Evaluation method for acidification blockage removal effect of low-permeability gas reservoir core

Abstract

The invention discloses a low-permeability gas reservoir core acidification plug removal effect evaluation method, and belongs to the technical field of gas field development, and the method comprises the following steps: S1, preparing a low-permeability gas reservoir core sample; s2, treating the low-permeability gas reservoir core sample to obtain a saturated formation water core; s3, performing a forward displacement test on the saturated formation water rock core, and determining the permeability of the rock core in a gas logging stable state before acidification; s4, carrying out a reverse displacement test on the saturated formation water rock core, and reversely injecting an acid solution to carry out an acidification reaction; s5, after the acidification reaction is finished, performing a forward displacement test on the saturated formation water rock core, and determining the permeability of the acidified rock core in a gas logging stable state; and S6, according to the permeability of the rock core in the gas logging stable state before and after acidification, the plug removal effect is evaluated, compared with joint evaluation of the gas phase and the liquid phase in the prior art, the whole process of the acidification plug removal effect of the low-permeability gas reservoir can be more truly reflected, the amplification process of the gas phase permeability of the low-permeability gas reservoir can be reflected, detection is convenient, and evaluation reliability is high.

Description

Technical Field

[0001] The invention relates to the technical field of gas field development, and in particular to a method for evaluating the acidification and plugging removal effect of a low-permeability gas reservoir core. Background Art

[0002] During the drilling and completion stages of low-permeability gas reservoir development, as well as in the later stages of production and development, the migration and precipitation of mineral ions in the formation water may cause reservoir contamination, pore throat blockage, and permeability reduction in the near-wellbore area, which will lead to reduced reservoir connectivity and thus affect production. After the near-wellbore area is polluted, acid removal is an effective means to repair the reservoir seepage condition and increase production capacity. Due to the large number of acid types and systems, the acid removal effect of different acids must be evaluated before on-site acid injection to provide a basis for the selection of on-site acid. At present, low-permeability gas reservoirs generally use real core saturated with formation water and then acid displacement to evaluate the acid removal effect. The core saturated with formation water is placed in the core holder, and the formation water is displaced at a forward flow rate of 0.5 ml / min. After the flow rate at the outlet is stable, the liquid permeability is measured, and then the acid solution 3 to 5 times the volume of the pore is injected in reverse. After holding the pressure for 1 to 2 hours, the formation water is displaced forward again, and the liquid permeability after acidification and plugging removal is measured. There may be gas and liquid phases in the pores of the forward displaced formation water, resulting in inaccurate absolute permeability measured by liquid, and the true acidification and plugging removal effect of the gas reservoir cannot be accurately characterized. At the same time, the evaluation of the acidification and plugging removal effect of the gas reservoir should be based on the gas phase permeability. The conventional evaluation methods in the past cannot characterize the acidification and plugging removal effect of the gas phase permeability of the gas reservoir. Summary of the invention

[0003] In view of the above problems, the present invention provides a method for evaluating the acidification and plugging removal effect of low-permeability gas reservoir cores. The acid displacement effect is evaluated by comparing the changes in gas permeability of the cores under the state of bound water saturation before and after acidification. The evaluation has high accuracy and is more in line with the change process of gas phase permeability of low-permeability gas reservoirs.

[0004] The technical solution of the present invention is: A method for evaluating the effect of acidification and plugging removal of low-permeability gas reservoir cores, comprising the following steps: S1. Prepare core samples of low permeability gas reservoirs; S2. Processing the core samples of low-permeability gas reservoirs to obtain saturated formation water cores; S3. Conduct a forward displacement test on the saturated formation water core to determine the permeability of the core gas test under the steady state before acidification; S4. Conduct a reverse displacement test on the saturated formation water core, and inject acid in the reverse direction to cause an acidification reaction; S5. After the acidification reaction is completed, a forward displacement test is conducted on the saturated formation water core to determine the permeability of the acidified core gas test in a stable state; S6. Evaluate the deblocking effect according to the permeability of the core gas test in a stable state before and after acidification obtained in step S3 and step S5.

[0005] In the step S1, the low permeability gas reservoir core sample is taken from the real formation core, and the sample needs to be dried for 23-25 ​​hours.

