RU2777702C1 - Method for determining the oil displacement coefficient at the pore scale based on 4d microtomography and a device for its implementation - Google Patents

Abstract

FIELD: oil industry.

SUBSTANCE: claimed group of inventions relates to the oil industry, namely, to special core studies for the design and analysis of the development of oil fields using various flooding systems. A method for determining the oil displacement coefficient at the pore scale based on 4D microtomography is proposed, which consists in making a cylindrical sample from the reservoir rock with a height of 4 to 6 mm and a diameter of 4 to 6 mm.; next, it is placed in an X-ray transparent mobile core holder of a device for X-ray computed microtomography of reservoir rocks, microtomography of a cylindrical sample is performed, followed by segmentation of the effective porosity structure. Next, a cylindrical sample is saturated with reservoir water or a model of reservoir water contrasted with X-ray-dense compounds, oil or oil models are filtered through the sample under specified thermobaric conditions until the water outlet from the sample stops. Next, repeated microtomography of the sample, registration of a three-dimensional image of the sample in the volume of the previously obtained digital model of the original sample, segmentation within the previously allocated effective porosity of the volumes of oil and residual reservoir water are carried out based on differences in X-ray density between the contrasted and non-contrasted phases. The volume of initial oil saturation is determined and oil or oil model is displaced by reservoir water or another displacing agent contrasted with X-ray-dense compounds, and repeated microtomography of the sample is performed, a three-dimensional image of the sample is recorded in the volume of the previously obtained digital model of the initial sample, segmentation within the previously allocated effective porosity of the volumes of residual oil or oil model based on differences in X-ray density between the contrasted and non-contrasted phases. The oil displacement coefficient β is calculated based on the ratio of the volumes of the initial oil saturation Vs.init., residual oil saturation Vs.resid. in the sample. A device for conducting 4D X-ray computer microtomography of reservoir rocks for the implementation of the claimed method, consisting of an X-ray computer microtomograph, a filtration unit, and an X-ray transparent mobile core holder, is also proposed. At the same time, the filtration plant includes a crimp pump, a pressure pump, a manifold for controlling the supply of liquid from the selected container, piston containers for injected liquids.

EFFECT: assessment of the change in oil saturation volumes directly inside the effective porosity of the sample during the oil displacement experiment, which makes it possible to determine the displacement coefficient with greater accuracy.

2 cl, 9 dwg

Description

The claimed group of inventions relates to the oil industry, namely, to special core studies for the design and analysis of oil field development using various waterflooding systems. The main feature of the invention is the assessment of the change in oil saturation volumes directly within the effective porosity of the sample during an oil displacement experiment based on the use of 4D X-ray computed microtomography.

Further in the text, the applicant gives the terms that are necessary to facilitate an unambiguous understanding of the essence of the claimed materials and to exclude contradictions and / or controversial interpretations when performing an examination on the merits.

Oil displacement coefficient (K _vn ) - the limiting value of oil recovery that can be achieved in laboratory conditions using this working agent during prolonged flushing of a rock sample [Intensification of development and enhanced oil recovery [Electronic resource]: electronic educational and methodological complex / A. V. Lysenkov; Federal State budgetary educational institution of higher education. prof. Education "Ufa State Oil Technical University" (FGBOU VPO UGNTU), Structural subdivision "Inst. additional prof. education" (SSP UGNTU "IDPO"). - Ufa: FGBOU VPO UGNTU, 2014].

4D microtomography is a technique where a three-dimensional computed microtomography volume containing a moving structure can be visualized over a period of time, creating a dynamic volume dataset [Kwong Y, Mel AO, Wheeler G, Troupis JM. Four-dimensional computed tomography (4DCT): A review of the current status and applications. J Med Imaging Radiat Oncol. 2015 Oct;59(5):545-54. doi: 10.1111/1754-9485.12326. Epub 2015 Jun 3. PMID: 26041442].

The equivalent pore diameter is the diameter of a ball whose volume corresponds to the pore volume [Hu, X., Hu, S., Jin, F., Huang, S. (Eds.), 2017. Physics of Petroleum Reservoirs, Springer Mineralogy. Springer Berlin Heidelberg, Berlin, Heidelberg. https://doi.org/10.1007/978-3-662-53284-3].

Extracted - subjected to hydrocarbon extraction in Soxhlet apparatuses [McPhee K., Reed J., Zubizaretta I. Laboratory core research: a guide to best practices // M. - Izhevsk: Institute for Computer Research, 2018. 924 p.].

One of the most important parameters determined in the course of special laboratory tests of the core is the oil displacement factor. It allows assessing the potential for oil recovery from the reservoirs of the field, therefore, the final forecast of technical and economic development indicators depends on its accuracy. In addition, the reliability of laboratory studies of the oil displacement efficiency affects the choice of new more effective displacement agents and flooding systems used to enhance oil recovery, such as polymer flooding, the use of surfactants, etc. Of particular importance is the ability to evaluate the displacement capacity on a pore scale , which allows understanding the physical features of the processes occurring in the pore space of the reservoir. There is now a growing understanding in the oil and gas industry that standard oil displacement efficiency estimation methods developed several decades ago are insufficient for a detailed understanding of the processes occurring in the reservoir under reservoir conditions.

According to the results of the study of the prior art, the applicant selected analogues that are closest to the claimed technical solution in terms of the totality of essential features.

There is a method [Balin V.P., Mokhova N.A., Sintsov I.A., Ostapchuk D.A. Determination of the oil displacement efficiency using the study of the structure of the pore space by capillarimetry // Territoriya "NEFTEGAS". 2017. No. 1-2. pp. 40-50] measurement of the oil displacement efficiency based on the study of the structure of the pore space of core samples by capillarimetry, for which capillarimetric studies should be carried out on core samples of the corresponding object using the centrifugation method or a semi-permeable membrane. The method takes into account the contribution to the filtration of pore channels of different sizes, corresponding to the proportion of pore volume in which mobile oil reserves are concentrated. Satisfactory convergence of the obtained results with the results of special flow experiments is shown.

