Shale oil reservoir sensitivity evaluation method

Abstract

The invention belongs to the field of oil and gas exploitation, and particularly relates to a shale oil reservoir sensitivity evaluation method. The shale oil reservoir sensitivity evaluation method comprises the following steps: (1) pretreating a rock core to obtain a dry rock core; (2) treating the dry rock core with different fluids, carrying out a rock core sensitivity experiment, and measuring the gas logging permeability of the wet rock core; and (3) comparing the gas logging permeability change H of the wet core, and evaluating the shale oil reservoir sensitivity. According to the shale oil reservoir sensitivity evaluation method, the limitation of an existing evaluation system is overcome, the flow velocity sensitivity, the water sensitivity, the salinity sensitivity, the acid sensitivity and the alkali sensitivity of an ultra-low-permeability shale oil reservoir can be accurately and safely evaluated from the permeability change angle, abnormal high pressure occurring when an industrial standard is adopted can be effectively avoided, and the shale oil reservoir sensitivity evaluation method is suitable for industrial production. The safety and reliability are higher.

Description

Technical Field

[0001] The invention belongs to the field of oil and gas exploitation, and in particular relates to a shale oil reservoir reservoir sensitivity evaluation method. Background Art

[0002] During the entire process of drilling, completion, downhole operations and oil and gas field exploitation, the phenomenon of decreased reservoir permeability is collectively referred to as reservoir damage. The essence of reservoir damage is the decrease in effective permeability, and reservoir damage is the result of the combined action of internal and external factors of the reservoir. The internal cause of reservoir damage refers to the inherent lithology, physical properties, pore structure, sensitivity and fluid properties of the reservoir; the external cause of reservoir damage refers to any external operating conditions that can cause changes in the original state of the reservoir microstructure and reduce the permeability of the reservoir during the construction process.

[0003] Indoor evaluation of reservoir damage (oil and gas layer damage) is an important means of understanding and evaluating reservoir damage by using various instruments and equipment to measure the change in permeability of oil and gas layer rocks before and after the action of external working fluids, or to measure the change in permeability before and after the physical and chemical environment of the oil and gas layer changes. Reservoir sensitivity evaluation is the main content of indoor evaluation of reservoir damage, usually including five sensitivity tests such as velocity sensitivity, water sensitivity, salt sensitivity, alkali sensitivity, and acid sensitivity. The purpose of reservoir sensitivity experiment is to clarify the sensitivity of reservoir permeability to changes in the external environment and the conditions for occurrence, and to provide a scientific basis for the design of various working fluids, analysis of reservoir damage mechanisms, and formulation of systematic reservoir protection technology plans.

[0004] At present, the existing quantitative evaluation methods for reservoir sensitivity to external fluids mainly implement the standard SY / T5358-2010 "Reservoir Sensitivity Flow Experiment Evaluation Method". Huang Ling et al. (2012) conducted sensitivity experimental evaluation on the Longmaxi Formation and Qiongzhusi Formation shales in the Sichuan Basin according to industry standards. The results show that shales have moderate to weak water sensitivity and moderate to strong alkali sensitivity. He Jingang et al. (2014) conducted sensitivity experiments on Longmaxi Formation shales according to existing industry standards. The results show that Longmaxi Formation shales have moderate to strong to strong alkali sensitivity and salt sensitivity. Salt sensitivity damage has little effect on rock micropores, while alkali sensitivity damage changes the microscopic pore structure of shale.

[0005] However, this standard applies to air permeability greater than 1×10 -3 μm 2The clastic reservoir rock samples are not suitable for ultra-low permeability shale reservoirs. The main reasons include: (1) It is impossible to establish an effective driving system for shale samples. If a large pressure difference is used to establish a driving system during the experiment, a large confining pressure needs to be applied to the rock sample. Due to the strong brittleness of shale samples, the rock sample is easily broken and cracked under the action of large confining pressure, which affects the accuracy of experimental measurements. (2) The permeability of shale matrix and bedding fractures is very low, and it takes a long time to achieve stable flow. During the measurement, experimental data such as pressure difference and flow rate need to be manually recorded, and the error is relatively large. (3) The existing standard SY / T5358-2010 "Reservoir Sensitivity Flow Experiment Evaluation Method" only evaluates the reservoir sensitivity to external fluids by measuring the relative change in the permeability of the rock sample before and after the action of the external fluid, and the evaluation index is single.

