RU2771453C1 - Method for studying the liquid permeability of core samples - Google Patents

Abstract

FIELD: oil and gas industry.

SUBSTANCE: invention relates to the oil and gas industry and can be used in the design of the development of oil fields. The method for studying the liquid permeability of core samples includes the combined effect of liquid filtration and effective stresses on core samples covered with a heat shrink sleeve, conducting a study of the permeability of samples under axial compression at the stages of deformation before and beyond the tensile strength at constant values of lateral pressure, while at the stages of elastic, inelastic and excessive deformation, the pore pressure is stepwise decreased and increased in the range that corresponds to the conditions of reservoir development, and the filtration of the liquid is continued at each stage until the permeability is stabilized, while the absolute permeability is determined under initial reservoir conditions, in extreme stress states that correspond to long-term, short-term and residual strength.

EFFECT: increase in the accuracy of determining the absolute permeability of the reservoir rock with cracks in the core sample.

1 cl, 6 dwg, 6 tbl

Description

The invention relates to the oil and gas industry and can be used in the design of the development of oil fields.

There is a known method for studying the porosity and permeability of core samples (RF patent No. 2625536, publ. 07/14/2017), which includes fluid filtration through core samples while exposing them to effective stresses of various magnitudes until the permeability of core samples is stabilized in at least three modes of exposure.

The disadvantage of this method is that it does not take into account the change in permeability during deformation of the samples due to the occurrence and development of cracks in them.

There is a known method for studying the permeability of core samples with cracks (RF patent No. 2620872, publ. 05/30/2017), including the combined effect of water filtration and effective stresses on core samples with a single crack, conducting a study of core samples with a cyclic increase and decrease in effective stresses and determining the value changes in fracture permeability due to elastic deformations of the core sample.

The disadvantage of this method is its low accuracy, due to the fact that the orientation of artificial cracks is not determined.

A known method for studying the influence of the stress-strain state of rocks on filtration characteristics (Kovkhuto A. M. et al. The influence of the stress-strain state of rocks on filtration characteristics and well flow rates (on the example of deposits of the Rechitsko-Vishansky zone of uplifts of the Pripyat trough) // Geology , Geophysics and Development of Oil and Gas Fields. - 2015. - No. 3. - P. 56-62.), which includes a study of the effect of changes in pressure reduction on the permeability of rock samples.

The disadvantage of this method is its low accuracy, due to the fact that the change in permeability is obtained by changing the compression pressure, and not the pore pressure.

There is a known method for studying the permeability of rocks in liquid during their deformation (Zemisev V.N., Kartashov Yu.M., Ilyinov M.D., Karmansky A.T., Kozlov V.A. Study of the permeability of rocks during their deformation. Mining geomechanics and mine surveying: Collection of scientific papers, St. Petersburg: VNIMI, 1999. - Ministry of Fuel and Energy of the Russian Federation, Russian Academy of Sciences, - pp. 65-69), including the combined effect on the sample of fluid filtration and bending loads; conducting a study of the sample at the stages of deformation before and beyond the limit of strength with a change (increase) in the filtration pressure and axial compression by a load element (spherical indenter or flat punch); determination of the dependence of the change in the permeability index (filtration coefficient) on the deformation of the sample.

The disadvantage of this method is the low accuracy due to the difficulty of measuring the deformations of thin flat samples during contact tests. The capabilities of the method are limited by determining the permeability of the reservoir rock under the combined action of tensile and compressive stresses under "pure shear" conditions.

There is a known method for studying the permeability of composite core samples with cracks (article: Yu J. et al., Triaxial test research on mechanical properties and permeability of sandstone with a single joint filled with gypsum //KSCE Journal of Civil Engineering. - 2016. - T. 20. - No. 6. - S. 2243-2252), taken as a prototype, which consists in the fact that core samples dressed in a heat-shrinkable shell with single artificial cracks are subjected to the combined effect of fluid filtration and effective stresses; conduct a study of the permeability of samples by axial compression at the stages of deformation before and beyond the ultimate strength at constant values of lateral pressure.

The disadvantage of this method is the low accuracy of determining the permeability of the reservoir rock with cracks on composite samples, due to the heterogeneity of the material composition and structure introduced by the artificial crack filler. Another disadvantage of the method is its complexity, due to the manufacture and testing of a series of prefabricated samples with artificial cracks oriented at different angles of inclination.

The technical result is to increase the accuracy of determining the absolute permeability of the reservoir rock with cracks in the core sample.

