A method for enhancing the mass transfer efficiency of coalbed methane in guar gum fracturing wells by heat injection

Abstract

The present invention relates to the technical field of the development of low-temperature oil and gas reservoirs such as coalbed methane, and specifically to a method for improving the mass transfer efficiency of coalbed methane in a guar gum fracturing well by heat injection, including analyzing the composition of reservoir water and preparing simulated reservoir water, conducting mining sensitivity experiments to determine appropriate experimental and operation parameters, and using the simulated reservoir water and the prepared acid solution and alkali solution to evaluate the velocity sensitivity, acid sensitivity, and alkali sensitivity of coal rock samples to determine appropriate injection and production rates, pH value ranges, etc. Since the coal seam itself contains a large amount of water, injecting hot water, especially hot water compatible with reservoir water, will not introduce new damage, can effectively increase the temperature of the reservoir and fluid, is beneficial to the gel breaking and backflow of the fracturing fluid, and can accelerate the desorption and diffusion of coalbed methane, thereby protecting and improving the multi-scale mass transfer capacity of coalbed methane.

Description

Technical Field

[0001] The invention relates to the technical field of development of low-temperature oil and gas reservoirs such as coalbed methane, and in particular to a method for improving the mass transfer efficiency of coalbed methane in a guar gum fracturing well by injecting heat. Background Art

[0002] Coalbed methane is an unconventional resource gas that coexists in the form of free and adsorbed states. Coalbed methane mass transfer needs to go through a multi-scale and cross-scale mass transfer process of desorption, diffusion and seepage. Coalbed methane development must protect the permeability and desorption capacity of coalbed methane, because only when the desorption capacity is protected or improved, there will be a steady supply of coalbed methane for exploitation in the later stage. Similarly, only when the permeability of the reservoir is protected or improved, the coal seam will be depressurized, coalbed methane will be desorbed, coalbed methane will have sufficient gas sources and good seepage channels, and coalbed methane exploitation will be efficient.

[0003] The pore structure characteristics are geological factors that affect the multi-scale mass transfer of coalbed methane. The more pores and fractures are developed, the better the pore structure is, the faster the coalbed methane desorption, diffusion, and seepage speeds are, and the greater the mass transfer rate is. Fracturing is currently the most effective method to improve the pore structure, so coalbed methane wells also use fracturing methods to increase coalbed methane production. However, due to the low reservoir temperature, the fracturing fluid cannot break the gel after entering the reservoir, causing a large amount of fracturing fluid to be unable to flow back and remain in the reservoir, resulting in liquid phase retention, polymer adsorption, fluid sensitivity and other damage to the reservoir, causing the desorption, diffusion and seepage capacity of coalbed methane to be greatly reduced. Therefore, a method of heat injection to reduce coalbed methane reservoir damage in fracturing wells and improve coalbed methane mass transfer efficiency was invented. Summary of the invention

[0004] The purpose of the present invention is to provide a method for reducing coalbed methane reservoir damage in fracturing wells by heat injection and improving the mass transfer efficiency of coalbed methane, so as to solve the problem raised in the above background technology that due to the low reservoir temperature, the fracturing fluid cannot break the gel after entering the reservoir, resulting in a large amount of fracturing fluid being unable to flow back and being retained in the reservoir, causing liquid phase retention, polymer adsorption, fluid sensitivity and other damages to the reservoir, resulting in a significant reduction in the desorption, diffusion and seepage capacity of coalbed methane.