[0006] Shale oil reservoirs have high organic matter content, complex mineral composition, small storage space, multiple micron-nano pore throats, low porosity and permeability, and reservoir development requires fracturing. During the fracturing process, it is easy to cause reservoir damage such as particle migration, salt precipitation crystallization, and sensitive mineral dissolution. Reservoir sensitivity data is an important basic data for protecting shale reservoirs and improving the development effect and benefits of shale oil reservoirs. Its accurate evaluation is of great significance for decision-making and technical research on shale oil reservoir protection.

[0007] Yao Shanglin et al. studied and revised the standard SY / T 5358-2010, which had problems such as unclear experimental steps and sensitivity determination range values ​​that were not in line with reality. However, this revision only focused on the stress sensitivity test, evaluation method and determination criteria of tight reservoir rocks, and could not be directly applied to the sensitivity performance evaluation of shale oil reservoirs. Summary of the invention

[0008] The purpose of the present invention is to overcome the deficiencies of the prior art and provide a shale oil reservoir reservoir sensitivity evaluation method, which is applicable to ultra-low permeability shale oil reservoirs, especially reservoirs with high clay mineral content, and can avoid abnormal high pressure when using industry standards, and is safer and more reliable. By comparing the change in gas permeability H of the reservoir before and after the core is treated with different liquids, the sensitivity of the shale oil reservoir can be safely and accurately evaluated, and the formulation of shale oil reservoir protection strategies can be guided to improve reservoir stability, and further support can be provided for the optimization of fracturing fluid performance, evaluation of fracturing effects, etc.

[0009] In order to achieve the above-mentioned purpose of the present invention, the specific technical solution adopted by the present invention is:

[0010] A shale oil reservoir reservoir sensitivity evaluation method, the shale oil reservoir reservoir sensitivity evaluation method comprising the following steps:

[0011] (1) core pretreatment to obtain a dry core;

[0012] (2) Dry cores were treated with different fluids, and core sensitivity experiments were performed to determine the gas permeability of wet cores;

[0013] (3) Compare the wet core gas permeability change H to evaluate the sensitivity of the shale oil reservoir. The gas permeability change H is obtained by formula (1):

[0014]

[0015] Among them, K1 and K2 are the gas permeabilities of the cores after treatment with different fluids, and H is the change in gas permeability.

[0016] Preferably, the diameter of the core in step (1) is 2.5 cm-4.0 cm, and the height of the core is 4 cm-10 cm.

[0017] Preferably, the core pretreatment in step (1) includes: grinding both end faces of the core flat and ensuring that the end faces are perpendicular to the side faces of the column, washing with a mixed solvent of benzene and ethanol, and drying.

[0018] Further preferably, the volume ratio of benzene to ethanol is 1:3-7; the drying temperature is 50° C.-100° C., and the drying time is 48 h-96 h.

[0019] Preferably, the core sensitivity in step (2) includes: flow rate sensitivity, water sensitivity, salinity sensitivity, acid sensitivity and alkali sensitivity; the method for determining the flow rate sensitivity includes the following steps:

[0020] S1, determination of the initial gas permeability K1 of the constant weight core;

[0021] S2, build the device, load the dry core, and apply initial ring pressure;

[0022] S3, change the gas flow rate, record the outlet flow rate, and calculate the core gas permeability K2;

[0023] S4. Calculate the gas permeability change H according to formula (1), analyze the velocity sensitivity of the reservoir and determine the critical velocity.

[0024] Preferably, S1, according to SY / T 5336, the initial gas permeability K1 of the constant weight core is measured with dry nitrogen;

[0025] S2. Build a device according to the core gas permeability measurement experimental device diagram, load the above-mentioned dry core, and apply initial ring pressure;

[0026] S3, by changing the inlet pressure in sequence (keeping the annular pressure 1.5MPa-2.0MPa higher than the inlet pressure), change the gas flow rate, after the pressure at each experimental point is stable for a period of time, record the outlet flow rate, and calculate the core gas permeability K2;

[0027] S4, stop the experiment, calculate the gas permeability change H according to formula (1), analyze the flow rate sensitivity of the reservoir and determine the critical flow rate;

[0028] Further preferably, the initial ring pressure in step S2 is 2MPa-10MPa, and the gas flow rate in step S3 is 0.1mL / s-2.0mL / s.

