saturated with said first fluid and for said sample fully saturated with said second fluid;
B) A plurality of successive injections into said sample of predetermined volumes of said second fluid are carried out, said sample being, prior to each of said successive injections, completely saturated in said first fluid, and measurement is made, for each of said predetermined volumes in said second fluid, said average saturation of said pore volume in said second fluid by means of said device for measuring an average saturation of said pore volume of said calibrated sample;
C) from said average saturation measurements of said pore volume in said second fluid carried out for each of said predetermined volumes of said second fluid injected into said sample, at least said pore volume of said sample is determined by determining a slope of a curve representative of the evolution of said volume in said second fluid as a function of said average saturation of said porous volume in said second fluid. A method according to claim 1, wherein said device for measuring an average saturation of said pore volume of said sample comprises means for measuring an intensity of a signal having passed through said sample. A method according to claim 2, wherein said calibration comprises measuring a first difference between an average intensity of said signal having passed through said sample completely saturated with said first fluid and an average intensity of said signal having passed through said sample completely saturated with said second fluid. Method according to one of Claims 2 or 3, in which the said average saturation of the said porous volume in the said second fluid is determined by measuring a second difference between an average intensity of the said signal having passed through the said sample completely saturated by the said first fluid and an average intensity said signal having passed through said sample comprising said predetermined volume of said second fluid, said second average intensity difference being normalized by said first average intensity difference. A method according to one of claims 2 to 4, wherein said means for measuring an intensity of a signal having passed through said sample comprises x-ray radiography means, said x-ray radiography means comprising at least one X-ray source and an X-ray detector. Method according to one of Claims 2 to 4, in which the said means for measuring an intensity of a signal having passed through the said sample comprises means for measuring by neutron rays. Method according to one of the preceding claims, wherein said first fluid is a brine and said second fluid is an oil. Process according to one of Claims 1 to 7, in which the said first and second fluids are brines with salinities at least 50 g/L apart. Method according to one of the preceding claims, in which the said plurality of successive injections comprises at least 5 successive injections, preferably 10 successive injections. Method according to one of the preceding claims, in which in step C), a porosity of said sample of said porous medium is also determined from said porous volume and dimensions of said sample of said porous medium.