[0006] In step S2, the saturated formation water core sample is obtained after the low permeability gas reservoir core sample is vacuum saturated in the sampled formation water solution, and the vacuum saturation time is 2.5-3.5 hours.

[0007] The step S3, forward displacement test is specifically as follows: Place the saturated formation water core in the core holder, adjust the confining pressure of the core holder to 2-3MPa, inject wet gas at an inlet pressure of 0.5-2MPa, and measure the gas permeability every 8-12 minutes after the injection time of 3 hours. When the error of the gas permeability measured for three consecutive times does not exceed 5%, it is considered that the gas permeability has reached stability; The calculation formula of the steady-state permeability of the core gas test before acidification is as follows: Where: K1 is the gas permeability before acidification, unit 10 3 um 2 ; p0 is atmospheric pressure, unit MPa; Q0 is flow rate under atmospheric pressure, unit cm 3 / s; A is the cross-sectional area of ​​the core, in cm 2 ;p 1b 、p 2b is the gas pressure before and after the displacement test pipeline before acidizing, both in units of 0.1 MPa; µ is the viscosity of the gas, in units of mPa·s; L is the length of the rock sample, in units of cm.

[0008] In step S4, the flow rate of reverse injection of acid solution is 0.4-0.6 ml / min, the volume of the injected acid solution is 3-5 times the pore volume of the core, and the acidification reaction time is not less than 2 hours.

[0009] The step S5, forward displacement test is specifically as follows: Keep the confining pressure unchanged, inject real sampled formation water forward at a flow rate of 0.4-0.6ml / min, and displace the formation water volume 3-5 times the pore volume of the core. Then inject wet gas at an inlet pressure of 0.3-0.8MPa. After the injection time of 3h, measure the gas permeability every 8-12 minutes. When the error of the gas permeability measured for 3 consecutive times does not exceed 5%, it is considered that the gas permeability has reached stability. At this time, the permeability of the core gas test in the stable state after acidification is confirmed; The calculation formula of the steady-state permeability of the core gas test after acidification is as follows: Where: K2 is the gas permeability after acidification, unit 103 um 2 ; p0 is atmospheric pressure, unit MPa; Q0 is flow rate under atmospheric pressure, unit cm 3 / s; A is the cross-sectional area of ​​the core, in cm 2 ;p 1a 、p 2a is the air pressure before and after the displacement test pipeline after acidizing, both in units of 0.1 MPa; µ is the viscosity, in units of mPa·s; L is the length of the rock sample, in units of cm.

[0010] In step S6, the evaluation formula for the blocking removal effect is as follows: Where: K1 is the gas permeability before acidification, and K2 is the gas permeability after acidification.

[0011] The beneficial effects of the present invention are: The acid displacement effect is evaluated by comparing the changes in gas permeability of the core before and after acidizing under the state of irreducible water saturation. Compared with the gas-liquid two-phase joint evaluation method in the prior art, this method can more truly reflect the increase in gas permeability during the entire process of acidizing and plugging removal in low-permeability gas reservoirs, and quantify the increase in gas permeability under the effects of pore expansion and permeability increase, irreducible water saturation and water lock damage reduction during the acidizing and plugging removal process. It is easy to detect and has high evaluation reliability. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Figure 1 It is a flow chart of a method for evaluating the effect of acidification and plugging removal of a low-permeability gas reservoir core according to an embodiment of the present invention; Figure 2 This is a schematic diagram of the change of permeability over time in an embodiment of a method for evaluating the effect of acidification and plugging removal in a low-permeability gas reservoir core according to an embodiment of the present invention. DETAILED DESCRIPTION

[0013] The embodiments of the present invention will be further described below in conjunction with the accompanying drawings.

[0014] Example

[0015] like Figure 1 As shown, a method for evaluating the plugging removal effect of low permeability gas reservoir core acidification includes the following steps: S1. Prepare core samples of low permeability gas reservoirs; S2. Processing the core samples of low-permeability gas reservoirs to obtain saturated formation water cores; S3. Conduct a forward displacement test on the saturated formation water core to determine the permeability of the core gas test under the steady state before acidification; S4. Conduct a reverse displacement test on the saturated formation water core, and inject acid in the reverse direction to cause an acidification reaction; S5. After the acidification reaction is completed, a forward displacement test is conducted on the saturated formation water core to determine the permeability of the acidified core gas test in a stable state; S6. Evaluate the deblocking effect according to the permeability of the core gas test in a stable state before and after acidification obtained in step S3 and step S5.