[0029] Further preferably, the method for determining water sensitivity, salinity sensitivity, acid sensitivity and alkali sensitivity comprises the following steps:

[0030] 1) Determine the initial gas permeability K1 of the constant weight core;

[0031] 2) The cores are vacuumed for 2h-8h and soaked in different liquids for 2h-24h;

[0032] 3) Build the device, load the core after the sensitivity experiment, and apply initial ring pressure;

[0033] 4) Introduce dry nitrogen to displace the core, and after the core breaks through, record the data and calculate the core gas permeability K2;

[0034] 5) Calculate the gas permeability change H according to formula (1) and analyze the sensitivity of the reservoir.

[0035] Preferably, 1) according to SY / T 5336, the initial gas permeability K1 of the constant weight core is measured using dry nitrogen;

[0036] 2) The cores are vacuumed for 2h-8h and soaked in different liquids for 2h-24h;

[0037] 3) Build the device according to the core gas permeability measurement experimental device diagram, load the core after the sensitivity experiment above, and apply initial ring pressure;

[0038] 4) Introduce dry nitrogen to displace the core (keep the annular pressure 1.5MPa-2.0MPa higher than the inlet pressure). After the core breaks through, continue to pass nitrogen for a period of time, record the data, and calculate the core gas permeability K2;

[0039] 5) Stop the experiment, calculate the change H of gas permeability according to formula (1), and analyze the sensitivity of the reservoir;

[0040] Preferably, the liquid in step 2) includes deionized water, formation water, brine, acid solution, and alkali solution; the initial ring pressure in step 3) is 2MPa-10MPa; the mineralization of the brine is 25%-100% of the mineralization of the formation water, the pH value of the alkali solution is 7-11, and the pH value of the acid solution is 1-6.

[0041] Further preferably, the brine is selected from one or both of KCl solution and NaCl solution with different formation water mineralization; the alkali solution is selected from one or both of KCl solution and NaCl solution with different pH values; and the acid solution is selected from one or both of KCl solution and NaCl solution with different pH values.

[0042] Preferably, the evaluation criteria for the sensitivity of the shale oil reservoir in step (3) are: the gas permeability change H≤5%, indicating that the shale reservoir is insensitive; the gas permeability change H is between 5%-30%, indicating that the shale reservoir has weak sensitivity; the gas permeability change H is between 30%-50%, indicating that the shale reservoir has medium to weak sensitivity; the gas permeability change H is between 50%-70%, indicating that the shale reservoir has medium to strong sensitivity; the gas permeability change H>70%, indicating that the shale reservoir has strong sensitivity.

[0043] Further preferably, the gas-tested permeability change H≤5%, indicating that the permeability of the core does not change much after the sensitivity experiment, that is, the reservoir is not sensitive to the specific fluid, and this factor causing reservoir damage can be ignored during subsequent reservoir protection and fracturing construction; as the gas-tested permeability change H increases, it indicates that after the sensitivity experiment, the reservoir permeability change increases, and the reservoir sensitivity to the specific fluid increases; when the gas-tested permeability change H>70%, it indicates that after the sensitivity experiment, the reservoir permeability decreases significantly, and this factor should be avoided as much as possible during subsequent reservoir protection and fracturing construction to avoid causing serious reservoir damage.

[0044] Compared with the prior art, the present invention has the following beneficial effects:

[0045] (1) The shale oil reservoir sensitivity evaluation method of the present invention overcomes the limitations of the existing evaluation system and can accurately and safely evaluate the flow rate sensitivity, water sensitivity, salinity sensitivity, acid sensitivity and alkali sensitivity of ultra-low permeability shale oil reservoirs from the perspective of permeability changes. It can effectively avoid the abnormal high pressure that occurs when using industry standards and is safer and more reliable.