[0005] To achieve the above object, the present invention provides the following technical solution: a method for improving the mass transfer efficiency of coalbed methane in guar gum fracturing wells by heat injection, comprising the following steps;

[0006] Step 1: Analyze the composition of reservoir water and prepare simulated reservoir water. Analyze the characteristics of the block reservoir water, mainly including the concentrations of Ca2+, Mg2+, Na+, K+, HCO3-, Cl-, and SO42-. According to the characteristics of the reservoir water and the test results, add the corresponding Ca2+, Mg2+, Na+, K+, HCO3-, Cl-, and SO42- ions to the produced water of the adjacent well reservoir (hereinafter referred to as adjacent well water);

[0007] Step 2: Mining sensitivity test, determine the appropriate experimental and operating parameters, use simulated reservoir water and prepared acid and alkali solution to evaluate the velocity sensitivity, acid sensitivity and alkali sensitivity of coal and rock samples, so as to determine the appropriate injection and production rate, pH value range, etc.;

[0008] Step 3: Determine the critical temperature of the reservoir, evaluate the temperature sensitivity of the coal rock, and determine the critical temperature L of the reservoir;

[0009] Step 4: Determine the temporary plugging pore diameter, conduct mercury injection test on the coal core mined on site, and the pore throat diameter d corresponding to the median pressure Pc50 value is called the temporary plugging pore diameter.

[0010] Step 5: Filter out the solid particles that may block the pores and use a filter to filter the reservoir water near the well water. Solid particles;

[0011] Step 6: Add alkali to the well water. According to the acid sensitivity and alkali sensitivity results, decide whether to add alkali (Na2CO3) to the filtered produced water. If the reservoir acid sensitivity is none to weak, then no alkali is needed. If the reservoir acid sensitivity is medium to strong, then add appropriate alkali (based on the acidity of the breaker) to the filtered well water.

[0012] Step 7: Heat the well water, using a heating device to heat the well water to T°C;

[0013] Step 8: Flow out and calculate the amount of injected water. Flow out the well that has just completed guar fracturing, and estimate the amount of fracturing fluid that has not been discharged.

[0014] Step 9: Inject hot water, use a fracturing pump at the same pump speed as the fracturing displacement to inject at least D tons of T℃ temporary well water into the development well;

[0015] Step 10: Shut in the well. Indoor experiments are conducted to study the time t corresponding to when the viscosity of the fracturing fluid decreases by 80-90% when the reservoir temperature is the critical temperature L. Because the heat transfer time needs to be considered, for safety reasons, 1.3t is used as the shut-in time, which generally does not exceed 24-48 hours. The purpose is to allow the water in the well to fully contact with the fracturing fluid and break the gel;

[0016] Step 11: Open the well and extract gas. Open the well and use extraction (natural blowing) to discharge the fracturing fluid, hot water and mixed liquid, and then start normal drainage and gas production operations. When draining and gas production operations, attention should be paid to the influence of velocity sensitivity.

[0017] Preferably, the method for analyzing the characteristics of the block reservoir water in step one is performed according to the method of the petroleum and natural gas industry standard "SY / T5523-2016 Oilfield Water Analysis Method".

[0018] Preferably, the experimental evaluation of step 2 is performed in accordance with the petroleum and natural gas industry standard "SY / T5358-2010 Reservoir Sensitivity Flow Experiment Evaluation Method".

[0019] Preferably, the step three requires that under critical temperature conditions, the gel breaking rate of the fracturing fluid is ≥85%.

[0020] Preferably, the temperature T°C in step seven is 90°C or higher.

[0021] Preferably, the calculation formula of step eight can be Q=G·C·(tg-th), which calculates the injection amount D of the well water with a temperature of T°C required to raise the temperature of the fracturing fluid in the reservoir to the critical temperature L of the reservoir. In the formula, Q is the mass of the liquid, kg; C is the specific heat capacity, J / (kg·℃); tg is the temperature after heating, ℃; th is the temperature before heating, ℃.

[0022] Compared with the prior art, the beneficial effects of the present invention are: since the coal seam itself contains a large amount of water, the injection of hot water, especially the injection of hot water compatible with reservoir water, will not introduce new damage, can effectively increase the reservoir and fluid temperature, is conducive to the degelation and return of the fracturing fluid, and can accelerate the desorption and diffusion of coal bed methane, thereby protecting and improving the multi-scale mass transfer capacity of coal bed methane. DETAILED DESCRIPTION

[0023] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0024] Embodiment 1:

[0025] Step 1: Analyze the composition of reservoir water and prepare simulated reservoir water. The produced water from a horizontal well in a block of Qinshui Basin was selected for main component analysis. The results showed that the concentrations of Ca2+, Mg2+, Na+, K+, HCO3-, Cl-, and SO42- in the reservoir water were 16.15mg / L, 12.24mg / L, 890.15mg / L, 43.29mg / L, 675mg / L, 366mg / L, and 7.15mg / L, respectively. According to the test results, A liter of simulated reservoir water was prepared by adding the following amounts of Ca2+, Mg2+, Na+, K+, HCO3-, Cl-, and SO42- to A liter of distilled water: 16.15Amg, 12.24Amg, 890.15Amg, 43.29Amg, 675Amg, 366Amg, and 7.15Amg, respectively.

[0026] Step 2: Mining sensitivity test to determine appropriate experimental and operating parameters. According to the petroleum and natural gas industry standard "SY / T5358-2010 Reservoir Sensitivity Flow Experiment Evaluation Method", acid and alkali solutions prepared with simulated reservoir water were used to evaluate the acid sensitivity and alkali sensitivity of coal rock samples. The results showed that the acid sensitivity of 15% HCl was weak, the alkali sensitivity was medium to strong, and the critical pH value was 9.

[0027] Step 3: Determine the critical temperature of the reservoir. The temperature sensitivity of coal rock was evaluated. The experimental results showed that the critical temperature was 50-70°C. According to the degradation efficiency of the breaker and the economic analysis of heat injection under different temperature conditions, the critical temperature of the reservoir was 50°C.

[0028] Step 4: Determine the pore diameter. Cast thin sections, mercury injection analysis, in-situ stress simulation experiments and other methods show that the diameter d of pores that contribute most to permeability is mainly distributed in the range of 6 to 10 μm;

[0029] Step 5: Filter out solid particles that may clog the pores. Use a filter to filter the well water and filter out solid particles with a particle size greater than 4μm.

[0030] Step 6: Add alkali to the well water. The reservoir is not acid-sensitive, so there is no need to add alkali to the filtered well water.

[0031] Step 7: Heat the well water. Use a heating device to heat the well water reservoir water to 90°C.

[0032] Step 8: Release and calculate the amount of injected water. For the guar fracturing well that has just been completed, it is estimated that 30% of the 600 cubic meters of fracturing fluid injected will be released, and the remaining 420 cubic meters of fracturing fluid will not be returned. For the convenience of calculation, it is assumed that the specific heat capacity of the fracturing fluid is consistent with that of water. According to the law of conservation of energy, the temperature of the fracturing fluid in the reservoir (reservoir temperature, about 25°C) is increased to 50°C, and at least 262.5 tons of well water with a temperature of 90°C needs to be injected.

[0033] Step 9: Inject hot water. Use a fracturing pump at a displacement of 10m3 / min (the same as the fracturing displacement) to inject at least 262.5 tons of 90°C well water into the development well K1;

[0034] Step 10: Shut down the well. Because the reservoir temperature is relatively low and the amount of fracturing fluid required is relatively large, the well is shut down for 48 hours to allow the injected well water to fully contact the fracturing fluid and break the gel.

[0035] Step 11: Open the well and extract gas. Open the well and use the extraction method to extract the fracturing fluid, hot water and mixed fluid. Then start the normal drainage and gas extraction operation. The drainage and gas extraction operation should pay attention to the influence of speed sensitivity.

[0036] Embodiment 2:

[0037] Step 1: Analyze the composition of reservoir water and prepare simulated reservoir water. The produced water from a reservoir in a block of Qinshui Basin was selected for main component analysis. The results showed that the concentrations of Ca2+, Mg2+, Na+, K+, HCO3-, Cl-, and SO42- in the reservoir water were 17.04mg / L, 8.96mg / L, 781.59mg / L, 100.02mg / L, 940mg / L, 655mg / L, and 9.03mg / L, respectively. According to the test results, A liter of simulated reservoir water was prepared by adding the following amounts of Ca2+, Mg2+, Na+, K+, HCO3-, Cl-, and SO42- to A liter of distilled water: 16.15Amg, 12.24Amg, 890.15Amg, 43.29Amg, 675Amg, 366Amg, and 7.15Amg, respectively.