[0016] The specific embodiments are as follows: S1. Prepare core samples of low permeability gas reservoirs; A real formation rock sample with a diameter of 2.5 cm and a length of 5.15 cm at a depth of 2400 m was placed in an oven and dried for 25 h; S2. Processing the core samples of low-permeability gas reservoirs to obtain saturated formation water cores; The real formation sampling core was vacuum saturated with the real sampling formation aqueous solution for 3 hours to obtain the core; S3. Conduct a forward displacement test on the saturated formation water core to determine the permeability of the core gas test under the steady state before acidification; The core saturated with formation water was placed in the core holder, and the confining pressure was adjusted to 2.5 MPa. The core displacement pressure was set to 0.6 MPa. Wet gas was continuously injected in the positive direction. The pressure difference ΔP during the displacement process was recorded by recording the pressure at both ends of the core holder, and the gas flow rate at different times was recorded at the outlet. After 3 hours, the permeability change was recorded every 10 minutes. When the permeability of three consecutive detection points did not change, it was taken as the gas permeability K1 in the bound water state before acidification. The gas permeability formula was used to obtain the permeability K1 values ​​of the core every ten minutes after three hours: 0.10, 0.17, 0.19, 0.21, 0.22, 0.24, 0.24, 0.25 md. It can be seen that a stable state has been reached at 240 minutes, so 0.24 md is selected as the permeability value of the core gas test in the stable state before acidification. The gas permeability formula is as follows: Where: K is the gas permeability, unit 10 3 um 2 ; p0 is atmospheric pressure, unit MPa; Q0 is flow rate under atmospheric pressure, unit cm 3 / s; A is the cross-sectional area of ​​the core, in cm 2 ; p1 and p2 are the air pressures before and after the displacement test pipeline, both in units of 0.1 MPa; µ is the viscosity, in units of mPa·s; L is the length of the rock sample, in units of cm; S4. Conduct a reverse displacement test on the saturated formation water core, and inject acid in the reverse direction to cause an acidification reaction; Acid was injected into the system and reversely displaced 3.5 times the pore volume of the core with acid. The system was maintained at a confining pressure of 2.6 MPa for 2 hours to allow the acidification reaction to fully occur. The sampled formation water was positively injected at a flow rate of 0.5 ml / min. The volume of the sampled formation water displaced was 5 times the pore volume of the core to drive out the residual acid in the core. S5. After the acidification reaction is completed, a forward displacement test is conducted on the saturated formation water core to determine the permeability of the acidified core gas test in a stable state; The core displacement pressure is still set to 0.6 MPa, and wet gas is continuously injected in the positive direction. The pressure difference ΔP during the displacement process is recorded at both ends of the outlet, and the gas flow rate at different times is recorded at the outlet. After 3 hours, the permeability change is recorded every 10 minutes. When the permeability of three consecutive detection points does not change, it can be taken as the gas permeability K2 under the bound water saturation after acidification. The gas permeability formula is used to obtain the permeability K2 values ​​of the core after acidification and plugging removal every 10 minutes after 3 hours: 0.43, 0.48, 0.52, 0.55, 0.59, 0.61, 0.61, 0.62 md, which can be seen that the stable state has been reached at 240 minutes, so 0.61 md is selected as the permeability value of the core gas test in the stable state after acidification and plugging removal. S6, evaluating the plugging removal effect according to the permeability of the core gas test in a stable state before and after acidification obtained in step S3 and step S5; The plugging removal rate is 154%, which shows that the acidification plugging removal will cause the pore expansion and permeability increase; and the gas phase permeability will increase due to the decrease of bound water saturation after the pore throat becomes larger. The results are shown in the following table and Figure 2 .

[0017]

[0018] The above-mentioned embodiments only express the specific implementation of the present invention, and the description thereof is relatively specific and detailed, but it cannot be understood as limiting the scope of the present invention. It should be pointed out that, for ordinary technicians in this field, several variations and improvements can be made without departing from the concept of the present invention, which all belong to the protection scope of the present invention.