[0046] (2) The evaluation method of the present invention is easy to operate and simple to calculate. It can provide important basic data for shale oil reservoir protection strategies, and can also optimize reservoir fracturing and transformation processes, providing important technical support for improving the efficient utilization of shale oil. BRIEF DESCRIPTION OF THE DRAWINGS

[0047] Figure 1 This is a graph showing the variation of core permeability with gas flow rate in Example 1;

[0048] Figure 2 This is a graph showing the variation of core permeability with formation water salinity in Example 3. DETAILED DESCRIPTION

[0049] The technical solutions in the embodiments of the present invention are further clearly described. The described embodiments are only a part of the present invention and are used to explain the present invention but not to limit the present invention. Therefore, other embodiments obtained by other technicians in the field without creative work all belong to the protection scope of the present invention.

[0050] Example 1

[0051] A core (named J-1) with a diameter of 2.5 cm and a height of 6 cm was taken from the Jimusar shale oil reservoir, and the flow rate sensitivity was evaluated according to the following steps:

[0052] (1) According to SY / T 5336, the initial gas permeability of the constant weight core measured with dry nitrogen is K1 = 1.53 × 10 -5 μm 2 , porosity

[0053] (2) Build a device according to the core gas permeability measurement experimental device diagram, load the above-mentioned dry core, and apply an initial ring pressure of 2.8 MPa;

[0054] (3) The gas flow rate was gradually changed from 0.1 mL / s to 1.8 mL / s by changing the inlet pressure in sequence (keeping the annular pressure 1.5 MPa-2.0 MPa higher than the inlet pressure). After the pressure at each experimental point was stable for a period of time, the outlet flow rate was recorded. The corresponding core permeability under each flow rate condition is as follows: Figure 1 As shown in Figure 2, when the gas flow rate is 1.8 mL / s, the core gas permeability K2 = 0.97 × 10 -5 μm 2 ;

[0055] (4) Stop the experiment and calculate the change in gas permeability H = 36.60% according to formula (1).

[0056]

[0057] It can be seen that the shale oil reservoir has a medium to strong velocity sensitivity, and the critical velocity is 2665m / d. During fracturing development and reservoir protection, special attention should be paid to avoid the impact of particle migration in the reservoir.

[0058] Example 2

[0059] Two cores (named J-2 and J-3) from the Jimusar shale oil reservoir were taken, both with a diameter of 2.5 cm and a height of 5 cm, and the water sensitivity was evaluated according to the following steps:

[0060] (1) According to SY / T 5336, the initial gas permeability K0 and porosity of constant weight cores J-2 and J-3 were measured using dry nitrogen. The results are listed in Table 1;

[0061] (2) Evacuate the cores J-2 and J-3 for 5 h, and soak the cores J-2 and J-3 in formation water and deionized water for 5 h, respectively;

[0062] (3) Build a device according to the core gas permeability measurement experimental device diagram, load the above two cores, and apply an initial ring pressure of 8.0 MPa;

[0063] (4) Dry nitrogen was introduced to displace the core (keeping the annular pressure 1.5MPa-2.0MPa higher than the inlet pressure). After the core broke through, considering that some free water in the core occupied the pores at this time, nitrogen was continued to be introduced to displace the free water in the core. After stabilization for 3.5 hours, the data was recorded and the gas permeability K1 and K2 of the J-2 and J-3 cores were calculated respectively. The results are listed in Table 1.

[0064] (5) Stop the experiment and calculate the gas permeability change H=86.40% according to formula (1). It can be seen that the shale oil reservoir has strong water sensitivity and is close to extremely strong water sensitivity. During the fracturing development process, it is not suitable to inject a large amount of water.

[0065] Table 1 Water sensitivity test results

[0066]

[0067] Example 3

[0068] Three cores (named J-4, J-5, and J-6) from the Jimusar shale oil reservoir were taken, each with a diameter of 2.5 cm and a height of 5 cm, and the salt sensitivity was evaluated according to the following steps:

[0069] (1) According to SY / T 5336, the initial gas permeability K0 and porosity of constant weight cores J-4, J-5, and J-6 were measured using dry nitrogen. The results are listed in Table 2;

[0070] (2) The cores J-4, J-5, and J-6 were vacuumed for 5 h, and the cores J-4, J-5, and J-6 were soaked in 75% mineralization formation water, 50% mineralization formation water, and 25% mineralization formation water for 5 h, respectively;

[0071] (3) Build a device according to the core gas permeability measurement experimental device diagram, load the three cores mentioned above, and apply an initial ring pressure of 8.0 MPa;

[0072] (4) Dry nitrogen was introduced to displace the core (keeping the annular pressure 1.5 MPa-2.0 MPa higher than the inlet pressure). After the core broke through, nitrogen was continued to be introduced for 3.5 h to stabilize. The data were recorded and the gas permeability of the cores J-4, J-5, and J-6 was calculated respectively. The results are listed in Table 2.

