SYSTEM AND METHOD FOR MEASURING A FLOW PROPERTY OF A FLUID WITHIN A POROUS MEDIUM

Abstract

The invention relates to a system and a method for measuring at least one flow property of at least one fluid within a porous medium. The measurement system (1) comprises at least one cell (2), means for injecting fluid(s) into the cell, and means for X-ray radiography (4, 5).

Description

1. The present invention relates to the field of measuring the flow properties of a fluid in a porous medium, particularly a porous medium originating from an underground formation. These measurements can be used, in particular, to determine the residual oil saturation of an underground formation, especially during the exploration and production of hydrocarbon wells, and particularly for enhanced oil recovery (EOR). It is currently estimated that for all active oil reservoirs, 60%–65% of the oil in place remains trapped. Various mechanisms are responsible for this trapping; for example, geological heterogeneity leads to non-homogeneous oil movement across the reservoir. But even at the local scale, the presence of interfacial tension between water and oil leads to capillary trapping of the oil phase at the pore centers in the case of water-wettable rocks.This trapping can represent up to 50% of the oil in place. Water-wettable reservoirs constitute roughly half of the world's reservoirs. Consequently, mobilizing the residual oil contained in the matrix under conditions of preferential water wettability is a real challenge. However, the use of surfactants injected in the aqueous phase can significantly reduce capillary trapping. Surfactants have the property of considerably lowering this tension, even virtually eliminating it. The use of surfactant additives was successfully tested on sandstones in the 1980s and is experiencing a resurgence of interest. Enhanced oil (hydrocarbon) recovery processes using surfactant injection represent strong potential because they allow for the release of a significant quantity of oil trapped in the rock pores. To characterize capillary detrapping, it is necessary to measure the relative properties of fluid flow within the porous medium. Currently, the methods used require several weeks to obtain this fluid flow characterization. Indeed, it is currently necessary to prepare rock samples with a pore volume sufficient for the fluids produced and collected at the outlet of the porous volume to be of sufficient volume to characterize the properties of the medium (typically several milliliters). Thus, typical sample sizes are on the order of 5 cm in diameter and 10 cm in length. The samples are initially saturated with water, then drained with oil, and finally swept with water to obtain residual oil saturation. Then, a volume of an ASP formulation (ASP stands for Alkaline, Polymer Surfactant) is injected, followed by a final water sweep.At the end of the sequence, the residual oil saturation is deduced from the total volume of oil produced at the outlet of the porous medium. Furthermore, in other technical fields, it may be useful to quickly characterize the flow of a fluid within a porous medium, particularly a porous medium formed by a polymer. The invention relates to a system and a method for measuring at least one flow property of at least one fluid within a porous medium. The measurement system comprises at least one cell, means for injecting fluid(s) into the cell, and X-ray radiography means. The use of X-ray radiography means in a cell, into which a fluid can be injected, allows for rapid, real-time measurements. The system and method according to the invention: The invention relates to a system for measuring at least one flow property of at least one fluid within a porous medium.The measurement system comprises at least one cell containing the porous medium, means for injecting the fluid into the cell, and X-ray radiography means comprising a source and a detector, the X-ray radiography means being capable of acquiring a plurality of 20 radiographs during the injection of the fluid. According to one embodiment of the invention, the measurement system comprises means for controlling the injection means. According to another embodiment, the measurement system comprises means for collecting and / or analyzing the radiographs obtained by the detector. Advantageously, the cell has a substantially cylindrical shape. Preferably, the diameter of the cell is substantially between 2 mm and 5 cm, and more preferably between 5 mm and 2 cm.According to one embodiment of the invention, said injection means comprise a plurality of fluids to be injected into said cell, simultaneously or sequentially. Advantageously, said injection means comprise at least one pump, at least one valve, and at least one pressure sensor. In one embodiment, said fluid is selected from an aqueous phase and an oil phase. In one feature, at least one aqueous phase comprises at least one additive, in particular a surfactant. In one design, said measurement system comprises means for positioning said cell. Furthermore, the invention relates to a method for measuring at least one flow