A method for evaluating water lock damage of tight sandstone

Abstract

This invention belongs to the field of petroleum development technology, specifically relating to a method for evaluating water-lock damage in tight sandstone. The method includes the following steps: after cleaning and drying, the core sample absorbs formation water under vacuum using capillary self-absorption; a first-stage water saturation gradient is established by increasing the vacuum level and / or absorption time; after completion, the core sample absorbs formation water under pressure; a second-stage water saturation gradient is established by increasing the pressure and / or pressure application time; and the gas phase permeability K at different water saturations is calculated. i The degree of waterlock damage is determined based on the rate of change of gas phase permeability. Through the experimental process of gradually increasing water saturation in the core, a full range of water saturation from 0% to 100% can be quickly and accurately established, which conforms to the production pattern of normal gas reservoirs. Moreover, no drying step is required, avoiding salt precipitation, and the degree of waterlock damage at different water saturation levels can be accurately evaluated.

Description

Technical Field

[0001] This invention relates to the field of petroleum development technology, and specifically to a method for evaluating water-lock damage in tight sandstone. Background Technology

[0002] Tight gas reservoirs are characterized by low reservoir permeability and rapid depletion of elastic energy, quickly transitioning to a low-efficiency, long-cycle solution gas drive development stage. Gas well production declines rapidly, development levels are low, and extraction is difficult, with water-locking damage being a common cause. The Tongnanba block of the Zhongyuan Oilfield is a "tectonic-lithological controlled fracture-porosity type" tight sandstone gas reservoir, belonging to unconventional natural gas, with proven reserves of 51.523 billion cubic meters. The average reservoir porosity is only 4.1%, and the permeability is less than 0.1 mD. It is characterized by rapid changes in reservoir sand bodies, large differences in fracture distribution, and high formation pressure coefficients. The formation water is calcium chloride type with a mineralization of approximately 110,000. These unique geological conditions bring a series of challenges in geology, drilling, and fracturing, making it difficult to achieve large-scale production reduction and efficient development in the Tongnanba block. Currently, water-locking is a common problem in the development of the Tongnanba Xujiahe gas reservoir and urgently needs to be addressed.

[0003] Whether in drilling, workover, or production operations, water-based working fluids or unidentified foreign liquids can infiltrate reservoir pores, causing water lock phenomena. This leads to a gradual increase in reservoir water saturation, a significant decrease in gas phase permeability, and impedes gas flow, potentially even causing well shutdowns. Therefore, accurately assessing the degree of water lock damage in a gas reservoir is a crucial prerequisite for conducting related engineering operations involving aqueous solutions. This improves the success rate of engineering operations during reservoir development, reduces the financial investment required for reservoir recovery and unblocking measures after water lock damage, and ultimately enhances the natural gas recovery rate and economic benefits of reservoir development.

[0004] A prerequisite for evaluating waterlock damage is the accurate and rapid determination of different water saturations, at which the waterlock damage can be evaluated. Currently, methods for establishing different water saturations include natural air drying, displacement, centrifugation, and baking. For example, Chinese patent CN107991335B, authorized on September 4, 2020, discloses a test method for evaluating waterlock damage in tight sandstone. This method involves saturating passivated and dried core samples, then centrifuging them. By using different centrifugation speeds, mobile and bound water saturations are obtained, establishing a gradually decreasing water saturation level. The rate of change in gas permeability relative to the initial formation water saturation level at different water saturations is calculated to evaluate the degree of waterlock damage. However, this patent's centrifugation method establishes different water saturations sequentially from high to low. In contrast, during normal gas reservoir production, water saturation gradually increases with water intrusion, which does not align with normal gas reservoir production processes.