[0073] (5) Stop the experiment and draw a graph based on the results in Table 1 and Table 2 to understand the change in permeability of the shale oil reservoir when it comes into contact with water of different salinity. The results are as follows: Figure 2 As shown in the figure, it can be seen that when the formation water salinity changes from 100% to 75%, the permeability of the core changes by H=56.0%, indicating that the permeability of the shale reservoir decreases significantly with the decrease of formation water salinity.

[0074] Table 2 Salinity sensitivity test results

[0075]

[0076] Example 4

[0077] Two cores (named J-7 and J-8) from the Jimusar shale oil reservoir were taken, both with a diameter of 2.5 cm and a height of 5 cm, and the acid sensitivity was evaluated according to the following steps:

[0078] (1) According to SY / T 5336, the initial gas permeability K0 and porosity of constant weight cores J-7 and J-8 were measured using dry nitrogen. The results are listed in Table 3;

[0079] (2) The cores J-7 and J-8 were vacuumed for 5 h, and the cores J-7 and J-8 were soaked in neutral KCl solution and pH=4 KCl solution for 5 h respectively;

[0080] (3) Build a device according to the core gas permeability measurement experimental device diagram, load the above two cores, and apply an initial ring pressure of 8.0 MPa;

[0081] (4) Dry nitrogen was introduced to displace the core (keeping the annular pressure 1.5 MPa-2.0 MPa higher than the inlet pressure). After the core broke through, nitrogen was continued to flow for 3.5 h to stabilize. The data were recorded and the gas permeability of the cores J-7 and J-8 was calculated respectively. K1=9.0×10 -4 μm 2 , K2=4.7×10 -3 μm 2 , the results are listed in Table 3;

[0082] (5) Stop the experiment and calculate the gas permeability change H = -80.85% according to formula (1). It can be seen that the shale oil reservoir has no acid sensitivity, and the acid sensitivity damage caused by acid entering the reservoir is small. In the process of reservoir development, an appropriate and simple acid formula can be used to improve the formation conditions.

[0083] Table 3 Acid sensitivity test results

[0084]

[0085]

[0086] Example 5

[0087] Four cores (named J-9, J-10, J-11, and J-12) from the Jimusar shale oil reservoir were taken, each with a diameter of 2.5 cm and a height of 6 cm, and the alkaline sensitivity was evaluated according to the following steps:

[0088] (1) According to SY / T 5336, the initial gas permeability K0 and porosity of constant weight cores J-9, J-10, J-11, and J-12 were measured using dry nitrogen. The results are listed in Table 4;

[0089] (2) The cores J-9, J-10, J-11, and J-12 were vacuumed for 5 h, and the cores J-9, J-10, J-11, and J-12 were soaked in a neutral KCl solution, a KCl solution with a pH value of 9, a KCl solution with a pH value of 11, and a KCl solution with a pH value of 13 for 5 h, respectively;

[0090] (3) Build a device according to the core gas permeability measurement experimental device diagram, load the above two cores, and apply an initial ring pressure of 8.0 MPa;

[0091] (4) Dry nitrogen was introduced to displace the core (keeping the annular pressure 1.5 MPa-2.0 MPa higher than the inlet pressure). After the core broke through, nitrogen was continued to be introduced for 3.5 h to stabilize. The data were recorded and the gas permeability of the cores J-9, J-10, J-11, and J-12 was calculated respectively. The results are listed in Table 4.

[0092] (5) Stop the experiment and calculate the change in gas permeability H≈0 according to formula (1). It can be seen that the shale oil reservoir has no alkali sensitivity. The alkaline liquid such as fracturing fluid used in the fracturing process will enter the reservoir and will not cause reservoir damage such as particle migration, and will not react with the rock minerals in the reservoir to cause them to dissolve and damage the reservoir.

[0093] Table 4 Alkali sensitivity test results

[0094]

[0095] The above detailed description is a specific description of one feasible embodiment of the present invention. The embodiment is not intended to limit the patent scope of the present invention. Any equivalent implementation or modification that does not deviate from the present invention should be included in the scope of the technical solution of the present invention.