Claims

1. A method for evaluating the effect of acidification and plugging removal in low permeability gas reservoir cores, characterized in that: The steps include: S1. Prepare core samples of low permeability gas reservoirs; S2. Processing the core samples of low-permeability gas reservoirs to obtain saturated formation water cores; S3. Conduct a forward displacement test on the saturated formation water core to determine the permeability of the core gas test under the steady state before acidification; S4. Conduct a reverse displacement test on the saturated formation water core, and inject acid in the reverse direction to produce an acidification reaction; S5. After the acidification reaction is completed, a forward displacement test is conducted on the saturated formation water core to determine the permeability of the acidified core gas test in a stable state; S6. Evaluate the deblocking effect according to the permeability of the core gas test in a stable state before and after acidification obtained in step S3 and step S5.

2. The method for evaluating the plugging removal effect of low permeability gas reservoir core acidification according to claim 1, characterized in that: In the step S1, the low permeability gas reservoir core sample is taken from the real formation core, and the sample needs to be dried for 23-25 ​​hours.

3. The method for evaluating the plugging removal effect of low permeability gas reservoir core acidification according to claim 1, characterized in that: In step S2, the saturated formation water core sample is obtained after the low permeability gas reservoir core sample is vacuum saturated in the sampled formation water solution, and the vacuum saturation time is 2.5-3.5 hours.

4. The method for evaluating the plugging removal effect of low permeability gas reservoir core acidification according to claim 1, characterized in that: The step S3, forward displacement test is specifically as follows: Place the saturated formation water core in the core holder, adjust the confining pressure of the core holder to 2-3MPa, inject wet gas at an inlet pressure of 0.5-2MPa, and measure the gas permeability every 8-12 minutes after the injection time of 3 hours. When the error of the gas permeability measured for three consecutive times does not exceed 5%, it is considered that the gas permeability has reached stability; The calculation formula of the steady-state permeability of the core gas test before acidification is as follows: Where: K1 is the gas permeability before acidification, unit 10 3 um 2 ; p0 is atmospheric pressure, unit MPa; Q0 is flow rate under atmospheric pressure, unit cm 3 / s; A is the cross-sectional area of ​​the core, in cm 2 ;p 1b 、p 2b is the gas pressure before and after the displacement test pipeline before acidizing, both in units of 0.1 MPa; µ is the viscosity of the gas, in units of mPa·s; L is the length of the rock sample, in units of cm.

5. The method for evaluating the plugging removal effect of low permeability gas reservoir core acidification according to claim 1, characterized in that: In step S4, the flow rate of reverse injection of acid solution is 0.4-0.6 ml / min, the volume of the injected acid solution is 3-5 times of the pore volume of the core, and the acidification reaction time is not less than 2 hours.

6. The method for evaluating the plugging removal effect of low permeability gas reservoir core acidification according to claim 1, characterized in that: The step S5, forward displacement test is specifically as follows: Keep the confining pressure unchanged, inject the sampled formation water forward at a flow rate of 0.4-0.6ml / min, and displace the sampled formation water volume 3-5 times the pore volume of the core to drive out the residual acid in the core. Then inject wet gas at an inlet pressure of 0.3-0.8MPa. After the injection time is 3h, measure the gas permeability every 8-12 minutes. When the error of the gas permeability measured for three consecutive times does not exceed 5%, it is considered that the gas permeability has reached stability. At this time, the permeability of the core gas test in the stable state after acidification is confirmed; The calculation formula of the steady-state permeability of the core gas test after acidification is as follows: Where: K2 is the gas permeability after acidification, unit 10 3 um 2 ; p0 is atmospheric pressure, unit MPa; Q0 is flow rate under atmospheric pressure, unit cm 3 / s; A is the cross-sectional area of ​​the core, in cm 2 ;p 1a 、p 2a is the air pressure before and after the displacement test pipeline after acidizing, both in units of 0.1 MPa; µ is the viscosity, in units of mPa·s; L is the length of the rock sample, in units of cm.

7. A method for evaluating the effect of acidification and plugging removal in low permeability gas reservoir cores according to any one of claims 1 to 6, characterized in that: In step S6, the evaluation formula for the blocking removal effect is as follows: Where: K1 is the gas permeability before acidification, and K2 is the gas permeability after acidification.

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