The disadvantage of the known method is its predominantly complementary nature of laboratory measurements in the analysis and justification of the displacement parameters. In the case of independent application, there is a need for a sufficient selection of a collection of samples by permeability in the absence of direct special laboratory studies at the facility.

An invention is known according to patent RU 2445604 "Method for reliably determining the displacement factor and relative phase permeabilities, field type", the essence is a method for reliably determining the displacement factor and relative phase permeabilities, including conducting field hydrodynamic studies of the well and geophysical measurements, characterized in that for the study choose an injection well with a vertical or directional wellbore that has come out of drilling or was previously used for oil production, penetrating the reservoir from the roof to the bottom; before the start of injection, geophysical surveys are carried out in the well in order to determine the initial profile of the distribution of the oil saturation factor; a working agent is started to be pumped into the well; at different points in time, bottomhole pressure is measured, the injectivity profile of the working agent along the formation section, as well as the distribution profile of the water saturation coefficient; measurements are stopped at the n-th stage, when the profiles of the water saturation coefficient repeat the measured profiles at the (n-1)-th stage; based on the results of this monitoring, the oil displacement factor is determined by the working agent, as well as the functions of relative phase permeabilities for each characteristic interval of the reservoir section.

At the same time, a distinctive feature of the known invention is the use of a vertical or directional well that has come out of drilling or was previously used for oil production, opening the layer from the roof to the sole. It implements a standard set of geophysical and hydrodynamic studies and determines the reservoir parameters, including the initial distribution of the oil saturation factor, along the entire opened section. Further, in the well, in the course of further research, geophysical methods are applied to control the dynamics of the current reservoir saturation (for example, the method of pulsed neutron-neutron logging with saline injection or other methods using labeled injection agents). The working agent is started to be pumped into the well. It is envisaged that the forthcoming monitoring of the injection process will include several stages in time. At each stage, based on geophysical studies, the current distribution of the oil saturation factor along the opened section, the injectivity profiles of the working agent and temperature along the entire section are determined. Throughout all stages, the change in bottomhole pressure is constantly recorded by one or more high-precision pressure gauges, if possible, placed at different elevations within the reservoir opening interval. The complex of geophysical studies ends at the n-th stage, when the oil saturation profile at the n-th stage coincides with the oil saturation profile at the previous (n-1st) stage. Based on the results of geophysical surveys, prior to the start of injection of the agent, the value of the oil saturation coefficient is determined in each i-th characteristic interval of the formation S _{n nachi} . The characteristic reservoir interval is understood as a lithologically homogeneous interlayer, within which there are stable values of parameters determined from geophysical data, including the value of the initial oil saturation coefficient S _{n nachi.} The profile of the oil saturation factor, interpreted according to the logging results, at the n-th stage of geophysical measurements makes it possible to determine the true limiting values of the displacement efficiency in each i-th characteristic interval (interlayer) К _vyti according to the following formula,

,

,

where S ⁿ _{n, i} - coefficient of residual oil saturation in the i-th interlayer at the end of the n-th stage of research. Estimates of _Kvyti obtained in this way correspond to the desired values for the intervals, the oil saturation of which at the beginning of the study corresponded to its maximum value 1-S _{in the rest} . However, the values of S _{nн, i} themselves can be used as values of the residual oil saturation to set the relative permeability curves for all the studied intervals.

The disadvantage of the known technical solution is the complexity of its implementation, which consists in multitasking, large financial costs and the impossibility of widespread use, since not every field site may have a well in which such an experiment can be carried out. Also noteworthy is the lack of comparison of field experiments with conventional laboratory studies.

The invention is known according to patent RU 2654315, the essence is a method for determining the coefficient of oil displacement of the Bashkir carbonate deposits of the Solikamsk depression, according to which standard cylindrical samples are made from the core of the real rock of the Bashkir carbonate deposits of the Solikamsk depression, they are extracted from oil, dried to stabilize the mass, weighed in a dry state , determine the coefficient of porosity Kp, the coefficient of absolute gas permeability Kprg and the viscosity of oil μ _n , under vacuum, 100% saturation of the sample with formation water or its model is performed, the saturated sample is weighed in air, the method of capillarimetry is used to displace water from the samples to the value of residual water saturation , determine the coefficient of residual water saturation Kow, then set the oil displacement coefficient Kw, characterized in that after 100% saturation of the sample with water under vacuum, it is additionally weighed in water, then the volumetric The value of the discriminant function Z of the sample is calculated by the method of discriminant analysis using the method of discriminant analysis:

Z=101.442-191.381⋅Kp+29.490⋅Kov+13.620⋅Kprg-35.118⋅ρ,

where Kp - porosity coefficient, d.u.;

Kow - coefficient of residual water saturation of the sample, units;

Kprg - coefficient of absolute gas permeability, µm ² ;

ρ - bulk density of the rock, g / cm ³ ,

according to the specified discriminant function Z, the rock class from which the sample is made is determined based on the following:

when Z>0, the rock is assigned to the first class;

at Z<0 - to the second class,

further, for a sample assigned to the first class, the oil displacement coefficient Kw ^M1 is calculated using the formula:

kW ^M1 \u003d 1.1483-5.6251⋅Kprg + 0.1718⋅μ _n +16.1795⋅ (Kprg / μ _n ) -0.4404⋅ρ-0.1534⋅Kov,

and for a sample assigned to the second class, the oil displacement coefficient Kw ^M2 is calculated using the following formula:

kW ^M2 \u003d 0.5712 + 0.1914⋅Kprg + 0.2823⋅ (Kprg / μ _n ),

where Kw ^M1 - oil displacement coefficient of the sample of the first class, units;

kW ^М2 - coefficient of oil displacement of the sample of the second class, units;

μ _n - oil viscosity, mPa⋅s.

The method according to claim 1, characterized in that the value of the oil displacement factor Kw for a particular reservoir is calculated from the average values for all samples of both classes of porosity coefficients Kp, absolute gas permeability Kprg and residual water saturation Kow, rock bulk density ρ and a known value oil viscosity μ _n .

