RU2748021C1 - Method for creating residual water saturation on a weakly cemented core for conducting flow studies - Google Patents

Abstract

FIELD: mining.

SUBSTANCE: invention relates to the field of laboratory studies of the joint movement of fluids through a sample of rocks the conduct of which is necessary for the technical and economic assessment of the calculation of reserves in the development of the field. To create residual water saturation, a device is used as the lower end piston in the core holder of the installation for conducting streaming studies, which includes a place for a semi-permeable membrane and three through hydraulic lines, two of which are designed to bypass the membrane, and one is for water outlet when creating a residual water saturation. In this case, a dried sample of poorly cemented rock is placed in a core holder, the sample is evacuated, followed by 100% saturation with formation water or its model, then, by closing the hydraulic channels, for the movement of fluids, supplying gas from above with a pressure of no higher than 12 atm, pore pressure is created and water is displaced from the sample through the membrane and the corresponding hydraulic channel. During all stages, the sample is scanned with gamma / X-ray sources, while the values ​​of the residual water saturation (C_rws) are calculated from the values ​​of the absorption coefficients when the sample is X-ray or gamma rays.

EFFECT: possibility of creating with the subsequent calculation of the residual water saturation (C_rws) by scanning the sample with X-rays or gamma rays without removing it and the membrane from the core holder of the installation, followed by flow studies - relative phase permeabilities or displacement coefficient.

1 cl, 2 tbl, 4 dwg

Description

The invention relates to the field of laboratory studies of the joint movement of fluids through a sample of rocks, the conduct of which is necessary for the technical and economic assessment of the calculation of reserves in the development of the field.

The pore space of the reservoir rock contains not only oil and / or gas, but also residual "relict" water, estimated by the coefficient of residual water saturation (hereinafter CWO).

Quo = Vv / Vpor,

where Vv - the volume of residual water in the pore space, cm ³ ; Vpore - the volume of all voids, cavities of the intergranular space of the rock, cm ³ .

The volume of residual water in the pores is created for petrophysical and other studies by indirect methods. As a rule, these are methods of stripping water from the pores of the sample using a centrifuge or using a semipermeable membrane.

The volume of water is calculated by the formula Vw = (m _in -m _dry ) / ρ _in ,

where m _in is the mass of the sample with residual water, g; m _dry is the mass of a dry sample, g; ρ _in - the density of water, g / cm ³ .

The pore volume is calculated by the formula

Vpore = (m _air -m _dry ) / ρ _in ,

where m _air is the mass of the sample with water in air.

The volume and structure of pores in poorly cemented rocks, under the influence of external pressure (weight of the overlying rocks), decreases and contracts. Moreover, if for strong, consolidated porous media the compressibility of the rock and pores can be neglected, then for loose, weakly cemented rocks, this is unacceptable. And since the compressibility of water in comparison with the compressibility of the pores can be neglected, and the residual water with the compressibility of the rock (pores) remains in the pore space without changing its volume, the error in determining the KVO can reach more than 50% (relative).

In the current research practice, the creation of a KVO is carried out in a core holder of a device intended for this, and then the sample is dismantled, transferred and installed into a core holder of another installation (by determining the phase permeabilities or displacement coefficient).

The current procedure for creating Quo and conducting experiments is for strong, well-consolidated core samples. For the case of a weakly consolidated, loose core, it is not applicable, since these samples are partially destroyed in the process of dismantling, moving, reassembling and imitating the load by reservoir pressure, crumbling outwardly, and losing mass.

The problem of studying poorly cemented rock samples is that after each contact / impact with it, it crumbles from the outside, loses some of the constituent grains of the skeleton, and accordingly its mass decreases, which should be constant during all measurements. Therefore, such samples must be placed in the core holder immediately after manufacture or after weighing, determination of porosity, gas permeability and not removed from it until the end of all measurements (Vп, Кв, etc.).

There are several known methods for creating residual water saturation in samples of weakly cemented rock (hereinafter referred to as a sample) directly in the core holders of the stands, which are subsequently used for conducting streaming studies.

There is a known method of creating residual water saturation according to standard technology by the centrifugation method - a method of displacement, drainage, partial replacement of water with an immiscible liquid (oil, oil) from strong reservoir rocks (method of G.I. Kolokolov) [ZapSibNIGNI, works, issue 6, Shishigin C .AND. "Methods and results of studying the reservoir properties of oil and gas horizons of the West Siberian province", publishing house "Nedra", Moscow 1968; OST 39-235-89 “Oil. Method for determining phase permeabilities in laboratory conditions with joint filtration "]. This method allows you to "expressly" obtain water saturation (hereinafter Kw), the value of which will be higher than the natural value of Kwo by 10-15%. Those. the water in the pore space of the sample will not all be residual. In part, it will be mobile, and its distribution along the length of the cylindrical sample will be gradient (non-uniform): at the entrance to the sample of the displaced fluid, the water saturation will be lower than at the exit.

Conducting streaming studies under such initial conditions, when the value of Kw is not equal to the value of Kwo, will be incorrect and the results of streaming studies will obviously contain errors.