The method is illustrated by the following figures:

fig. 1 - plots of the maximum effective stress (σ ₁ - _Rthr ) on the relative axial strain ε ₁ of the sample and the absolute permeability K on the relative axial strain ε ₁ ;

fig. 2 - graphs of dependence of absolute permeability K on the value of pore pressure (P _pore ) in the sample at initial reservoir conditions, in limit stress states corresponding to long-term, short-term and residual strength;

fig. 3 - plots of the maximum effective stress (σ ₁ - _Rthr ) on the relative axial strain ε ₁ of the sample and absolute permeability K on the relative axial strain ε ₁ ;

fig. 4 - graphs of dependence of absolute permeability K on the value of pore pressure Р _pores in the sample at initial reservoir conditions, in limit stress states corresponding to long-term, short-term and residual strength;

fig. 5 - plots of the maximum effective stress (σ ₁ - _Rthr ) on the relative axial strain ε ₁ of the sample and the absolute permeability K on the relative axial strain ε ₁ ;

fig. 6 - plots of dependence of absolute permeability K on the value of pore pressure (Р _pore ) in the sample at initial reservoir conditions, in limit stress states corresponding to long-term, short-term and residual strength.

The method is carried out as follows. At the first stage, at least two core samples are taken from the productive horizon, which are identical in origin. Core samples are extracted, for example in a SOX-5000 equipment, and dried, for example in a Binder FED 115 equipment, to remove any liquids and impurities present in the pore space. One of the prepared specimens is tested on a uniaxial compression press, such as MTS Insight, to obtain the dependence of the relative longitudinal deformation on the magnitude of the axial load. Further, using software, such as Microsoft Excel, an experimental strain curve is built, which is necessary to determine the range of values of the long-term, short-term and residual strength of the sample, according to the readings of the longitudinal strain gauges and the dynamometer.

The second sample, saturated with liquid, is tested under the combined effect of liquid filtration and effective stresses to study the permeability of the sample under axial compression at the stages of deformation before and beyond the ultimate strength at constant values of lateral pressure, in a triaxial chamber, for example, GCTS RTR 1500, which allows to determine change in fluid permeability. Prior to testing, the specimen is wrapped in a heat-shrinkable sheath to prevent penetration of the working fluid of the technical oil used to create hydrostatic lateral pressure. Then the sample is placed in the installation and brought to the actual conditions of occurrence. To do this, a predetermined hydrostatic pressure is created in the triaxial compression chamber, the value of which is selected from the range of values corresponding to the horizontal stress in the real massif, and the initial pore pressure corresponding to the initial reservoir pressure. Axial load, hydrostatic and pore pressure values are set and controlled via software.

Further, at the stage corresponding to the initial reservoir conditions, filtration tests of the sample are carried out at a constant value of axial deformation to the ultimate strength, at constant values of lateral pressure and a stepwise decrease and increase in pore pressure in the range corresponding to the reservoir development conditions. Fluid filtration at each stage continues until the permeability stabilizes. The change in liquid permeability from effective stresses in various mechanical states due to the gradual development of systems of microcracks in the sample is investigated.

Further, by increasing the axial deformation, at the stage of elastic deformation, filtration tests of the sample are carried out at a constant value of axial deformation up to the long-term strength limit at constant values of lateral pressure and a stepwise decrease and increase in pore pressure in the range corresponding to the reservoir development conditions. Fluid filtration at each stage continues until the permeability stabilizes. The change in liquid permeability from effective stresses in various mechanical states due to the gradual development of systems of microcracks in the sample is investigated.

Further, by increasing the axial deformation, at the stage of inelastic deformation, filtration tests of the sample are carried out at a constant value of axial deformation up to the short-term strength limit at constant values of lateral pressure and a stepwise decrease and increase in pore pressure in the range corresponding to the reservoir development conditions. Fluid filtration at each stage continues until the permeability stabilizes. The change in liquid permeability from effective stresses in various mechanical states due to the gradual development of macrocrack systems in the sample is investigated.

Further, by increasing the axial deformation and destroying the sample, at the stage of ultimate deformation, filtration tests of the sample are carried out at a constant value of axial deformation after the short-term strength limit at constant values of lateral pressure and a stepwise decrease and increase in pore pressure in the range corresponding to the reservoir development conditions. Fluid filtration at each stage continues until the permeability stabilizes. Investigate the change in fluid permeability from effective stresses in various mechanical states due to through cracks.