[0038] Step 2: According to the petroleum and natural gas industry standard "SY / T5358-2010 Reservoir Sensitivity Flow Experiment Evaluation Method", acid and alkali solutions prepared with simulated reservoir water are used to evaluate the acid sensitivity and alkali sensitivity of coal rock samples. The acid sensitivity of 15% HCL is strong, the alkali sensitivity is weak to medium, and the critical pH value is 10.

[0039] Step 3: Determine the critical temperature of the reservoir. The temperature sensitivity of coal rock was evaluated. The experimental results showed that the critical temperature was 60-80°C. According to the degradation efficiency of the debonding agent and the economic analysis of heat injection under different temperature conditions, the critical temperature of the reservoir was 60°C.

[0040] Step 4: Determine the pore diameter. Cast thin sections, mercury injection analysis, in-situ stress simulation experiments and other methods show that the diameter d of pores that contribute most to permeability is mainly distributed in the range of 12 to 20 μm;

[0041] Step 5: Filter out solid particles that may block the pores. Use a filter to filter the reservoir water near the well water to filter out solid particles with a particle size greater than 8μm.

[0042] Step 6: Add alkali to the well water. According to the actual situation that the acid sensitivity of HCl is medium to strong, the alkali sensitivity is weak to medium, and the critical pH value is 10, add an appropriate amount of Na2CO3 to the filtered well water reservoir water.

[0043] Step 7: Heating the well water: Use a heating device to heat the well water reservoir water to 100°C.

[0044] Step 8: Release and calculate the amount of injected water. For the guar fracturing well that has just been completed, it is estimated that 30% of the 500 cubic meters of fracturing fluid injected will be released, and the remaining 350 cubic meters of fracturing fluid will not be returned. For the convenience of calculation, it is assumed that the specific heat capacity of the fracturing fluid is consistent with that of water. According to the law of conservation of energy, the temperature of the fracturing fluid in the reservoir (reservoir temperature, about 25°C) is increased to 60°C, and 310 tons of well water with a temperature of 100°C needs to be injected.

[0045] Step 9: Inject hot water. After the blowdown is completed, use a fracturing pump to inject at least 310 tons of 100°C well water into the development well at a displacement of 10m3 / min (the same as the fracturing displacement);

[0046] Step 10: Shut down the well. Because the reservoir temperature is low and the amount of fracturing fluid is large, the well is shut down for 48 hours to wait for the injected well water to fully contact with the fracturing fluid and break the gel.

[0047] Step 11: Open the well and extract gas. Open the well and use the extraction method to extract the fracturing fluid, hot water and mixed fluid. Then start the normal drainage and gas extraction operation. The drainage and gas extraction operation should pay attention to the influence of speed sensitivity.

[0048] Combining Example 1 and Example 2, it is found that since the coal seam itself contains a large amount of water, injecting hot water, especially injecting hot water compatible with reservoir water, will not introduce new damage, can effectively increase the reservoir and fluid temperature, and is beneficial to the degelation and backflow of the fracturing fluid. At the same time, it can accelerate the desorption and diffusion of coal bed methane, thereby protecting and improving the multi-scale mass transfer capacity of coal bed methane.

[0049] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above and that the present invention can be implemented in other specific forms without departing from the spirit or essential features of the present invention. Therefore, the embodiments should be considered exemplary and non-restrictive in all respects, and the scope of the present invention is defined by the appended claims rather than the above description, and it is intended that all changes falling within the meaning and scope of the equivalent elements of the claims be included in the present invention.