Known semi-dynamic method for studying the formation of the initial equilibrium saturation with fluids (water and oil) of the pore space set forth in [Lenormand R. And Eisenzimmer A. // "A Novel Method for the Determination of Water / Oil Capillary pressures of Mixed Wetability Samples". SCA 9322 (1993)]. In it, the Quo is created by "washing" the end of the rock sample with oil while pumping formation water (or the model of formation water) inside the sample from the opposite end. This method belongs to the method of studying capillary characteristics.

The disadvantage of this method is the duration of the formation of KVO, and further continuation of experiments (without moving the core sample) only for the oil-water system. For tests in oil-gas, gas-water systems, the semi-dynamic method of creating a KVO on a weakly cemented core is inapplicable.

Also known is a universal multifunctional device for determining various characteristics of rock samples [RU 2343281 C1 IPC E21C 39/00, publ. 01/10/2009], with a core holder, the design of which allows placing a semi-permeable membrane to create a Quo.

The disadvantage of this device is that it does not functionally provide for carrying out on a rock sample, after the creation of the Quo, subsequent streaming studies without removing a semipermeable membrane. And this is unacceptable for "loose" rocks, since part of the rock layer (sand, silt, clay) adhering to the membrane will be extracted together with the membrane.

A technical problem is the development of a method for creating residual water saturation of the sample directly in the core holder of the installation, in which, subsequently, the joint movement of any fluids will be studied without removing the semipermeable membrane. At the same time, redistributing the flow of fluids bypassing the membrane, it does not create hydraulic barriers and does not interfere with the flow experiment.

The technical result of the proposed solution consists in the possibility of creating with the subsequent calculation of water saturation (KVO), by the method of X-ray or gamma ray transillumination of the sample, without removing it and the membrane from the core holder of the installation, followed by conducting streaming studies (relative phase permeabilities or displacement coefficient).

The specified technical result is achieved by the fact that the proposed device is used to create residual water saturation, the design of which includes a place for a semi-permeable membrane and three through hydraulic lines, as a lower end piston in the core holder of the installation for conducting streaming studies. The dried sample of poorly cemented rock is placed in a core holder in the arrangement according to FIG. 3: sample in a cuff, with end pistons (a semi-permeable membrane is glued in the lower one). The sample is evacuated, followed by 100% saturation with formation water (or its model). Then, by closing the hydraulic channels 2, supplying gas from above with a pressure not exceeding 12 atm., Pore pressure is created and water is displaced from the sample through the membrane and channel 3. During all stages, the sample is scanned by gamma / X-ray sources and the Quo value is calculated from the reference values.

In the proposed method, before carrying out the flow experiment (to determine the relative phase permeabilities or the coefficient of displacement / replacement of one fluid by another) on a rock sample, the residual water saturation is created by the semipermeable membrane method directly in the core holder of the flow installation. Further, in the same core holder, the actual flow experiment is carried out, without any movement, manipulation with a rock sample and a membrane.

A feature of the claimed invention is a method for creating and calculating the value of KVO, taking into account the compressibility of a weakly cemented, "loose" rock sample when it is placed in the core holder of the installation at reservoir pressure and reservoir temperature, as well as close to the natural, natural distribution of residual water inside the pore space of the reservoir due to capillary forces.

The essence of the invention lies in the fact that after complete saturation of the sample with water (Kw = 100%), with the help of a semipermeable membrane placed in the end plunger of the core holder and gas injection at a pressure of up to 1.5 MPa, residual water saturation (Kw) is created.

The device is a cylindrical piston (bottom end plunger) of the core holder of the installation with a recess (on the surface of contact with the sample) for a semi-permeable membrane, made eccentrically (for the possibility of making hydraulic outlets), and two separate hydraulic through holes / outlets passing through the proposed design. Hydraulic outlets are designed for the movement of fluids (oil, gas, water) through the sample, bypassing the membrane, during the flow experiment.

The proposed technical solution is illustrated by the figures.

FIG. 1 shows a sectional view of the device, where numbers indicate: 1 - a place (depression) for a semi-permeable membrane with a diameter of 30 mm (when using a sample with a diameter of 38 mm (1.5 '')); 2 - channels (hydraulic lines) for the movement of fluids bypassing the membrane; 3 - channel (hydraulic line) for water outlet when creating a Quo; 4 - case.

FIG. 2 shows a general view of the device.

FIG. 3 shows a general view of the device assembled with a rock sample, collar and end plungers.

FIG. 4 shows the dependence of the values of Kwo on permeability.

The method is carried out as follows.

A cylindrical weakly cemented rock sample prepared in accordance with GOST 26450.0, dried at a temperature of (70 ± 2) ° C [GOST 26450.1] is placed in a rubber cuff 5, and end pistons 4 and 7 are placed from the ends of the sample. Cuff 5 serves to isolate the sample from the fluid, which creates hydraulic pressure, this is achieved by enclosing the edges of the end plunger collar. In the lower end piston there is a semi-permeable membrane in the recess 1. Further, the assembly: a rock sample in the collar 5 with two end pistons 4 and 7 is placed in the core holder of the installation for conducting streaming studies. When using a sample with retained saturation with formation fluids (made from a core sampled using an isolated technology), it is placed in the collar immediately after production (drilling, extrusion), and then into the core holder. And all further studies, including sample preparation, are carried out directly in the core holder, without removing the sample until the end of the flow study.