Thus, applicable to improve the accuracy of determining the oil displacement efficiency from rocks represented by the Bashkir carbonate deposits of the Solikamsk depression, which consists in a combination of the capillarimetry method and the discriminant analysis method. Standard cylindrical samples are prepared, they are extracted from oil, dried to stabilize the mass, weighed in a dry state, the porosity coefficient and gas permeability coefficient and oil viscosity are determined. Next, under vacuum, the sample is 100% saturated with formation water or its model, and the sample saturated with water is weighed in air. After that, the method of capillarimetry produces the displacement of water from the samples to the value of the residual water saturation, as a result of which the coefficient of residual water saturation is determined. After saturating the sample with water up to 100% under vacuum, it is additionally weighed in water, and the bulk density of the rock that makes up the sample is determined. The discriminant analysis method calculates the value of the discriminant function of the sample. Based on the discriminant function, the rock class to which the sample belongs is determined and the displacement coefficient is calculated oil, taking into account the class of the rock. The proposed method makes it possible to determine the oil displacement efficiency with an accuracy that is 5.9 times higher than the accuracy of determining this indicator according to the standard method based on OST 39-195-86, and thus provides the most accurate representation of the distribution of oil reserves.

The disadvantage of the known technical solution is its focus only on the Bashkir carbonate rocks of the Solikamsk depression, and the need for extensive research for other geological objects.

The invention is known according to the patent WO2013180593A1 "Method for determining the oil and gas saturation factor by a well logging complex based on pulsed neutron logging methods", the essence is a method for determining the hydrocarbon content using a well logging complex (GIS) based on pulsed neutron-neutron logging (PNL) or pulsed neutron gamma logging (INGK) in the future (INL), which consists in carrying out measurements by the INK method and calculating the macroscopic cross section for the absorption of thermal neutrons of the rock, determining the macrocomponent composition of rocks, including porosity, using the well logging complex, while calculating the macroscopic cross section for the absorption of thermal neutrons by formation water and hydrocarbons their elemental composition and density are used, and the calculation of hydrocarbon saturation is carried out according to the dependence for gas

and for oil

∑в ∑нг - макроскопическое сечение поглощения тепловых нейтронов компонентов горной породы, остаточной воды, пластовой воды и углеводородной фазы;where ∑i,

∑v ∑ng - macroscopic cross section of thermal neutron absorption of rock components, residual water, formation water and hydrocarbon phase;

Ki, Kp, Kow, Kw and Kng, are the concentration of the i-th macrocomponent, coefficients of porosity, residual water saturation, current water and oil and gas saturation;

Σu and Σn - macroscopic cross section of gas and oil, respectively,

characterized in that in order to calculate the macroscopic cross sections of thermal neutron absorption by macrocomponents that form the solid phase of rocks, a collection of core samples from reference wells is additionally prepared, on which measurements of the mineral, elemental composition of the samples and sample weight loss during heating are made, a mineral-component model of the rock is formed and calculate the macroscopic absorption cross-sections of thermal neutrons for each macrocomponent that forms the solid phase of the rock.

Thus, the known method as a whole consists in carrying out measurements by the method of pulsed neutron logging methods (PIN) and calculating the macroscopic cross section for the absorption of thermal neutrons of the rock, determining the macrocomponent composition of rocks, including porosity, using the well logging complex, while calculating the macroscopic cross section for the absorption of thermal neutrons of the reservoir water and hydrocarbons use their elemental composition and density, and the calculation of hydrocarbon saturation is carried out according to the dependence for gas and oil. To calculate the macroscopic absorption cross sections of thermal neutrons by macrocomponents that form the solid phase of rocks, a collection of core samples from reference wells is additionally prepared, on which the measurements of the mineral and elemental composition of the samples and the weight loss of the sample during heating are performed, a mineral-component model of the rock is formed, and macroscopic absorption cross sections are calculated thermal neutrons for each macrocomponent that forms the solid phase of the rock. The invention improves the accuracy of determining the content of hydrocarbons.

The disadvantage of the known technical solution is the indirect nature of the data obtained, which cannot be used for direct experimental determination of the displacement efficiency. In addition, its use complicates the laboriousness and significant time costs for collecting a collection of core samples, conducting studies of the material and mineral composition, and weight loss upon heating.

Known analogue in relation to the device [Beletskaya, A., Chertova, A., Abashkin, V., Willberg, D., Korobkov, D., Yakimchuk, I., & Dovgilovich, L. Image-Based Evaluation of Retained Proppant Pack Permeability // SPE Russian Petroleum Technology Conference, 2018 SPE-191663-18RPTC-MS, P. 1-14]. The essence of the known technical solution is a system consisting of an X-ray computed microtomograph, a radiolucent mini-core holder, a crimping pump and a feed pump. The radiolucent mini-core holder consists of a rubber cuff in which the sample is fixed, a lower fixed end cap with a channel inside and an upper movable end cap with a channel inside, which are installed to the ends of the sample; the top and bottom covers hold the X-ray transparent tube. The bottom cover has tubing connections from the squeezing pump and the feed pump, and the top cover has an outlet for filtered fluid. The radiolucent mini-core holder is installed in the holder of the X-ray computed microtomograph between the X-ray tube and the detector.

One of the disadvantages of the known device is the limited scanning area when using elongated samples, which do not allow to study the changes occurring in the entire volume of the sample. For example, it is described that due to design limitations of the known technical solution, the resulting digital model represents only 0.38 units of the actual sample length of 13.7 mm with a diameter of 7.8 mm.

Another significant disadvantage of the known device is the impossibility of filtering two or more liquids through the sample, both simultaneously and sequentially, without disassembling the system. The latter leads to the depressurization of the system and the ingress of air first into the tubes supplying the liquid, and then into the sample. This makes it impossible to carry out a qualitative experiment, since air appears in the pore structure of the sample along with the studied phases.

In addition, the analog is not equipped with a mobile heating element, with the ability to determine the heating temperature of a mobile X-ray transparent core holder and a built-in thermocouple, with the ability to detect the temperature of the medium inside the mobile X-ray transparent core holder, which does not allow modeling the thermal conditions of the reservoir during the experiment.