[0005] Chinese patent CN112710595B, authorized on October 28, 2022, discloses a method for evaluating water-lock damage in gas reservoirs. After saturating the core sample with formation water, capillary pressure and centrifuge speed for different water saturation levels are determined through theoretical calculations. Under these parameters, the core sample is centrifuged to the desired saturation level. Through examples, a series of water saturation levels from high to low (90%, 80%, 70%, 60%, 50%, 40%, 30%, 20%) are established, and the damage coefficient Ds of gas phase permeability is calculated to determine the degree of damage to gas phase permeability due to water-lock damage. However, this patent also uses a centrifugation method to sequentially establish different water saturation levels from high to low, which does not conform to the normal production process of gas reservoirs; furthermore, the theoretical formula requires many parameters, leading to deviations in the calculated values.

[0006] Chinese patent application CN102288732A, published on December 21, 2011, discloses a method for rapidly evaluating water lock in ultra-low permeability gas reservoirs. The method involves gas displacement to separate 100% water-containing samples into gas and liquid components. The water saturation of the sample is calculated based on the amount of water displaced, and the gas permeability (including movable water) at that water saturation level is also calculated. After displacement, the sample containing only immobile water is dried, and the gas permeability (including immobile water) during drying is calculated. The degree of water lock damage is determined based on the rate of change in gas permeability. This patent employs both displacement and drying methods, establishing different water saturations sequentially from high to low, which does not conform to the normal production process of gas reservoirs. Furthermore, to simulate water-locking conditions in actual reservoirs, the saturated water used for establishing water saturation in the core sample is typically formation water from the same layer of the reservoir being evaluated, possessing a certain degree of mineralization. The drying method, however, easily leads to salt precipitation from the formation water. Since tight sandstone itself has relatively low porosity and permeability, salt precipitation will block reservoir channels, affecting the accurate measurement of permeability. Other methods, such as air-drying, are time-consuming, and displacement methods are difficult to apply and inaccurately measure water content. Summary of the Invention

[0007] This invention provides a method for evaluating water-lock damage in tight sandstone, which solves the problems of existing evaluation methods that establish water saturation from high to low, which does not conform to the actual normal production process of gas reservoirs, resulting in large deviations between permeability test results and reality, and the drying method is prone to salt precipitation, leading to inaccurate permeability tests.

[0008] To solve the above-mentioned technical problems, the technical solution of the method for evaluating water-locking damage in dense sandstone according to the present invention is as follows:

[0009] A method for assessing water-locking damage in dense sandstone includes the following steps:

[0010] (1) Take actual reservoir cores from the gas reservoir, clean and dry them, and test the core length L, initial gas phase permeability K0, core cross-sectional area A, and weight m0.

[0011] (2) The core from step (1) is used to absorb formation water under vacuum by capillary self-absorption. By increasing the vacuum degree and / or absorption time, the first stage water saturation gradient is established based on the core weight change rate. After completion, the core is used to absorb formation water under pressure. By increasing the pressure and / or pressure application time, the second stage water saturation gradient is established based on the core weight change rate.

[0012] (3) Calculate the gas phase permeability K of the core at different water saturation levels based on Darcy's law for gases. i The rate of change of gas phase permeability under different water saturation levels is calculated based on K0, and the degree of water lock damage is judged based on the rate of change of gas phase permeability.

[0013] This invention improves upon existing technologies and provides a method for evaluating water-lock damage in tight sandstone. By evacuating a dried core sample and gradually increasing the vacuum level and the time the core absorbs formation water, the water saturation of the core gradually increases. On this basis, the core is then pressurized to absorb formation water, further increasing the water saturation. This method can quickly and accurately establish a full range of water saturation from 0% to 100%, conforming to the production patterns of normal gas reservoirs. It can accurately grasp the water-lock damage patterns of gas reservoirs and provide a basis for decision-making in gas reservoir-related engineering operations.

[0014] The evaluation method provided by this invention avoids the use of a centrifuge, simplifies the experimental setup, and features a simple process, reliable principle, easy operation, high precision, and significant effect. Furthermore, it eliminates the need for a drying step, preventing salt precipitation and enabling accurate evaluation of the degree of waterlock damage at different water saturation levels.