property of at least one fluid within a porous medium, wherein said measurements are performed using said measurement system according to one of the preceding features.Advantageously, these flow properties are selected from the average saturation of the fluid, and / or the saturation profile of the fluid, and / or the pressure difference of the fluid within the sample. Preferably, a plurality of radiographs are taken during the injection of the fluid. In one embodiment, a plurality of radiographs are taken at regular intervals, substantially between 0.1 and 5 seconds. In one embodiment of the invention, a residual saturation curve is plotted as a function of the capillary scan number using these measurements.15 According to one embodiment, X-ray measurements are performed using said measuring system during the following injection steps within said cell: a) oil is injected at at least two different flow rates; b) water is injected at at least two different flow rates; and c) an aqueous phase comprising at least one additive is injected at at least two different flow rates. A brief description of the figures: Other features and advantages of the system and the method according to the invention, 25 will become apparent from the following description of non-limiting examples of embodiments, with reference to the accompanying figures described below. Figure 1 illustrates a measuring system according to one embodiment of the invention. Figure 2 illustrates the average oil saturation and pressure difference for an example of a method according to the invention.Figure 3 illustrates the oil saturation profile along the sample during the drainage phase at several time points for the example in Figure 2. Figure 4 illustrates the oil saturation profile along the sample during the surfactant scanning phase at several flow rates for the example in Figure 2. Figure 5 represents the residual saturation measured with the system as a function of the number of capillaries scanned for two types of rock for the example in Figure 2. Detailed Description of the Invention: The present invention relates to a system for measuring at least one flow property of at least one fluid within a porous medium. The flow properties may include, in particular, the average fluid saturation and / or the fluid saturation profile and / or the fluid pressure difference within the porous medium, etc.The flow properties can depend on the operational parameters as well as the flow rates of each fluid. The fluid can be of any type; it may include, in particular, an aqueous phase, an oil phase, etc. When the fluid includes an aqueous phase, it may also contain at least one additive, for example, a surfactant. Surfactants have the property of significantly reducing the interfacial tension between water and oil, or even virtually eliminating it. The porous medium can be of any type; it may be, for example, a sample of rock taken from an underground formation, or it may be a porous polymer. In the case where the porous medium is rock, it is possible to characterize the fluid flow(s) within the underground formation.15 The measurement system according to the invention comprises: at least one cell suitable for containing the porous medium, the cell may be called a sample holder, the measurement system may include several cells in order to carry out simultaneous measurements, means for injecting the fluid into the cell, the injection means are suitable for circulating at least one fluid in the cell, for example, the injection means may include at least one fluid reservoir, at least one pipe connecting the reservoir to the cell, at least one pump, at least one valve and at least one pressure sensor, means for X-ray radiography, the X-ray radiography means 25 include at least one X-ray source, and an X-ray detector, preferably a flat panel detector, the cell being positioned between the source and the detector, the radiography means are suitable for carrying out a plurality of radiographs when injecting at least one fluid into the cell.X-ray radiography allows for the measurement of 30 fluid flow properties within the porous medium contained in the cell. Indeed, through calibration, it is possible to correlate the gray level of the obtained radiographic image to the saturation in the porous medium. To achieve this, a preliminary image can be acquired with the sample saturated with at least one fluid to be injected into the porous medium. Thus, it is possible to obtain so-called "reference" radiographs, which can be compared with the radiographs taken during the injection process. For example, in the case of a measurement related to a rock sample, a first radiograph can be taken of a sample saturated with water, and then a second radiograph can be taken of a sample saturated with oil.The use of multiple radiographs also allows for rapid (on the order of an hour, compared to the several weeks required for current measurements) and real-time measurement. For safety reasons, the radiography equipment and the cell can be located inside an X-ray shielding booth, while the other components of the measurement