Closest to the claimed technical solution, chosen by the applicant as a prototype, is a method for determining the oil displacement efficiency, which is regulated by the industry standard OST 39-195-86 “Oil. Method for determining the oil displacement efficiency by water in laboratory conditions”, which is widely used in the territory of the Russian Federation. The essence of the prototype is a method that consists in determining the completeness of the extraction of oil from a sample of oil-bearing rock by filtering water through it until the output fluid is almost completely water cut. At the same time, the oil displacement conditions are as close as possible to reservoir ones due to the use of reservoir or model fluids with the obligatory creation and maintenance of reservoir temperature and pressure.

The disadvantages of the prototype are:

1 - low accuracy of the results obtained, which is a consequence of the limitation of the minimum possible determination of the volume of the displaced liquid in 0.1 ml, which makes it necessary to increase the size of the composite core model;

2 - the need to use exclusively extracted core samples, which leads to a change in the wettability properties of the sample and to a distortion of the oil displacement coefficients;

3 - the impossibility to evaluate the efficiency of the displacing abilities of certain agents on a pore scale, to determine the size and geometric features of the pores from which the most and least effective displacement occurs.

The technical result of the claimed technical solution is an estimate of the change in oil volumes in the process of displacement directly inside the effective pore space of the sample (in situ) based on the application of 4D microtomography and calculation of the oil displacement efficiency, which leads to the elimination of the shortcomings of the prototype, namely:

1 - to a greater accuracy in determining the displacement efficiency, compared with the standard approach. According to OST 39-195-86, the minimum volume of displaced liquid is determined using a measuring burette, the measurement accuracy of which is at least 0.1 cm ³ . In this regard, in standard displacement experiments, in order to increase the volume of effective porosity, they try to maximize the volume of a composite sample, sometimes exceeding 1 m. Among other things, this procedure requires the presence of special elongated core holders, the selection of a large number of standard samples and a certain order of their arrangement. Accordingly, the scaling factor P, which reflects the measurement accuracy, will depend on

the ratio of the volume of effective porosity of the sample V _eff. to the given minimum value V _min :

2 - the possibility of using the described method for working with both extracted and non-extracted samples;

3 - the possibility of evaluating the effectiveness of the displacing abilities of various agents on a pore scale, determining the size of the pores from which the most and least effective displacement occurs.

The essence of the claimed technical solution is a method for determining the oil displacement factor on a pore scale based on 4D microtomography, which consists in the fact that a cylindrical sample with a height of 4 to 6 mm and a diameter of 4 to 6 mm is made from the reservoir rock; then it is placed in an X-ray transparent mobile core holder of a device for performing X-ray computer microtomography of reservoir rocks, microtomography of a cylindrical sample is performed, followed by segmentation of the effective porosity structure, then the cylindrical sample is saturated with formation water or a model of formation water contrasted with X-ray dense compounds, oil or oil model is filtered through a sample under given thermobaric conditions until the moment when water exits from the sample stops; then, a repeated microtomography of the sample is performed, a three-dimensional image of the sample is recorded in the volume of the previously obtained digital model of the original sample, segmentation within the previously identified effective porosity of the volumes of oil and residual formation water based on differences in X-ray density between the contrasted and non-contrasted phases, and the volume of initial oil saturation is determined; then, the oil or the oil model is displaced by reservoir water or another displacing agent contrasted with X-ray dense compounds, and a repeated microtomography of the sample is performed, a three-dimensional image of the sample is recorded in the volume of the previously obtained digital model of the original sample, segmentation within the previously selected effective porosity of the volumes of residual oil or the oil model based on differences in X-ray density between the contrasted and non-enhanced phase; carry out the calculation of the displacement coefficient β of oil based on the ratio of the volumes of the initial oil saturation Vн.nach. residual oil saturation Vн.rest. in the sample according to the formula:

A device for performing 4D X-ray computed microtomography of reservoir rocks for implementing the method according to claim 1, consisting of an X-ray computed microtomograph, a filtration unit, and a radiolucent mobile core holder; at the same time, the filtration unit includes a crimping pump, a pressure pump, a manifold for controlling the supply of liquid from the selected container, piston containers for pumped liquids connected to the radiolucent mobile core holder by supply pipes arranged in such a way that they allow the radiolucent mobile core holder to freely rotate around its axis , and shut-off valves installed on the supply pipes; the radiolucent mobile core holder is placed in the holder between the x-ray tube and the detector of the x-ray computed microtomograph; X-ray transparent mobile core holder consists of a base with three internal channels, polymer tubes with dividers at the ends, a silicone cuff with the possibility of fixing a cylindrical sample of reservoir rock in its cavity, an external X-ray transparent tube and an upper locking cap with an outlet channel; supply tubes from containers are connected to the base at the entrance to the channels, the supply tube from the compression pump is connected to a separate entrance of the base; polymer tubes are installed on top in the central part of the base, between which a silicone cuff is placed; the ends of the polymer tubes with dividers face the silicone cuff; the radiolucent tube is installed on the base in such a way that the polymer tubes and the silicone cuff remain inside it; on top of the radiolucent tube fixed upper locking cap with an outlet channel connected to a polymer tube; the device is equipped with a mobile heating element, made along the outer contour of the X-ray transparent tube, with the possibility of determining the heating temperature of the mobile X-ray transparent core holder, and a thermocouple located at the base of the mobile X-ray transparent core holder, with the possibility of detecting the temperature of the medium inside it.

The claimed technical solution is illustrated in Fig.1 - Fig.7.

On FIG. 1 shows a diagram of the claimed device for filtration experiments based on 4D microtomography, where:

1 - X-ray computed microtomograph,

2 - filtration unit,

3 – radiolucent mobile core holder,

4 - crimp pump,

5 - injection pump,

6 - manifold,

7, 8, 9 - piston containers,

10 - supply pipes,

11 - holder,

12 - shut-off valves,

13 - x-ray tube,

14 - detector,

15 - base,

16 - three internal channels,

17 - polymer tubes,

18 - silicone cuff,

19 - cylindrical sample of reservoir rock,

20 - radiolucent tube,

21 - top locking cover,

22 - output channel,

23 - mobile heating element,

24 - thermocouple.