Then, according to the indications of the displacement of the pore pressure piston, using software, for example, Microsoft Excel, the amount of fluid filtered through the sample is calculated and putting this value into the Darcy formula for a rectilinear-parallel flow, the absolute permeability is determined in the initial reservoir conditions, in stressed states of long, short-term and residual strength, and change in the permeability of the sample due to microcracks, macrocracks and through cracks according to the formulas:

,

,

where K ₀ - effective permeability of the core sample corresponding to the initial reservoir conditions;

K _L - effective permeability of the core sample corresponding to the stress state of long-term strength (obtained at the stage of elastic deformation);

K _F - effective permeability of the core sample, corresponding to the stress state of short-term strength (obtained at the stage of inelastic deformation);

K _R - effective permeability of the core sample, corresponding to the residual strength (obtained at the stage of beyond limiting deformation);

ΔK _L - change in the permeability of the sample due to microcracks;

ΔK _F - change in the permeability of the sample due to macrocracks;

ΔK _R - change in the permeability of the sample due to through cracks;

The obtained values of the absolute permeability of the core sample, determined under the initial reservoir conditions and in the ultimate stress states corresponding to long-term, short-term and residual strength, can be considered as estimated indicators of the permeability of the reservoir rock, undisturbed, weakened by systems of microcracks, macrocracks or through cracks, respectively. Which are used to adjust the main parameters of the field development, for example, the cumulative oil production during its design.

The method is illustrated by the following example. For experimental studies, 4 terrigenous rock samples were selected and prepared. For this group of samples, the uniaxial compressive strength was determined σ _com =27.4 MPa.

Based on the test results, plots of the dependence of the maximum effective stress (σ ₁ - P _pore ), where σ ₁ - P _pore - the maximum effective stress, equal to the difference between the maximum main normal stress σ ₁ and pore pressure P _pore from the relative axial deformation ε ₁ of the sample and absolute permeability K versus relative axial strain ε ₁ (Fig. 1), and plots of absolute permeability K versus pore pressure P _pores in initial reservoir conditions, in ultimate stress states corresponding to long-term, short-term and residual strength (Fig. 2).

The test results on Sample No. 1 are presented in Tables 1 and 2.

Table 1. Sandstone absolute permeability values at three points (at maximum pore pressure (at the beginning of the test), at the minimum pore pressure, at the maximum pore pressure (at the end of the test))

° K ₀ , ⋅10 ^-3 µm ² K _L , ⋅10 ^-3 µm ² K _F , ⋅10 ^-3 µm ² K _R , ⋅10 ^-3 µm ² Sample No. 1 1,800 1.352 1.111 0.902 1.251 1.117 0.895 0.790 1.673 1.119 0.943 0.755

Table 2. Change in absolute permeability due to microfractures, macrofractures and through cracks

ΔK _L , ⋅10 ^-3 µm ² ΔK _F , ⋅10 ^-3 µm ² ΔK _R , ⋅10 ^-3 µm ² With a decrease in pore pressure Sample No. 1 -0.191 -0.342 -0.445 With increasing pore pressure Sample No. 1 -0.236 -0.371 -0.472

It has been established that with a decrease/increase in the pore pressure in the sample at the stage of elastic deformation, it causes the development of a system of microcracks and, accordingly, contributes to a decrease in absolute permeability relative to absolute permeability at initial reservoir conditions by 0.191/0.236⋅10 ^-3 µm ² . Similarly, a decrease / increase in pore pressure at the stage of inelastic deformation at stresses exceeding the limit of long-term strength contributes to the gradual coalescence of shear and shear microcracks and, accordingly, contributes to a decrease in absolute permeability relative to absolute permeability at initial reservoir conditions by 0.342/0.371⋅10 ^-3 μm ² . At the limiting stage of deformation, a decrease in pore pressure contributes to the development of through cracks and, accordingly, contributes to a decrease in absolute permeability relative to absolute permeability at initial reservoir conditions by 0.445/0.472⋅10 ^-3 µm ² .

Based on the test results, graphs of the dependence of the maximum effective stress (σ ₁ - Р _pore ) on the relative axial strain ε ₁ of the sample and absolute permeability K on the relative axial strain ε ₁ (Fig. 3), and plots of the dependence of absolute permeability K on the value of pore pressure ( Р _pore ) in the initial reservoir conditions, in the ultimate stress states corresponding to long-term, short-term and residual strength (Fig. 4).

The test results for Sample No. 2 are presented in Tables 3 and 4.

Table 3. Sandstone absolute permeability values (at maximum pore pressure (at start of test), minimum pore pressure, at maximum pore pressure (at end of test))

° K ₀ , ⋅10 ^-3 µm ² K _L , ⋅10 ^-3 µm ² K _F , ⋅10 ^-3 µm ² K _R , ⋅10 ^-3 µm ² Sample #2 1.337 1.079 0.980 0.802 1.171 0.891 0.812 0.646 1.156 1.037 0.823 0.662

Table 4. Change in absolute permeability due to microfractures, macrofractures and through cracks

ΔK _L , ⋅10 ^-3 µm ² ΔK _F , ⋅10 ^-3 µm ² ΔK _R , ⋅10 ^-3 µm ² With a decrease in pore pressure Sample #2 -0.214 -0.285 -0.423 With increasing pore pressure Sample #2 -0.171 -0.297 -0.438