[0015] To further simplify the method for determining water saturation, preferably, the method for determining water saturation in step (2) is as follows:

[0016] S i =(m i -m0) / m0;

[0017] Among them, S i For water saturation, 0 ≤ S i ≤1; m i This represents the weight of the core after it has absorbed formation water.

[0018] In order to further improve the efficiency of establishing water saturation and quickly establish water saturation with a certain gradient, preferably, in step (2), after the water saturation growth rate is less than 1% during the establishment of the first stage water saturation gradient, the vacuum degree and / or absorption time are no longer increased; and after the water saturation growth rate is less than 1% during the establishment of the second stage water saturation gradient, the pressure and / or pressurization time are no longer increased.

[0019] In order to further establish a comprehensive water saturation and refine the water saturation gradient, preferably, the range of water saturation established in step (2) is 10% to 100%; the growth rate of water saturation is 3% to 30%.

[0020] To further simplify the operation of core saturated water, preferably, in step (2) of establishing the first stage of water saturation gradient process, the core is first placed in a sealed container and vacuumed, and then the core is submerged by injecting formation water into the sealed container, and the formation water is absorbed by the capillary self-absorption of the core.

[0021] To further simplify the core pressurization and saturation water operation, preferably, in step (2) of establishing the second stage water saturation gradient process, the core is immersed in the formation water and pressure is applied to the formation water so that the core absorbs the formation water under pressure.

[0022] To further simplify the core saturation water operation, preferably, step (2) is carried out in a core saturation device, which includes an intermediate container, which is connected to a liquid holding device, a vacuum pump and a piston container; the piston container is connected to a pressure pump.

[0023] To further improve the accuracy of gas phase permeability calculation, preferably, the gas phase permeability K in step (3) is... i The calculation method is as follows:

[0024]

[0025] Where, μ g L is the gas viscosity; Q is the core length; P1 is the core inlet pressure; P2 is the core outlet pressure; A is the core cross-sectional area; P a Atmospheric pressure.

[0026] Preferably, the method for calculating the rate of change of gas phase permeability in step (3) is as follows:

[0027]

[0028] Among them, I i K0 is the rate of change of gas phase permeability of the core at a certain water saturation level; K0 is the initial gas phase permeability of the core .... i This represents the gas phase permeability of the core at a certain water saturation level. Attached Figure Description

[0029] Figure 1 This is a schematic diagram of the core saturation device in Embodiment 1 of the present invention;

[0030] Figure 2 The curves showing the gas phase permeability variation of six types of core samples in Example 1 of this invention at different water saturation levels are shown.

[0031] Figure 3 The figures show the variation curves of water-lock damage rates of the six types of core samples in Example 1 of this invention under different water saturation levels. Detailed Implementation

[0032] The technical concept of the method for evaluating water-lock damage in dense sandstone according to the present invention is as follows:

[0033] Current techniques typically involve gradually reducing the water saturation of a core sample from 100% saturation through centrifugation to establish a water saturation level from high to low. However, in actual waterlocking, the degree of water intrusion increases slowly. Current methods for establishing water saturation do not align with normal gas reservoir production processes, leading to inaccuracies in assessing the extent of waterlocking damage. Furthermore, the drying method is prone to salt precipitation, resulting in poor accuracy in gas phase permeability testing, which is detrimental to accurately assessing waterlocking damage.

[0034] This invention utilizes vacuum extraction and capillary self-absorption to saturate formation water. By controlling the vacuum level and self-absorption time, the water saturation is gradually increased. Based on this, pressure is applied to further increase the water saturation. By controlling the pressure and time, the water saturation is gradually increased, establishing a full range of water saturation from 0% to 100%, from extremely low to extremely high levels, which conforms to the production patterns of normal gas reservoirs. Furthermore, it avoids centrifugation and drying operations, preventing salt precipitation and improving the accuracy of gas phase permeability testing.

[0035] The method for evaluating the water-locking damage of dense sandstone in this invention is as follows:

[0036] (1) Drill a cylindrical core of the actual gas reservoir, clean and dry it, and test the core length L, initial gas phase permeability K0, core cross-sectional area A, and weight m0.