On FIG. Figure 2 shows segmented volumes of porosity on the X-ray density section: 2a - absolute porosity, 2b - effective porosity.

On FIG. Figure 3 shows X-ray density sections for different stages of the experiment: 3a – initial dry sample, 3b – after saturation with contrasted water, 3c – after filtering kerosene, 3d – after displacement of kerosene with water.

On FIG. Figure 4 shows the volumes of the initial (4a) and residual (4b) oil saturations (shown in orange) on an X-ray dense section of the sample.

On FIG. 5 presents Table 1, which shows the general characteristics of the used sample of Berea sandstone from Example 1.

On FIG. 6 shows Table 2, which shows the results of comparing the claimed method with the prototype according to Example 1.

On FIG. 7 shows diagrams:

7a - distribution of the percentage volume of effective porosity over the equivalent pore diameter;

7b is a diagram of distribution of initial oil saturation (7-1) and residual oil saturation after displacement (7-2) by equivalent effective porosity diameters.

In the diagrams, the X-axis indicates the equivalent pore diameters in mm, and the Y-axis indicates the fraction of the volume of the effective porosity of the sample in percent.

On FIG. Figure 8 shows X-ray density sections showing the main experimental steps in determining the oil displacement efficiency of a polymer composition on a pore scale according to Example 2: 8a - initial dry sample, 8b - after saturation with contrasted formation water, 8c - after saturation with oil at residual water saturation, 8d - after displacement oil with a contrasting polymer composition.

On FIG. 9 shows Table 3, which shows the results of comparing the oil displacement coefficients for two polymer compositions of three different concentrations according to Example 2, proving the comparability of the obtained values of the oil displacement coefficients by the polymer compositions of the claimed method with the prototype.

Further, the applicant provides a description of the claimed technical solution.

The claimed technical result is achieved by developing the claimed method for determining the oil displacement factor on a pore scale based on 4D microtomography, which consists in studying the processes of oil displacement directly in the pore space of reservoirs.

The claimed method consists of 4 stages:

Stage 1 (preparatory) consists of assembling the claimed device (Fig. 1). The claimed device consists of three main components:

- X-ray computed microtomograph 1,

- filtration unit 2,

- X-ray transparent mobile core holder 3.

Filtration unit 2 includes a compression pump 4 and a pressure pump 5, a manifold 6 to control the flow of liquid from the selected container, piston containers 7, 8, 9 for pumped liquids connected to an X-ray transparent mobile core holder 3 by inlet tubes 10 located in such a way that they allow free rotate the radiolucent mobile core holder 3 in the holder 11 around its axis, and the stopcocks 12 installed on the inlet tubes.

The radiolucent mobile core holder 3 consists of a base 15 with three internal channels 16, polymer tubes 17 with dividers at the ends, a silicone cuff 18 with the possibility of fixing a cylindrical sample of reservoir rock 19 in its cavity, a radiolucent tube 20 and an upper locking cap 21 with an outlet channel 22 ; supply tubes 10 from containers 7-9 are connected to the base at the entrance to the channels, a supply tube 23 from the compression pump 4 is connected to a separate entrance of the base 15; on top in the central part of the base 15, polymer tubes 17 are installed, between which a silicone cuff 18 is placed; the ends of the polymer tubes 17 with dividers face the silicone cuff 18; the radiolucent tube 20 is installed on the base 15 so that the polymer tubes 17 and the silicone cuff 18 remain inside it; on top of the radiolucent tube 20, the upper locking cap 21 with the output channel 22 is fixed, connected to the polymer tube 17; the device is equipped with a mobile heating element 23, made along the outer contour of the X-ray transparent tube 18, with the possibility of determining the heating temperature of the mobile X-ray transparent core holder, and a thermocouple 24 installed in the base 13 of the mobile X-ray transparent core holder 3 , with the possibility of detecting the temperature of the medium inside it.

For testing, a cylindrical sample of the reservoir rock 19 with a diameter of 4 to 6 mm and a length of 4 to 6 mm is used, which is placed in the cavity of the silicone cuff 18 with the possibility of fixing it by crimping the silicone cuff with liquid from the crimp pump 4.

Thus, the resulting assembly of polymer tubes 17 with a cylindrical sample of reservoir rock 19 in a silicone cuff 18 is installed inside the radiolucent tube 20 on the base 15.

Further, the liquids necessary for the experiment (formation water or formation water model, formation oil or formation oil model and displacing agent) are pumped into piston containers 7, 8, 9 and sequentially forced through the supply pipes until they exit the base into the polymer tube.

The 2nd stage is that they carry out:

- microtomographic survey of a dry cylindrical sample of the reservoir rock,

- reconstruction of the 3D model,

- segmentation of the pore space,

- selection of the volume of effective porosity in the direction of filtration,

- determination of the coefficients of total and effective porosity based on the ratio of the allocated volumes to the volume of the entire sample.

The shooting parameters for each sample are selected by the operator separately, depending on the X-ray density characteristics of the minerals in the sample. In this case, the resolution of microtomographic imaging should be sufficient to characterize most of the filtration pores of the sample, i.e. the voxel size of the microtomographic image should be less than the equivalent diameters of the main transport pores of the sample. As a rule, for most sandy and carbonate reservoirs, it lies in the range from 2 to 10 microns.

3rd stage includes:

- saturation of the sample with formation water or its model, which are contrasted with special X-ray absorbing compounds, as a rule, including iodine in their composition,

- then the oil or oil model is filtered through a water-saturated cylindrical sample,

- then carry out microtomography of the sample,

- reconstruction of the 3D model,

- registration of a three-dimensional volume in the volume of a previously removed dry sample.

- due to the difference in absorbing abilities between the contrasted and non-contrasted liquid phases in the volume of effective porosity, oil and water phases are segmented

- determine the volumes of the selected phases of oil and water and the ratio of their volumes to the volume of effective porosity calculate the initial oil saturation and residual water saturation.