system, particularly the fluid injection means, can be located outside the shielding booth. Furthermore, the measurement system can include the following elements, alone or in combination: control means for injection means; for example, the control means are capable of controlling at least one pump, at least one valve, and at least one flow meter, etc.The control means may include a computer system, means for collecting and / or analyzing radiographs obtained by the detector, and means for collecting and / or analyzing measurements obtained from radiographs. For example, the means for collecting and / or analyzing measurements may include a computer system; in this case, the means for collecting and / or analyzing measurements may also serve as visualization means. The means for collecting and / or analyzing measurements may, in particular, link the gray level of each radiograph image to the saturation in the porous medium. Means for moving the cell may be capable of moving the cell by translations in two horizontal directions and in one vertical direction. Thus, it is possible to adapt the measurement system to different cell dimensions.In cases where the measurement system includes both control means for the injection means and means for collecting and / or analyzing radiographs, a single computer system can perform both functions. According to one embodiment of the invention, the cell of the measurement system may have a substantially cylindrical shape. Preferably, the cell has small dimensions (referred to as a mini-sample) compared to the conventional dimensions used for oil saturation measurements. These conventional dimensions are on the order of 5.35 cm in diameter and 10 cm in length. According to one possible cell design, the cell diameter (or the diameter in which the cell is inscribed when it is not cylindrical) is substantially between 2 mm and 3 cm, preferably between 5 mm and 2 cm. For example, the cell diameter may be approximately 1 cm.The length (height) of the cell can be between 5 and 50 mm, and can be, for example, approximately 20 mm. The reduced dimensions of the cell compared to conventionally used samples allow for faster measurements, particularly because injection times can be shorter. Thanks to the shorter experimental times, the mini-samples also allow for multiple experiments and statistical analysis of the desired results. According to one embodiment of the invention, the cell can be designed to operate at temperatures close to 150°C and pressures close to 150 bar. The fluid injection means in the cell can be adapted to inject a single fluid. Alternatively, the injection means can be adapted to inject a plurality, for example, from 2 to 5, fluids into the cell.In this case, the injection of the different fluids can be carried out sequentially or simultaneously. The ability to inject different fluids allows for specific injection sequences to determine certain flow properties within the sample. For example, when studying a surfactant for an EOR process, it is possible to inject three fluids: water, oil, and an aqueous phase containing a surfactant. Furthermore, the fluid injection means in the cell may include means for adjusting the fluid injection rate, such as a flow meter, to perform flow-dependent measurements. In addition, the fluid injection means may include at least one pressure regulator to control the pressure of the fluid injected into the cell.25 According to one embodiment of the invention, the X-ray radiography means can be provided to perform radiographs at regular intervals during the fluid injection(s). The regular interval can be between 0.1 and 5 seconds, and can be, for example, 1 second. Thus, performing radiographs at regular intervals makes it possible to regularly monitor the flows in the cell, which allows for real-time monitoring of the flows in the sample. Figure 1 illustrates, schematically and without limitation, a measurement system according to one embodiment of the invention. The measurement system 1 comprises a cell 2 which contains a rock sample (not shown), or any other porous medium. The cell 2 is placed in an X-ray shielding booth 3. The booth 3 also contains X-ray radiography means which include an X-ray source 4 and an X-ray detector 5.Cell 2 is placed on a support that can be moved 3047315 7 along three axes (schematically represented by arrows). The measurement system 1 also includes means 7 for injecting fluid into the cell. The injection means 7 are provided with four fluids. The injection means 7 are connected to the cell by four pipes. The injection means 7 are located outside the X-ray shielding booth 5 3. In addition, the measurement system 1 includes a computer system 6. The computer system 6 is connected to the detector 5 and to the injection means 7. The computer system 6 serves as a control means for the injection means 7 and as a means for collecting and analyzing the radiographs obtained by the detector 5. The computer system 6 is located outside the shielding booth 3. 