It has been established that with a decrease/increase in the pore pressure in the sample at the stage of elastic deformation, it causes the development of a system of microcracks and, accordingly, contributes to a decrease in absolute permeability relative to absolute permeability at initial reservoir conditions by 0.214/0.171⋅10 ^-3 µm ² . Similarly, a decrease/increase in pore pressure at the stage of inelastic deformation at stresses exceeding the limit of long-term strength contributes to the gradual coalescence of shear and shear microcracks and, accordingly, contributes to a decrease in absolute permeability relative to absolute permeability at initial reservoir conditions by 0.285/0.297⋅10 ^-3 μm ² . At the transcendental stage of deformation, a decrease in pore pressure promotes the development of through cracks and, accordingly, contributes to a decrease in absolute permeability relative to absolute permeability at initial reservoir conditions by 0.423/0.438⋅10 ^-3 µm ² .

Based on the test results, plots of the maximum effective stress (σ ₁ - P _pore ) on the relative axial strain ε ₁ of the sample and absolute permeability K on the relative axial strain ε ₁ (Fig. 5) are plotted, and plots of the dependence of the absolute permeability K on the pore pressure value ( P _pore ) in the initial reservoir conditions, in the ultimate stress states corresponding to long-term, short-term and residual strength (Fig. 6).

The test results for Sample No. 3 are presented in Tables 5 and 6.

Table 5—Absolute permeability values of sandstone (at maximum pore pressure (at start of test), minimum pore pressure, at maximum pore pressure (at end of test))

° K ₀ , ⋅10 ^-3 µm ² K _L , ⋅10 ^-3 µm ² K _F , ⋅10 ^-3 µm ² K _R , ⋅10 ^-3 µm ² Sample #3 1.066 0.832 0.722 0.425 0.591 0.507 0.460 0.406 0.924 0.781 0.523 0.319

Table 6—Change in absolute permeability due to microfractures, macrofractures and through cracks

ΔK _L , ⋅10 ^-3 µm ² ΔK _F , ⋅10 ^-3 µm ² ΔK _R , ⋅10 ^-3 µm ² With a decrease in pore pressure Sample #3 -0.192 -0.287 -0.498 With increasing pore pressure Sample #3 -0.150 -0.351 -0.521

It has been established that with a decrease/increase in the pore pressure in the sample at the stage of elastic deformation, it causes the development of a system of microcracks and, accordingly, contributes to a decrease in absolute permeability relative to absolute permeability at initial reservoir conditions by 0.192/0.150⋅10 ^-3 µm ² . Similarly, a decrease/increase in pore pressure at the stage of inelastic deformation at stresses exceeding the limit of long-term strength contributes to the gradual coalescence of shear and shear microcracks and, accordingly, contributes to a decrease in absolute permeability relative to absolute permeability at initial reservoir conditions by 0.287/0.351⋅10 ^-3 μm ² . At the limiting stage of deformation, a decrease in pore pressure contributes to the development of through cracks and, accordingly, contributes to a decrease in absolute permeability relative to absolute permeability at initial reservoir conditions by 0.498/0.521⋅10 ^-3 µm ² .

The proposed method for studying the fluid permeability of core samples makes it possible to determine the value of the absolute permeability of the rock in the initial reservoir conditions, in stressed states of long-term, short-term and residual strength. Determine the change in the absolute permeability of the sample due to microcracks, macrocracks and through cracks.

Claims (11)

A method for studying the fluid permeability of core samples, which includes exposing core samples dressed in a heat-shrinkable shell to the combined effect of fluid filtration and effective stresses, conducting a study of the permeability of samples under axial compression at the stages of deformation before and beyond the ultimate strength at constant values of lateral pressure, characterized in that that at the stages of elastic, inelastic and transcendental deformation, the pore pressure is stepwise reduced and increased in the range that corresponds to the reservoir development conditions, and the fluid filtration is continued at each stage until the permeability is stabilized, while determining the absolute permeability at the initial reservoir conditions, in the limit stress states , which correspond to long-term, short-term and residual strength, and the change in the permeability of the sample due to microcracks, macrocracks and through cracks is determined by the formulas:

, where K ₀ - effective permeability of the core sample corresponding to the initial reservoir conditions; K _L - effective permeability of the core sample, corresponding to the stress state of long-term strength, obtained at the stage of elastic deformation; K _F - effective permeability of the core sample, corresponding to the stress state of short-term strength, obtained at the stage of inelastic deformation; K _R - effective permeability of the core sample, corresponding to the residual strength, obtained at the stage of beyond limiting deformation; ΔK _L - change in the permeability of the sample due to microcracks; ΔK _F - change in the permeability of the sample due to macrocracks; ΔK _R - change in the permeability of the sample due to through cracks.

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