At the 4th stage , oil is displaced by formation water or another agent by filtering it through the sample. In this case, the displacing agent must be contrasted with X-ray absorbing compounds. The stoppage of residual water output is controlled by X-ray method.

After that, the filtration process is stopped and repeated microtomography of the sample, reconstruction of the 3D model and repeated spatial registration of the obtained digital model in the volume of the original sample are carried out.

Further, in the volume of effective porosity, the contrasted and non-contrasted phases are segmented, thus determining the volumes of residual oil saturation.

If necessary, additional stages of additional oil displacement can be carried out, including the use of various agents to increase oil recovery. In this case, it is important to observe the same frequency of contrasting between the displacing and displaced fluid used in the early stages of the experiment.

The oil displacement coefficient β is determined based on the ratio of the volumes of the initial oil saturation V _{n. early} and residual oil saturation V _n.res in the sample according to the formula:

The claimed method for determining the oil displacement efficiency at the pore scale based on 4D microtomography is illustrated by the following examples , which does not limit its scope.

Example 1 Implementation of a method for determining the oil displacement efficiency on a pore scale based on 4D microtomography on a reference sandstone from the Berea formation and comparison of the results obtained with the results obtained on the same sandstone according to OST 39-195-86 “Oil. Method for determination of oil displacement efficiency by water in laboratory conditions”. The size of the cylindrical sample is 4 x 4 mm, saturation with the formation water model, filtering the oil model, displacement of the formation water with the model.

At the first stage, the claimed device for 4D microtomography was assembled, including an X-ray computed microtomograph, a filtration unit, and a radiolucent mobile core holder.

The filtration unit included a pressurizing pump and a pressure pump, a manifold for controlling the supply of liquid from the selected container, piston containers for injected liquids connected to the radiolucent mobile core holder by inlet tubes located in such a way that they allow the radiolucent mobile core holder to freely rotate in the holder around its axis, and shut-off valves installed on the supply pipes. The X-ray transparent mobile core holder was installed in the holder between the X-ray tube and the detector of the X-ray computed microtomograph.

The radiolucent mobile core holder included a base with three internal channels, polymer tubes with dividers at the ends, a silicone cuff with the possibility of fixing a cylindrical sample of reservoir rock in its cavity, an external radiolucent tube, and an upper locking cap with an outlet channel. The inlet pipes from the containers were connected to the base at the entrance to the channels, the inlet pipe from the compression pump was connected to a separate entrance of the base. From above, in the central part of the base, polymer tubes were installed, between which a silicone cuff was placed. The ends of the polymer tubes with dividers were directed towards the sample in the silicone sleeve. The radiolucent tube was installed on the base so that the polymer tubes and the silicone cuff remained inside it. On top of the radiolucent tube, an upper locking cap with an outlet channel was fixed, which was connected to a polymer tube. The device is equipped with a mobile heating element, made along the outer contour of the X-ray transparent tube, with the possibility of determining the heating temperature of the mobile X-ray transparent core holder, and a thermocouple installed in the base of the mobile X-ray transparent core holder , with the possibility of detecting the temperature of the medium inside it.

A model of formation water (deionized water with CsI contrast) was pumped into piston containers 7, 9, and a model of oil (kerosene) was pumped into container 8, which were sequentially forced through the supply pipes until they exited the base into a polymer tube.

A cylindrical sample 4 mm in diameter and 4 mm high was cut from the extracted Berea sandstone, placed in a thin silicone cuff, on which polymer tubes with dividers at the ends were placed on both sides. An external radiolucent tube was placed on top of the column. The experiment was carried out at 23°C, a crimping pressure of 3 MPa, and a liquid flow rate of 0.05 ml/min.

At the second stage, after installing a cylindrical sample in a radiolucent core holder, microtomography of a dry sample was performed with a resolution of 8.8 μm. Reconstruction of the 3D model. Next, the absolute porosity was segmented in the sample, based on the ratio of the allocated volume to the volume of the entire sample, the total porosity coefficient was determined, which amounted to 18.3% (Fig. 2a). Based on the connection of pores along the vertical axis of the cylinder Z, the effective porosity was segmented, based on the ratio of the allocated volume to the volume of the entire sample, the effective porosity coefficient was determined, which amounted to 17.05% (Fig. 2b).

At the third stage, the sample was saturated with a model of formation water, which is deionized water contrasted with CsI salts (80 g/l). Repeated microtomography was performed to control saturation. After that, the oil model was filtered through the sample in the form of kerosene (about 10 pore volumes) until the water exit from the sample stopped. Next, we performed microtomography and segmentation on the obtained 3D images of volumes of oil and residual water models in the volume of effective porosity, determined the value of residual water saturation and initial oil saturation.

At the fourth stage, kerosene was replaced by a formation water model representing deionized water counterstained with CsI salts (80 g/l). After that, microtomography was again carried out and segmentation on the obtained 3D images of the volumes of kerosene and water in the volume of effective porosity with the determination of residual oil saturation.

The results of microtomography after each of the described stages are shown in Fig.3: 3a - X-ray section of a dry sample, 3b - X-ray section of a sample saturated with a contrasted model of formation water, 3c - X-ray section of a sample with residual water saturation after oil filtration, 3d - X-ray section of a sample after oil displacement by the contrasted reservoir water model. The volumes of initial (4a) and residual (4b) oil saturations identified on X-ray density sections are shown in Fig. 4 and amounted to 46.70% and 27.77% of the volume of effective porosity, respectively. The displacement coefficient was determined by formula (2).

To compare the results obtained , a prototype experiment was carried out on a sample of Berea sandstone with reservoir properties, presented in Table 1 in Fig. 5. For the study, a standard core sample of a cylindrical shape with a diameter of 37 mm and a length of 90 mm was used. The working fluids are deionized water with a contrast agent CsI (80 g/l); kerosene was used as a model of oil. The viscosity of the aqueous solution was 0.971 Pa⋅s, kerosene - 1.49 Pa⋅s.