After connecting the core holder to the hydraulic lines of the installation, hydraulic pressure is created with the process fluid (oil or water) in the core holder for all-round crimping of the sample. Create thermobaric conditions (hereinafter TBU) for the measurement, i.e. pore pressure and temperature. The pore pressure is created with the help of gas, which will subsequently be used for conducting the flow study, and the core holder is heated to the formation temperature. Scanning along the height of the cylindrical sample is carried out to obtain a baseline signal J of _{100% GAS} . The core holder is cooled and the pore pressure is reduced to atmospheric pressure.

The sample and the membrane, without removing from the core holder, are evacuated and saturated to 100% with formation water (or its model). Pore pressure is created with water and the signal J is _{100% water} . Then the core holder is cooled and the pore pressure is reduced to atmospheric pressure.

The outlets (with the corresponding valve / valve) of channels 2 are closed and at the opposite end (upper) pore pressure is created (up to the required value, or traditionally up to 1.2 MPa) using gas - nitrogen. Water from the sample through the membrane will come out through channel 3. If water stops leaving channel 3, it means that the water saturation of the sample has reached the KVO value. By the values of intermediate scans of the sample by the method of gamma or X-ray transmission, it is also possible to control and judge the state of the Quo. Further, after reaching Kvo in the sample, channel 2 is opened and, by injecting gas-nitrogen, pore pressure is created and the core holder is heated, thereby creating a TBM. Scan the sample to obtain J _i at TBU.

Using the obtained values of the sample scans, the Kvo value at TBU is calculated according to the formula of the semilogarithmic Bouguer-Lambert law:

The obtained KvO value, by the proposed method, does not need correction coefficients for water expansion and rock compressibility, pore volume, when the sample is placed in the TBU (in the core holder) (imitation of reservoir conditions).

To check the reliability of the proposed method, methodical work was carried out with a well-consolidated sample of quartz sandstone Berea. The choice of this sample was dictated by the need to remove it from the core holder, after creating a residual water saturation, to control the KvO value by the gravimetric method. The specified sandstone is practically not compressed and does not "lose" grains of sand during any standard manipulations, movements of the sample. In the course of the methodical work, the measurements of the residual water saturation were compared by three methods: Quo by gamma transmission of the sample with TBU (in a core holder), Quo measured by the gravimetric method, and Quo measured by the volumetric method. The results of the work are presented in the table.

The Quo value by the volumetric method was calculated using the formula:

Quo = 100⋅ (V _pores -V _samples ) / V _pores ,

where V _samples - the volume of water in the test tube, released from the sample, when creating a KVO by the method of a semipermeable membrane in the proposed device.

As can be seen from the table, the Quo values according to the results of gamma transmission have a relative error of 4.1%, in comparison with the gravimetric method (as the most accurate). The Quo value obtained by the volumetric method, in relation to the weight method, has a relative error of 9.1%.

To test the proposed method, creating a KVO in a core holder and measuring the KVO value at TBR for a weakly cemented rock sample, 5 samples of a weakly cemented rock were selected. The main characteristics of the samples, the results of the creation and measurement of the KVO are presented in Table 2.

FIG. 4 it can be seen that the values obtained and measured according to the claimed method (with TBR) are closer to the reservoir dependence of the KVO values on the permeability.

Thus, the obtained Quo value obtained with the inventive method of creating residual water saturation on a weakly cemented core for conducting streaming studies does not need correction factors to take into account the temperature expansion of water and the compressibility of a weakly cemented rock when placing / loading a sample in thermobaric conditions.

Claims (3)

A method of creating residual water saturation by the method of a semipermeable membrane for samples of weakly cemented rocks directly in the core holder of the installation, followed by flow studies with the joint movement of formation fluids without removing the sample and membrane from the core holder, characterized by the fact that to create residual water saturation as a lower end piston as part of the core holder Installations for streaming research use a device that includes a place for a semi-permeable membrane and three through hydraulic lines, two of which are designed to bypass the membrane, and one is for water to escape when creating residual water saturation, while a dried sample of weakly cemented rock is placed in a core holder, a sample is evacuated followed by 100% saturation with formation water or its model, then, by closing the hydraulic channels for the movement of fluids, supplying gas from above with a pressure not exceeding 12 atm, create a pressure and water is displaced from the sample through the membrane and the corresponding hydraulic channel, during all stages the sample is scanned by gamma / x-ray sources, while the values of the residual water saturation (КВо) are calculated from the values of the absorption coefficients J when the sample is transmitted with x-ray or gamma rays using the formula

where J _{100% GAS} is the absorption coefficient at 100% gas saturation of the sample, J _{100% water} is the absorption coefficient at 100% water saturation of the sample, J _i is the current value of the absorption coefficient at KVO.

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