RU2781042C1 - Method for determining the elastic properties of rocks of different saturation of core samples of gas fields - Google Patents

Abstract

FIELD: elastic properties determining.

SUBSTANCE: invention relates to a method for determining the elastic properties of rocks of different saturation of core samples from gas fields. The method consists in selecting a sample of a rock core from a gas field, conducting a preliminary evaluation of its integrity, and then evaluating the elastic-strength properties by placing it in a facility for geomechanical tests. The evaluation of elastic-strength properties is carried out under thermobaric conditions without bringing the sample to destruction, after the first cycle of the experiment at the initial given water saturation, the change in the water saturation of the sample is performed by supplying gas with a differential pressure of not more than 1.5 MPa. The necessary water saturation is created in stages by displacing water from the sample with gas through a semi-permeable membrane, after changing the water saturation of the sample, the elastic-strength properties are re-evaluated under thermobaric conditions without bringing the sample to failure. The evaluation of elastic-strength properties is carried out at a given number (at least three) of points of different water saturation for further use of the information obtained in geomechanical modeling.

EFFECT: correct evaluation of the elastic-strength properties of rock at various stages of gas field development.

1 cl, 5 dwg

Description

The invention relates to the field of laboratory studies of static elastic characteristics of rock samples, which is necessary to assess the impact of flooding processes in the development of deposits.

In the pore space of the rock of the gas reservoir, when it is completely filled, it contains gas and residual water, which is estimated by the coefficient of residual water saturation. In this case, it is assumed that the gas fills all possible pore space.

In the process of field development, the level of gas-water contact rises and the pore space filled with gas is partially replaced by formation water, as a result of which elastic and strength properties occur, which can lead to its destruction in the bottomhole formation zone and sanding, as well as other consequences.

In current practice, studies of elastic characteristics are carried out on samples with natural saturation or 100% saturated with fluid (water, kerosene, etc.), while the assessment of the current water saturation of the samples is taken from the data of studies of neighboring samples (except for 100% water-saturated ones) [McPhee K., Reed J., Zubizaretta I. "Laboratory core research: a guide to best practices" - M. - Izhevsk: Institute for Computer Research, 2018. (pp. 756-757, pp. 551-555)].

The most preferred are 2 main methods for creating residual water saturation in samples for geomechanical tests, namely the method of a semipermeable membrane in a group capillarimeter or in an individual capillarimeter [McPhee C., Reid J., Zubizaretta I. "Laboratory core research: a guide to best practices" - M. - Izhevsk: Institute for Computer Research, 2018 (pp. 756-757, pp. 551-555)]. At the same time, in both cases, when removing from the devices, the current water saturation in the samples can change under the influence of atmospheric conditions.

The technical problem is to develop a method for creating the required water saturation of the sample directly in the core holder of the geomechanical testing facility.

The technical result of the proposed solution lies in the possibility of creating a certain water saturation (Kw) of a rock sample with subsequent calculation by the volumetric method, by taking into account the volume of fluid released from the initial volume of the pore space, without extracting it and the membrane from the core holder of the installation, followed by geomechanical studies (determination of static elastic properties).

This technical result is achieved by the fact that to create water saturation, it is supposed to use a modernized lower loading plate as part of the core holder of the installation for geomechanical studies, the design of which includes a place for a semi-permeable membrane and a through hole of a hydraulic line with a connection to a pump for evaluating the released water.

In the proposed method, before conducting a geomechanical test (to determine the static elastic characteristics) on a rock sample, water saturation is created by the method of a semi-permeable membrane directly in the core holder of the geomechanical installation. Further, in the same core holder, at each stage of gas pressure, an experiment is carried out to determine the static elastic characteristics, without extracting a rock sample and a membrane.

A feature of the claimed invention is a method for assessing static elastic characteristics at the current value of Kvo, taking into account the compressibility of the rock sample when placed in the core holder of the installation at reservoir pressure and reservoir temperature.

The essence of the invention lies in the fact that after the sample is completely saturated with water (Kv = 100%), using a semi-permeable membrane placed in the lower loading plate and stepwise injection of gas at a pressure of up to 1.5 MPa, different water saturations (Kv) are created and elastic characteristics are evaluated at the current water saturation.

The device is a lower plate for the destruction of the core holder of the installation with a recess (at the contact surface with the sample) for a semi-permeable membrane, and a separate hydraulic through hole/outlet passing through the proposed design. The hydraulic outlet is designed to move water through the membrane while creating water saturation.

The proposed technical solution is illustrated by figures.

In FIG. 1 shows a graph of the dependence of static elastic characteristics on water saturation.

In FIG. 2 shows a sectional view of the loading plate, where the numbers indicate: 1 - a place (recess) under a semi-permeable membrane with a diameter of 30 mm (using a sample with a diameter of 38 mm (1.5'')); 2 - channel (hydraulic line) for water outlet when creating water saturation; 3 - body.

In FIG. 3 shows a general view of the device.

In FIG. 4 is a general view of the device assembled with a rock sample, a heat shrink tube and end plungers.

The method is carried out as follows.