The experimental setup includes plunger pumps, tubes, pressure sensors, a Hasler-type core holder with a working diameter of 38 mm and a length of 120 mm, tankers with working fluids and 0.1 ml measuring burettes [GOST 29251-91 "Laboratory glassware". Moscow, 1991].

Determination of the porosity coefficient was carried out according to [GOST 26450.1-85 “Rocks of mountains. Method for determining the coefficient of open porosity by liquid saturation. Moscow, 1985].

Water permeability was determined according to the method described in [McPhee K., Reid J., Zubizaretta I. Laboratory core research: a guide to best practices // M. - Izhevsk: Institute for Computer Research, 2018. 924 p.].

The experiment was carried out under thermobaric conditions, similar to the previous Example. The displacement ratio was calculated according to OST 39-195-86 using the formula:

where β is the coefficient of oil displacement by water; V _n.beginning – the volume of oil initially contained in the sample is defined as the difference between the volumes of voids and residual water; V _n. is the volume of oil displaced by water.

The results of the experiments performed are presented in Table 2 in Fig. 6. The results obtained demonstrate comparable values of the initial and residual oil saturation, as well as displacement factors, which allows us to speak about their reliability.

In addition, for the test sample were obtained:

- distribution diagram of the percentage volume of effective porosity by equivalent pore diameter (Fig. 7a), demonstrating that more than 90% of the effective porosity of the sample is occupied by pores with an equivalent diameter of 0.2-0.5 mm.

– and distribution diagram of initial oil saturation 1 and residual oil saturation after displacement 2 as a percentage of the volume of effective porosity by equivalent pore diameters (Fig. 7b). The difference between values 1 and 2 makes it possible to determine that the most effective displacement of the oil model occurred from pores with an equivalent diameter of 0.3-0.5 mm.

Example 2 Implementation of a method for determining the oil displacement efficiency of a polymer on a pore scale based on 4D microtomography on a carbonate reservoir and comparing the results obtained with the results according to OST 39-195-86 “Oil. Method for determination of oil displacement efficiency by water in laboratory conditions”. The size of the cylindrical sample is 6 x 6 mm, saturation with reservoir water, reservoir oil filtration, displacement with a polymer composition.

At the first stage, the claimed device of the technological installation for 4D microtomography was assembled, including an X-ray computed microtomograph, a filtration unit, and a radiolucent mobile core holder. The filtration unit included a pressurizing pump and a pressure pump, a manifold for controlling the supply of liquid from the selected container, piston containers for injected liquids connected to the radiolucent mobile core holder by inlet tubes located in such a way that they allow the radiolucent mobile core holder to freely rotate in the holder around its axis, and shut-off valves installed on the supply pipes. The X-ray transparent mobile core holder was installed in the holder between the X-ray tube and the detector of the X-ray computed microtomograph.

The radiolucent mobile core holder included a base with three internal channels, polymer tubes with dividers at the ends, a silicone cuff with the possibility of fixing a cylindrical sample of reservoir rock in its cavity, an external radiolucent tube, and an upper locking cap with an outlet channel. The inlet pipes from the containers were connected to the base at the entrance to the channels, the inlet pipe from the compression pump was connected to a separate entrance of the base. From above, in the central part of the base, polymer tubes were installed, between which a silicone cuff was placed. The ends of the polymer tubes with dividers were directed towards the sample in the silicone sleeve. The radiolucent tube was installed on the base so that the polymer tubes and the silicone cuff remained inside it. On top of the radiolucent tube, an upper locking cap with an outlet channel was fixed, which was connected to a polymer tube. The device is equipped with a mobile heating element, made along the outer contour of the X-ray transparent tube, with the possibility of determining the heating temperature of the mobile X-ray transparent core holder, and a thermocouple installed in the base of the mobile X-ray transparent core holder , with the possibility of detecting the temperature of the medium inside it.

Formation water was injected into the piston container 7 (Fig. 1), contrasting KI, formation oil was pumped into the piston container 8 (Fig. 1), and the polymer composition was pumped into the piston container 9 (Fig. 1). The liquids were sequentially pressed through the supply tubes until they exited the base into the polymer tube.

Next, cylindrical samples 6 mm in diameter and 6 mm high were cut out of a carbonate non-extracted reservoir of one of the high-viscosity oil fields, placed in a thin silicone cuff, on which polymer tubes with dividers at the ends were put on both sides. An external radiolucent tube was placed on top of the column. The experiment was carried out at 23°C, a crimping pressure of 11 MPa, and a liquid flow rate of 0.05 ml/min.

At the second stage, microtomography of dry samples was performed with a resolution of 8.5 µm. Reconstructed 3D models. Next, the absolute porosity was segmented in the samples, and the coefficients of the total porosity were determined based on the ratio of the allocated volume to the volume of the entire sample. Based on the connection of pores along the vertical axis of the cylinder Z, the effective porosity was segmented; on the basis of the ratio of the allocated volume to the volume of the entire sample, the effective porosity coefficients were determined.

At the third stage, the samples were saturated with formation water from the field, contrasted with KI salts (100 g/l), and repeated microtomography was performed to control saturation. After that, formation oil was pumped through the sample (about 10 pore volumes) until formation water exit from the sample stopped. The next step was microtomography and segmentation on the obtained 3D images of reservoir oil volumes and contrasted reservoir water in the volume of effective porosity, the values of residual water saturation and initial oil saturation were determined.

At the fourth stage, oil was displaced by polymer compositions of two types at three different concentrations - 1000, 1500 and 2000 mg/l, contrasted with KI salts (100 g/l). Next, microtomography was again carried out and segmentation on the obtained 3D images of the volumes of residual oil in the volume of effective porosity. The complete sequence of steps for one sample in the form of changes in the same X-ray image is shown in Fig. 8: 8a - X-ray section of a dry sample, 8b - X-ray section of a sample saturated with contrasted formation water, 8c - X-ray section of the sample after saturation with oil at residual water saturation, 8d - after displacement of oil with a contrasted polymer composition. The oil displacement coefficients were determined by formula (2).