[0037] (2) Prepare formation water and core saturation device. The core saturation device includes an intermediate container, which is connected to a liquid holding device, a vacuum pump and a piston container respectively; the piston container is connected to a pressure pump.

[0038] Multiple cores of different lithologies were placed in an intermediate container. The container was evacuated to a vacuum level at room temperature using a vacuum pump for the first 24–48 hours to ensure all air was removed from the cores. After evacuation, formation water from a liquid container was injected into the intermediate container, and capillary self-absorption was used to saturate the cores for 1–2 hours. The cores were then removed, the surface of the formation water was wiped dry, and the cores were weighed (m1). The minimum water saturation S1 was calculated using the formula S1 = (m1 - m0) / m0. From the second evacuation onwards, the cores were immersed in formation water to saturate the formation water for 2–48 hours, gradually increasing the saturation time, evacuation time, and capillary self-absorption time to obtain different water saturations S2, S3, etc. When the rate of change in water saturation was less than 1%, and the saturation effect of evacuation was no longer significant, the evacuation time and saturation time were no longer increased. At this point, the maximum water saturation was S1. j Proceed to the next step.

[0039] With a water saturation of S jThe core sample was immersed in an intermediate container filled with formation water. A pressure pump in the core saturation device pressurized the liquid in a piston container (the piston container containing formation water near the intermediate container). Pushing the piston pressurized the formation water, which was then injected into the intermediate container, further pressurizing the formation water there. This pressure caused the core sample to absorb formation water. The pressurization pressure ranged from 0 to 10 MPa, and the pressurization time ranged from 0 to 5 hours. By gradually increasing the pressurization pressure and time, different water saturation levels (S) were obtained. j S j+1 ...S n The pressure and pressurization time are no longer increased until the water saturation rate increases by less than 1%, at which point the water saturation is established.

[0040] (3) Calculate the gas phase permeability K at different water saturation levels based on Darcy's law for gases, as established in step (2). i The calculation method is as follows:

[0041]

[0042] Where, μ g L is the gas viscosity; Q is the core length; P1 is the core inlet pressure; P2 is the core outlet pressure; A is the core cross-sectional area; P a Atmospheric pressure; i is an integer from 1 to n. Gas phase permeability K i The parameters in the calculation method can be measured using existing water-lock devices, which are referenced in patent CN112710595B. Different water saturation levels S1, S2, S3...S are obtained. n The gas phase permeability K1, K2, K3...K n .

[0043] The rate of change of gas phase permeability (i.e., waterlock damage rate) under different water saturation levels is calculated based on K0, and the degree of waterlock damage is determined based on the rate of change of gas phase permeability. The calculation method for the rate of change of gas phase permeability is as follows:

[0044]

[0045] Among them, I i K0 is the rate of change of gas phase permeability of the core at a certain water saturation level; K0 is the initial gas phase permeability of the core .... i This represents the gas phase permeability of the core at a certain water saturation level.

[0046] The degree of damage caused by waterlocks is determined with reference to existing technologies. The corresponding relationship between different degrees of damage and the damage rate of waterlocks is shown in Table 1.

[0047] Table 1. Correspondence between different degrees of damage and waterlock damage rate

[0048]

[0049]

[0050] The present invention will be further described in detail below with reference to specific embodiments.

[0051] I. Specific Embodiments of the Method for Evaluating Water Lock Damage in Tight Sandstone Provided by the Present Invention

[0052] Example 1

[0053] The method for evaluating the water-lock damage of dense sandstone in this embodiment is as follows:

[0054] (1) Cylinder cores of actual gas reservoirs at different depths and from different wells within the study area were drilled. After cleaning and drying, the diameter D (to calculate the cross-sectional area A), length L, initial gas phase permeability K0, and weight m0 of the cores were tested. The performance test results of the cores are shown in Table 2.