Cylindrical rock sample 4 (Fig. 4), prepared in accordance with GOST 26450.0 with a diameter of 38 mm with a sample length to diameter ratio of 2.0, pre-extracted, dried and 100% water-saturated, is placed in a heat shrink tube 5 (Fig. 4 ), at the ends of the sample, the upper and lower load plates 3.1 and 3.2 are installed (Fig. 4). Shrink film 5 (Fig. 4) is designed to isolate the sample from the liquid to create crimping pressure. A semi-permeable membrane 1 is located in the lower loading plate in a recess (Fig. 4). Further, the sensors of longitudinal and transverse deformations are mounted. The assembled structure is placed in the strength chamber 10 (Fig. 5) of the installation for the study of the geomechanical properties of rocks. Next, the hydraulic lines and sensor connectors are connected and configured. The strength chamber is filled with a crimping liquid and a crimping pressure is created using pump 6 (Fig. 5) equal to 1.0 MPa to achieve a snug fit of the heat shrink tube. With the help of a counterpressure pump 7 (Fig. 5) with a difference of not more than 0.5 MPa, filtration is carried out through the lower destruction plate 3.2 (Fig. 5) until liquid appears at the outlet of the shut-off valve (cock) 9 (Fig. 5). The shut-off valve is closed, after which the initial thermobaric conditions are created, namely pore pressure, swaging pressure and temperature.

To do this, the crimp and pore pressures are increased to a pore pressure of 10 MPa, while the crimp value during the increase should always be 1 MPa higher than the pore pressure. Thus, at the first stage, the pore pressure is 10 MPa, and the crimp pressure is 11 MPa. After stabilization of the values of the pore and crimp pressures, as well as strain measurement sensors, the strength chamber is heated to the required temperature, while using pumps, constant values of the pore and crimp pressure are maintained.

When stable temperatures, pore and crimp pressures are reached, the crimp pressure is gradually increased to the required value, while the rate of increase should be such that when the liquid is displaced from the sample, there is no significant increase in pore pressure, i.e. the pore pressure should not exceed 10.5 MPa. In the process of increasing the swaging pressure, the pore pressure and temperature are controlled, and the amount of the squeezed-out liquid is recorded. The amount of liquid squeezed out is necessary to calculate the volume of the pore space in the sample under thermobaric conditions.

Next, static elastic characteristics are determined by creating an axial load and evaluating the axial and radial deformation of the sample, while the pore and crimp pressure, temperature are maintained constant. It is also necessary to observe the condition - the values of the axial load must be in the area of elastic deformations without transition to the zone of plastic deformations. Next, you need to reduce the value of the axial load to the original value.

Using the gas cylinder reducer 8 (Fig. 5), pressure is created in the gas supply line 11 (Fig. 5). The gas pressure should be in the range from 10.1 to 11.5 MPa, i.e. more than the pore pressure, but not more than 1.5 MPa of differential pressure (limitation of the membrane 1 (Fig. 5)). The shut-off valve is opened, the process of displacing water with gas (creating water saturation) is started, while using a counter-pressure pump, the amount of liquid squeezed out is fixed. In the primary process of liquid gas displacement, the required water saturation is achieved by controlling the required amount of displaced liquid, which can be estimated in advance, knowing the volume of the pore space under thermobaric conditions and the volume of liquid in the line to the sample:

where V _W1 - the required amount of displaced fluid to achieve the required water saturation, cm ³ ;

V _p - the volume of the pore space in thermobaric conditions, cm ³ ;

S ₁ - required water saturation, d.u.;

V _l ^_ the volume of liquid in the line to the sample, cm ³ .

Further, similarly to 100% water saturation, static elastic characteristics are determined.

Subsequently, a series of similar stages is carried out (of the order of 3-5), while the required amount of displaced liquid to achieve the required water saturation is determined by the formula:

where V _W1 - the required amount of displaced fluid to achieve the required water saturation, cm ³ ;

V _p - the volume of the pore space in thermobaric conditions, cm ³ ;

S ₁ - required water saturation, d.u.

The last water saturation point is reached at a differential pressure of 1.5 MPa (membrane limitation), thus reaching the minimum water saturation Kvo (residual water saturation).

The end result of the tests carried out will be a graph of the dependence of static elastic characteristics on water saturation (Fig. 1).

Claims (11)

A method for determining the elastic properties of rocks of different saturation of core samples from gas fields, based on a series of tests on a single sample under thermobaric conditions, while the creation of the necessary water saturation occurs directly in the installation for conducting geomechanical tests using the semipermeable membrane method, characterized in that water saturation is created using the lower loading plate, the design of which provides a place for installing a semi-permeable membrane, while the prepared 100% water-saturated rock sample in the assembly with strain sensors is placed in the strength chamber, then thermobaric conditions are created and the volume of the pore space of the sample and the static elastic characteristics of the 100% water-saturated sample, then, by supplying gas with a differential pressure of not more than 1.5 MPa, the necessary water saturation is gradually created by displacing water from the sample with gas through a semipermeable membrane, moreover, at each stage, the static elastic characteristics are evaluated, and the water saturation values are calculated from the volume of the squeezed out liquid, and in the primary process of liquid displacement by gas, the water saturation is determined by the formula:

where V _W1 - the required amount of displaced fluid to achieve the required water saturation, cm ³ ; V _p - the volume of the pore space in thermobaric conditions, cm ³ ; S ₁ - required water saturation, d.u.; V _zhl - the volume of liquid in the line to the sample, cm ³ , and at the subsequent stages of liquid displacement by gas, water saturation is determined by the formula:

where V _W1 - the required amount of displaced fluid to achieve the required water saturation, cm ³ ; V _p - the volume of the pore space in thermobaric conditions, cm ³ ; S ₁ - required water saturation, d.u.

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