10 The present invention also relates to a method for measuring at least one flow property of at least one fluid within a porous medium.The measurement method according to the invention is based on the use of the measurement system according to the invention. The method according to the invention may comprise the following steps: 15. A porous medium to be analyzed is placed in the cell of the measurement system; 20. One or more fluids are injected into the cell by means of the fluid injection means of the measurement system; during the injection, a plurality of X-ray radiographs are taken using the radiography means of the measurement system; and 20. At least one flow property is determined by means of the radiographs. By means of the method according to the invention, at least one of the following properties can be measured: the average saturation of a fluid injected into the porous medium, the saturation profile of a fluid injected into the porous medium, the pressure difference of the fluid 25 injected into the porous medium, etc.Furthermore, using these values, residual saturation (e.g., with oil) can be measured for different scavenging (injection) rates in an aqueous phase containing a surfactant. It is then possible to plot a residual saturation curve as a function of the capillary number. The curve representing the evolution of residual oil saturation as a function of the capillary number is called the Capillary Desaturation Curve (CDC). It can represent the amount of oil that can be produced by an EOR process through the injection of an aqueous composition containing at least one surfactant. The CDC therefore plays an important role in an EOR process. This curve depends, in particular, on the nature of the rock.35 Thus, the process according to the invention can be used in an enhanced oil recovery (EOR) process, in which the process according to the invention is used with the measurement system according to the invention to determine the formulation of the composition (water and at least one additive, including a surfactant) injected into the underground formation. In this case, the porous medium used corresponds to a rock sample taken from the underground formation in which the EOR process is implemented. 5 According to one embodiment of the invention, X-ray radiography means can be provided to perform radiographs at regular intervals during the fluid injection(s). The regular interval can be between 0.1 and 5 seconds, and can be, for example, 1 second.Thus, taking radiographs at regular intervals allows for the continuous monitoring of flows within the cell, enabling real-time tracking of flows in the sample. According to one embodiment, in which the benefit of injecting a surfactant into a rock is analyzed, the process according to the invention may comprise the following injection sequence: oil drainage with several different flow rates, i.e., injection of oil to drain the water present in the sample, for example at three different flow rates; water scouring at several different flow rates, i.e., injection of water to remove excess oil, for example at two different flow rates. The water scouring can be carried out at a very low flow rate or even zero; in this case, the water sweeps through the porous medium solely by capillary forces, a process known as spontaneous imbibition.Injection of an aqueous phase comprising a surfactant at various increasing flow rates, for example, eight different flow rates, until total oil desaturation. During these four steps, radiographs of the cell are taken. Furthermore, this sequence may include the following steps: - injection of a scanning water volume, and - rinsing and cleaning of the sample. Alternatively, the method according to the invention may include the simultaneous injection of water and oil at different flow rates. These combined injections allow, in particular, a measurement of relative permeability. Example: The system and method according to the invention are implemented during a sequence intended to characterize the evolution of residual oil saturation during different scans with water and a surfactant at different flow rates. A sandstone sample with a diameter of 10 mm and a length of 19 mm is used in one of the cells.Initially, the sample is 100% saturated with water. The injection sequence performed by the measurement system illustrated in Figure 1 is as follows: - E1: Oil drainage at three different flow rates, - E2: Spontaneous imbibition, - E3: Water sweep at two given flow rates, and - E4: Surfactant injection in the aqueous phase at eight increasing flow rates until total oil desaturation. During the experiment, an X-ray image is recorded every second, allowing, after calibration, the acquisition of both the average oil saturation and the saturation profile along the sample. The pressure difference over time is also measured. The average saturation S0 and the pressure difference dp (mbar) as a function of time T (s) are shown in Figure 2 for the injection sequence E1 to E4. The entire cycle is completed in 4000 seconds, or slightly more than one hour.It is observed that the injection of surfactant in the aqueous phase allows for the extraction of all the oil present in the sample: at the end