To compare the results obtained under the same temperature and pressure conditions described above, experiments were carried out to determine the oil displacement coefficients of the above polymer compositions according to the prototype on composite columns of standard samples (cylinders 30 mm long and 30 mm in diameter) of a carbonate reservoir, from the ends of which samples for 4D- microtomography. The oil displacement coefficients were calculated according to the prototype according to formula 3. The results of the determined oil displacement coefficients by polymer compositions for the claimed method and the prototype are presented in Table 3 in FIG. 9. The discrepancies between the two methods are in the range of 0.02 - 0.11. The correlation coefficient between the values obtained by the claimed method and the prototype is 92.74%, which indicates the reliability of the proposed method.

Thus, from the above, we can conclude that the applicant has achieved the claimed technical result , namely:

1 – the ability to determine the oil displacement factor on the pore scale with greater accuracy due to the high resolution of microtomography has been achieved.

As shown in Example 1, the extrusion scale factor based on the prototype on the standard sample with the characteristics shown in FIG. 5, and calculated by the formula (1) will be P _Art. = 188.2. Let the minimum determined volume in the proposed method based on 4D microtomography be equal to the volume of a cube consisting of 8 voxels, i.e. for the experiment described below will be 8×8.8 ³ = 5451.776 μm ³ . The exact volume of the sample according to microtomography was 86.8674 mm ³ . Therefore, the scale factor determined by formula (1) will be about P _micro = 2.7 × 10 ⁶ , which is more than 14,000 times higher than P _st. Thus, the method based on 4D microtomography due to high resolution has a higher accuracy in determining the volume of liquid phases compared to the prototype.

2 - since the method is based not on calculating the volumes of fluid leaving the sample, but on determining the volumes of contrasted and non-contrasted phases directly inside the effective porosity of the sample, the described method can be used to work with both extracted and non-extracted samples. According to Example 1, the results of applying the claimed method with an extracted sample are presented, according to Example 2, the results of applying the claimed method with non-extracted samples are presented.

3 - it was possible to evaluate the effectiveness of the extrusion abilities of agents on a pore scale, to determine the sizes of the pores from which the most and least effective displacement occurs, which was demonstrated in Example 1 and in Fig. 7b.

The claimed technical solution meets the "novelty" criterion for inventions, since no technical solutions have been identified from the studied prior art that have the claimed set of features that ensure the achievement of the claimed results.

The claimed technical solution meets the "inventive step" criterion for inventions, since it is not obvious to a specialist in this field of science and technology, since the claimed technical solution provides the ability to simultaneously implement several tasks (the possibility of determining the oil displacement efficiency on a pore scale with a greater accuracy, the ability to work with both extracted and non-extracted collector samples, the ability to assess the characteristics of the pores from which the most or least effective displacement occurs) with higher consumer properties.

The claimed technical solution meets the criterion of "industrial applicability", as it can be implemented at any specialized enterprise using standard equipment, well-known domestic materials and technologies.

Claims (3)

1. A method for determining the oil displacement factor on a pore scale based on 4D microtomography, which consists in making a cylindrical sample from a reservoir rock with a height of 4 to 6 mm and a diameter of 4 to 6 mm, then it is placed in an X-ray transparent mobile core holder of the device to perform X-ray computed microtomography of reservoir rocks, microtomography of a cylindrical sample is carried out, followed by segmentation of the effective porosity structure, then the cylindrical sample is saturated with formation water or a model of formation water contrasted with X-ray dense compounds, oil or oil model is filtered through the sample under specified thermobaric conditions until the moment of stopping water exit from the sample, then a repeated microtomography of the sample is carried out, a three-dimensional image of the sample is recorded in the volume of the previously obtained digital model of the original sample, segmentation, within the previously selected effective porosity, volumes of oil and of residual formation water based on differences in X-ray density between the contrasted and non-contrasted phases, the volume of initial oil saturation is determined, then the oil or oil model is displaced with formation water or another displacing agent contrasted with X-ray dense compounds, and repeated microtomography of the sample is performed, a three-dimensional image of the sample is recorded in volume previously obtained digital model of the original sample, segmentation, within the previously identified effective porosity, volumes of residual oil or oil model based on differences in x-ray density between the contrasted and non-contrasted phases, the displacement coefficient β of oil is calculated based on the ratio of the volumes of the initial oil saturation Vn.nach. , residual oil saturation Vн.rest. in the sample according to the formula:

. 2. A device for performing 4D X-ray computed microtomography of reservoir rocks for implementing the method according to claim 1, consisting of an X-ray computed microtomography scanner, a filtration unit, an X-ray transparent mobile core holder, while the filtration unit includes a crimp pump, a pressure pump, a manifold for supply control liquids from the selected container, piston containers for pumped liquids connected to the radiolucent mobile core holder by inlet pipes located in such a way that they allow the radiolucent mobile core holder to freely rotate around its axis, and stopcocks installed on the inlet pipes; the radiolucent mobile core holder is placed in the holder between the x-ray tube and the detector of the x-ray computed microtomograph; X-ray transparent mobile core holder consists of a base with three internal channels, polymer tubes with dividers at the ends, a silicone cuff with the possibility of fixing a cylindrical sample of the reservoir rock in its cavity, an external X-ray transparent tube and an upper shut-off cover with an outlet channel, attached to the base at the entrance to the channels supply tubes from containers, a supply tube from a crimping pump is connected to a separate entrance of the base, polymer tubes are installed on top in the central part of the base, between which a silicone cuff is placed; the ends of the polymer tubes with dividers face the silicone cuff; the radiolucent tube is mounted on the base in such a way that the polymer tubes and the silicone cuff remain inside it, on top of the radiolucent tube there is a top locking cap with an outlet channel connected to the polymer tube; the device is equipped with a mobile heating element, made along the outer contour of the X-ray transparent tube, with the possibility of determining the heating temperature of the mobile X-ray transparent core holder, and a thermocouple located at the base of the mobile X-ray transparent core holder, with the possibility of detecting the temperature of the medium inside it.

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