Claims

1. A method for improving the mass transfer efficiency of coalbed methane in a guar gum fracturing well by heat injection, characterized in that, including; Step 1: Analyze the composition of reservoir water and prepare simulated reservoir water, analyze the characteristics of reservoir water in the block, mainly including the concentrations of Ca2+, Mg2+, Na+, K+, HCO3-, Cl-, SO42-, and according to the characteristics of reservoir water and the test results, add the corresponding Ca2+, Mg2+, Na+, K+, HCO3-, Cl-, SO42- ions to the produced water from the adjacent well reservoir (referred to as adjacent well water, the same below); Step 2: Conduct a sensitivity experiment for production, determine appropriate experimental and operation parameters, and evaluate the velocity sensitivity, acid sensitivity, and alkali sensitivity of coal rock samples using simulated reservoir water and prepared acid and alkali solutions to determine appropriate injection and production rates, pH value ranges, etc.; Step 3: Determine the critical temperature of the reservoir, conduct a temperature sensitivity evaluation of coal rock, and determine the critical temperature L of the reservoir; Step 4: Determine the temporary plugging pore diameter, conduct a mercury injection experiment on the coal cores produced on site, and the pore throat diameter d corresponding to the median pressure Pc50 value is called the temporary plugging pore diameter, Step Five: Filter out solid particles that may clog the pores. Use a filter to filter the reservoir water of the adjacent well water, and filter out solid particles with a particle size of ; Step 6: Add alkali to the adjacent well water. According to the acid sensitivity and alkali sensitivity results, decide whether to add alkali (Na2CO3) to the filtered produced water. If the acid sensitivity of the reservoir is none to weak, alkali may not be added. If the acid sensitivity of the reservoir is medium to strong, add an appropriate amount of alkali (based on neutralizing the acidity of the breaker) to the filtered adjacent well water; Step 7: Heat the adjacent well water, use a heating device to heat the adjacent well water to T °C; Step 8: Flow back and calculate the amount of injected water. Flow back the well that has just completed guar gum fracturing, and estimate the amount of fracturing fluid that has not flowed back; Step 9: Inject hot water. Use a fracturing pump to inject at least D tons of adjacent well water at T °C into the development well at the same pump speed as the fracturing displacement; Step 10: Shut in the well. Conduct indoor experiments to study the time t corresponding to a 80 - 90% reduction in the viscosity of the fracturing fluid when the reservoir temperature is the critical temperature L. Since heat transfer time needs to be considered, for safety reasons, use 1.3t as the shut-in time, generally not exceeding 24 - 48 hours, with the aim of allowing the adjacent well water to fully contact and break the fracturing fluid; Step 11: Open the well and produce gas. Open the well and use drainage and extraction (natural flow back) to discharge the fracturing fluid, hot water, and mixture, and then start normal drainage and gas production operations. During drainage and gas production operations, attention should be paid to the influence of velocity sensitivity.

2. A method for improving the mass transfer efficiency of coalbed methane in a guar gum fracturing well by heat injection according to claim 1, characterized in that: The method for analyzing the characteristics of reservoir water in step 1 is carried out according to the method of the petroleum and natural gas industry standard "SY / T5523 - 2016 Analysis Method of Oilfield Water".

3. A method for improving the mass transfer efficiency of coalbed methane in a guar gum fracturing well by heat injection according to claim 1, characterized in that: The experimental evaluation in step 2 is carried out according to the petroleum and natural gas industry standard "SY / T5358 - 2010 Evaluation Method for Reservoir Sensitivity Flow Experiment".

4. A method for improving the mass transfer efficiency of coalbed methane in a guar gum fracturing well by heat injection according to claim 1, characterized in that: In Step 3, it is required that the gel-breaking rate of the fracturing fluid ≥ 85% under the critical temperature condition.

5. The method for improving the mass transfer efficiency of coalbed methane in a guar gum fracturing well by heat injection according to claim 1, characterized in that: the T °C in Step 7 is 90 °C or a higher temperature.

6. The method for improving the mass transfer efficiency of coalbed methane in a guar gum fracturing well by heat injection according to claim 1, characterized in that: the calculation formula in Step 8 can be Q = G·C·(tg - th), to calculate the injection volume D of the well water at the critical temperature L of the reservoir when the temperature of the fracturing fluid in the reservoir is raised to T °C; where Q is the liquid mass, kg; C is the specific heat capacity, J / (kg·°C); tg is the temperature after heating, °C; th is the temperature before heating, °C.

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