[0055] Table 2. Performance test results of the core samples

[0056]

[0057] (2) Prepare formation water (mineralization 110,000) and core saturation apparatus, such as... Figure 1 As shown, the core saturation device includes an intermediate container 3, with a metal screen tube 4 suspended on the upper side inside the intermediate container 3. The metal screen tube 4 is used to install six cores 5. The intermediate container 3 is connected to a glass container 2, a vacuum pump 1, and a piston container 5. The piston container 5 is connected to a pressure pump 7. The upper side of the piston of the piston container 5 is filled with formation water, and the lower side is filled with pure water or formation water. It also includes valves 8, 9, 10, 11, and 12.

[0058] Six cores (5) of different lithologies were placed into an intermediate container (3) through a metal sieve tube (4). A vacuum pump (1) was used to evacuate the intermediate container (3) to a vacuum level at room temperature for 24 hours initially to ensure all air was removed from the cores. After evacuation, formation water from a glass container (2) was injected into the intermediate container (3). Capillary self-absorption was used to saturate the cores for 1 hour. The cores were then removed, the formation water on the surface was wiped dry, and the cores were weighed (m1). The minimum water saturation (S1) was calculated using the formula S1 = (m1 - m0) / m0. From the second evacuation onwards, cores (5) were immersed in formation water to saturate the formation water for 2 to 48 hours. The saturation time, evacuation time, and capillary self-absorption time were gradually increased to obtain different water saturations (S2, S3, etc.). When the rate of change in water saturation was less than 1% and the saturation effect of evacuation was no longer significant, the evacuation time and saturation time were no longer increased. At this point, the maximum water saturation was S1. j Proceed to the next step.

[0059] With a water saturation of S j The core sample was immersed in an intermediate container 3 filled with formation water. A pressure pump 7 in the core saturation device pressurized the liquid in the lower part of a piston container 6. The piston container 6, containing formation water on the side closest to the intermediate container 3, pushed the piston to pressurize the formation water, which was then injected into the intermediate container 3. This further pressurized the formation water in the intermediate container 3, causing the core sample to absorb formation water under pressure. The pressurization pressure ranged from 0 to 10 MPa, and the pressurization time ranged from 0 to 5 hours. By gradually increasing the pressurization pressure and time, different water saturation levels S were obtained. j S j+1 ...S n The pressure and pressurization time are no longer increased until the water saturation rate increases by less than 1%, at which point the water saturation is established.

[0060] (3) Calculate the gas phase permeability K at different water saturation levels based on Darcy's law for gases, as established in step (2). i The calculation method is as follows:

[0061]

[0062] Where, μ g L is the gas viscosity; Q is the core length; P1 is the core inlet pressure; P2 is the core outlet pressure; A is the core cross-sectional area; P a Let $\mathbf{n}$ be the atmospheric pressure; $i$ be an integer from 1 to $n$. This yields different water saturation values ​​S1, S2, S3...S$. n The gas phase permeability K1, K2, K3...K n It is important to note that after measuring the gas permeability at each water saturation level, the rate of change of weight should be measured again to obtain the new water saturation levels S1', S2', S3'...S. n 'Calculate the average water saturation S before and after the gas phase permeability measurement.' i "" represents the current water saturation level.

[0063] The results of gas phase permeability tests at different water saturation levels are shown in Table 3 and Figure 2 As shown, Figure 2 The curves show the gas phase permeability variation of six types of core samples at different water saturation levels.

[0064] Table 3. Vapor phase permeability at different water saturations

[0065]

[0066]

[0067] The rate of change of gas phase permeability (i.e., waterlock damage rate) under different water saturation levels is calculated based on K0, and the degree of waterlock damage is determined based on the rate of change of gas phase permeability. The calculation method for the rate of change of gas phase permeability is as follows:

[0068]

[0069] Among them, I i K0 is the rate of change of gas phase permeability of the core at a certain water saturation level; K0 is the initial gas phase permeability of the core .... i This represents the gas phase permeability of the core at a certain water saturation level.

[0070] The test results of waterlock damage rates under different water saturation levels are shown in Table 4 and... Figure 3 As shown, Figure 3 The curves show the variation of waterlock damage rate of six types of core samples under different water saturation levels.