of the sequence, S0=0. Simultaneously, the pressure difference increases within the sample. Saturation profiles for each sequence are also obtained from each measured radiograph. The processing performed is a moving average along the image. Combined with calibration, the oil saturation can be obtained along the sample every second. Figures 3 and 4 provide two examples of oil saturation profiles S0 as a function of the sample depth x (mm) obtained during the drainage phase E1 and during the scanning phase with surfactant E4. In these figures, the sample boundaries are symbolized by dashed vertical lines, and the direction of injection is indicated by an arrow.Figure 3 shows the evolution of saturation S0 at different times: t=20 s, t=40 s, t=70 s, t=150 s, and t=700 s. As expected, oil saturation increases during the drainage phase. Figure 4 shows the evolution of saturation S0 at the end of different sequences of water and 30 surfactant injections at different flow rates, denoted S4, S5, S6, S1, S8, and S1 (for Surfactant Injection: S1), with S14: 0.16 cm3 / min, S15: 0.25 cm3 / min, S16: 0.5 cm3 / min, S17: 1 cm3 / min, S18: 2 cm3 / min, and S11: 0.02 cm3 / min. The resulting profiles allow for the measurement of residual saturation for each water and surfactant E4 sweep stage. It is then possible to plot the residual Sor / Sor* saturation curve (with Sor residual saturation and Sor* normalized residual saturation) as a function of the scanning capillary number Nc (figure 5), an important piece of data for any EOR study.The capillary number Nc is the ratio of the average sweep speed times the viscosity of water divided by the interfacial tension between the water / surfactant system and the oil. In Figure 5, two curves are obtained for two different sandstone samples with the same experimental sequence: Bentheimer sandstone and Clashach sandstone. Thus, in about an hour, it is possible to obtain a precise CDC curve, unlike conventional methods which can require several weeks.

Claims

Claims 1) System for measuring at least one flow property of at least one fluid within a porous medium, said measuring system (1) comprises at least one cell (2) containing said porous medium, means (7) for injecting said fluid into said cell (2), and X-ray radiography means (3) comprising a source (4) and a detector (5), characterized in that the diameter of said cell (2) is substantially between 2 mm and 5 cm and in that said X-ray radiography means (3) are capable of carrying out a plurality of radiographs during the injection of said fluid at regular intervals, substantially between 0.1 and 5 seconds. 2) System according to claim 1, wherein said measuring system comprises means for controlling said injection means (6). 3) System according to one of the preceding claims, in which said measuring system comprises means for collecting and / or analyzing (6) said radiographs obtained by said detector. 4) System according to one of the preceding claims, in which said cell (2) has a substantially cylindrical shape. 5) System according to claim 4, wherein the diameter of said cell (2) is substantially between 5 mm and 2 cm. 6) System according to one of the preceding claims, wherein said injection means (7) comprise a plurality of fluids to be injected into said cell (2), simultaneously or sequentially. 7) System according to one of the preceding claims, in which said injection means (7) comprise at least one pump, at least one valve and at least one pressure sensor. 8) System according to one of the preceding claims, in which said fluid is chosen from an aqueous phase and an oil phase. 9) System according to claim 8, in which at least one aqueous phase comprises at least one additive, in particular a surfactant. 10) System according to one of the preceding claims, in which said measuring system comprises means for positioning said cell (2). 11) Method for measuring at least one flow property of at least one fluid within a porous medium, in which said measurements are carried out by means of said measuring system (1) according to one of the preceding claims, by means of a plurality of radiographs during the injection of said fluid at regular intervals, substantially between 0.1 and 5 seconds. 12) Method according to claim 11, wherein said flow properties are chosen from the average saturation of said fluid, and / or the saturation profile of said fluid and / or the pressure difference of said fluid within said sample. 13) Method according to one of claims 11 to 12, in which a residual saturation curve is plotted as a function of the capillary scanning number by means of said measurements. 14) Method according to one of claims 11 to 13, in which the measurements are carried out by X-ray radiography using said measuring system (1) during the implementation of the following injection steps within said cell: a) oil is injected at at least two different flow rates; (b) water is injected at at least two different flow rates; and c) an aqueous phase comprising at least one additive is injected at at least two different flow rates.

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