Claims

1. A shale oil reservoir reservoir sensitivity evaluation method, characterized in that: The shale oil reservoir reservoir sensitivity evaluation method comprises the following steps: (1) core pretreatment to obtain a dry core; (2) Dry cores were treated with different fluids, and core sensitivity experiments were performed to determine the gas permeability of wet cores; (3) Compare the wet core gas permeability change H to evaluate the sensitivity of the shale oil reservoir. The gas permeability change H is obtained by formula (1): Among them, K1 and K2 are the gas permeabilities of the cores after treatment with different fluids, and H is the change in gas permeability.

2. The shale oil reservoir sensitivity evaluation method according to claim 1, characterized in that: The diameter of the core in step (1) is 2.5 cm-4.0 cm, and the height of the core is 4 cm-10 cm.

3. The shale oil reservoir sensitivity evaluation method according to claim 1, characterized in that: The core pretreatment in step (1) includes: grinding both end faces of the core flat and ensuring that the end faces are perpendicular to the side faces of the column, washing with a mixed solvent of benzene and ethanol, and drying.

4. The shale oil reservoir sensitivity evaluation method according to claim 3, characterized in that: The volume ratio of benzene to ethanol is 1:3-7; the drying temperature is 50° C.-100° C., and the drying time is 48h-96h.

5. The shale oil reservoir sensitivity evaluation method according to claim 1, characterized in that: The core sensitivity in step (2) includes: velocity sensitivity, water sensitivity, salinity sensitivity, acid sensitivity and alkali sensitivity; the method for determining the velocity sensitivity includes the following steps: S1, determination of the initial gas permeability K1 of the constant weight core; S2, build the device, load the dry core, and apply initial ring pressure; S3, change the gas flow rate, record the outlet flow rate, and calculate the core gas permeability K2; S4. Calculate the gas permeability change H according to formula (1), analyze the velocity sensitivity of the reservoir and determine the critical velocity.

6. The shale oil reservoir sensitivity evaluation method according to claim 5, characterized in that: The initial ring pressure in step S2 is 2MPa-10MPa, and the gas flow rate in step S3 is 0.1mL / s-2.0mL / s.

7. The shale oil reservoir sensitivity evaluation method according to claim 5, characterized in that: The method for determining water sensitivity, salinity sensitivity, acid sensitivity and alkali sensitivity comprises the following steps: 1) Determine the initial gas permeability K1 of the constant weight core; 2) The cores are vacuumed for 2h-8h and soaked in different liquids for 2h-24h; 3) Build the device, load the core after the sensitivity experiment, and apply initial ring pressure; 4) Introduce dry nitrogen to displace the core, and after the core breaks through, record the data and calculate the core gas permeability K2; 5) Calculate the gas permeability change H according to formula (1) and analyze the sensitivity of the reservoir.

8. The shale oil reservoir reservoir sensitivity evaluation method according to claim 7, characterized in that: The liquid in step 2) includes deionized water, formation water, brine, acid and alkali; the initial ring pressure in step 3) is 2MPa-10MPa; the mineralization of the brine is 25%-100% of the mineralization of the formation water, the pH value of the alkali is 7-11, and the pH value of the acid is 1-6.

9. The shale oil reservoir reservoir sensitivity evaluation method according to claim 8, characterized in that: The brine is selected from one or both of KCl solutions and NaCl solutions with different formation water mineralizations; the alkali solution is selected from one or both of KCl solutions and NaCl solutions with different pH values; and the acid solution is selected from one or both of KCl solutions and NaCl solutions with different pH values.

10. The shale oil reservoir sensitivity evaluation method according to claim 1, characterized in that: The evaluation criteria for the sensitivity of the shale oil reservoir in step (3) are as follows: the gas permeability change H≤5%, indicating that the shale reservoir is insensitive; the gas permeability change H is between 5%-30%, indicating that the shale reservoir has weak sensitivity; the gas permeability change H is between 30%-50%, indicating that the shale reservoir has medium to weak sensitivity; the gas permeability change H is between 50%-70%, indicating that the shale reservoir has medium to strong sensitivity; the gas permeability change H>70%, indicating that the shale reservoir has strong sensitivity.

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