[0071] Table 4. Waterlock damage rate at different water saturation levels

[0072]

[0073] from Figure 3 It can be seen that the degree of damage to formation permeability caused by core water saturation gradually increases with increasing water saturation; the higher the original permeability, the less damage occurs at the same water saturation. For gray fine sandstone, samples Y32 and Y27 have the lowest permeability, with a damage level of 60% at 50% water saturation, indicating moderate to severe damage. Samples Y23 and Y24, at 50% water saturation, show damage levels of 21% and 39% respectively, indicating weak to moderately weak damage. Water-locking damage is more pronounced in low-permeability reservoirs.

[0074] Finally, it should be noted that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for evaluating water-locking damage in dense sandstone, characterized in that, Includes the following steps: (1) Take actual reservoir cores from the gas reservoir, clean and dry them, and test the core length L, initial gas phase permeability K0, core cross-sectional area A, and weight m0. (2) The core from step (1) absorbs formation water under vacuum by capillary self-absorption. By increasing the vacuum degree and / or absorption time, a first-stage water saturation gradient is established based on the core weight change rate. After completion, the core absorbs formation water under pressure. By increasing the pressure and / or pressure application time, a second-stage water saturation gradient is established based on the core weight change rate. (3) Calculate the gas phase permeability K of the core at different water saturation levels according to Darcy's law for gases. i The rate of change of gas phase permeability under different water saturation levels is calculated based on K0, and the degree of water lock damage is judged based on the rate of change of gas phase permeability.

2. The method for evaluating water-lock damage in dense sandstone as described in claim 1, characterized in that, The method for determining the water saturation in step (2) is as follows: S i =(m i -m0) / m0; Among them, S i For water saturation, 0 ≤ S i ≤1; m i This represents the weight of the core after it has absorbed formation water.

3. The method for evaluating water-lock damage in dense sandstone as described in claim 1, characterized in that, In step (2), once the water saturation rate increase is less than 1% during the establishment of the first stage water saturation gradient, the vacuum degree and / or absorption time shall not be increased; once the water saturation rate increase is less than 1% during the establishment of the second stage water saturation gradient, the pressure and / or pressurization time shall not be increased.

4. The method for evaluating water-lock damage in dense sandstone as described in claim 1, characterized in that, The range of water saturation established in step (2) is 10% to 100%; the growth rate of water saturation is 3% to 30%.

5. The method for evaluating water-lock damage in dense sandstone as described in claim 1, characterized in that, Step (2) In the process of establishing the first stage water saturation gradient, the core is first placed in a sealed container and vacuumed. Then, formation water is injected into the sealed container to submerge the core, and the formation water is absorbed by the capillary self-absorption of the core.

6. The method for evaluating water-lock damage in dense sandstone as described in claim 1, characterized in that, Step (2) In the process of establishing the second stage water saturation gradient, the absorption of formation water involves immersing the core in formation water and applying pressure to the formation water, so that the core absorbs formation water under pressure.

7. The method for evaluating water-lock damage in dense sandstone as described in claim 1, characterized in that, Step (2) is carried out in a core saturation device, which includes an intermediate container, which is connected to a liquid holding device, a vacuum pump and a piston container; the piston container is connected to a pressure pump.

8. The method for evaluating water-lock damage in dense sandstone as described in claim 1, characterized in that, The gas phase permeability K in step (3) i The calculation method is as follows: ； Where, μ g L is the gas viscosity; Q is the core length; P1 is the core inlet pressure; P2 is the core outlet pressure; A is the core cross-sectional area; P a Atmospheric pressure.

9. The method for evaluating water-lock damage in dense sandstone as described in claim 1, characterized in that, The method for calculating the rate of change of gas phase permeability in step (3) is as follows: ； Among them, I i K0 is the rate of change of gas phase permeability of the core at a certain water saturation level; K0 is the initial gas phase permeability of the core .... i This represents the gas phase permeability of the core at a certain water saturation level.

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