Natural gas-assisted oil shale self-heat-generation in-situ conversion development method

Abstract

The invention discloses a natural gas-assisted oil shale self-heat-generation in-situ conversion development method, and belongs to the field of oil shale in-situ exploiting.The method comprises the steps that an inverse seven-point well pattern is arranged in a natural gas-assisted oil shale self-heat-generation in-situ conversion development target area to serve as a development well pattern, reservoir transformation is conducted on a reservoir of the target area, and a fracture network is formed; high-temperature nitrogen is injected through the injection well to locally preheat the oil shale reservoir, then normal-temperature air is injected into the injection well to cool the bottom of the injection well, and finally the normal-temperature air and natural gas are separately injected into the bottom of the injection well through the double-layer oil pipe. Under the assistance of natural gas, residues generated after kerogen thermal cracking are subjected to an oxidation reaction to release heat, an oil shale reservoir is heated, meanwhile, natural gas oxidation can effectively supplement insufficient heat release of low-oil-content oil shale ground storage residues in oxidation, successful triggering and operation of a self-heat-generation reaction are guaranteed, and the oil shale reservoir is protected from being damaged. And efficient output of oil-gas cracking products of the oil shale reservoir is realized.

Description

Technical Field

[0001] The invention discloses a natural gas-assisted oil shale autothermal in-situ conversion development method, belonging to the field of oil shale in-situ mining. Background Art

[0002] In-situ conversion technology aims to achieve in-situ conversion of organic matter in oil shale, thereby improving the development efficiency and utilization value of oil shale resources. In-situ conversion technology for oil shale involves direct heating, pressurization or other chemical treatments in the oil shale reservoir to cause chemical reactions such as pyrolysis and cracking of the organic matter in the oil shale, and convert it into recoverable oil and gas resources. This technology avoids the cumbersome steps of ground mining, transportation and processing required in the traditional oil shale mining process, and directly converts it underground, thereby greatly reducing mining costs and environmental impacts. In-situ conversion is mainly divided into in-situ combustion heating technology, conduction heating technology, convection heating technology, and radiation heating technology. However, these technologies all have the problem of poor economic benefits.

[0003] In response to this problem, in the prior art, a method for extracting shale oil and gas by in-situ local chemical method of oil shale (see patent document with publication number CN103790563A) proposes to heat oxygen-containing gas and hydrocarbon gas recovered from production wells and then mix and inject them into production wells to form local chemical reactions in the oil shale layer to produce oil and gas. The self-heating in-situ conversion development method of low-mature organic-rich shale (see patent document with publication number CN114017032A) proposes to first inject hot air into the ground for local preheating, and then inject air at room temperature to trigger the self-heating reaction of organic matter, so as to achieve convection heating of the reservoir to produce oil and gas. The self-heating in-situ conversion technology of oil shale has the advantages of low development cost and the development of deep oil shale resources. It aims to achieve efficient and environmentally friendly utilization of oil shale and is an important development trend of the oil shale industry. However, there is a problem of insufficient heat donors in the thermal cracking residues in low-oil-content oil shale reservoirs, which makes it difficult for the oxidation heat release to maintain the continuous advancement of the autogenous heat reaction. The oil shale autogenous heat in situ conversion technology is not suitable, and the energy return rate of the autogenous heat in situ conversion technology still has room for further improvement. Summary of the invention

[0004] In view of the above problems existing in the prior art, the purpose of the present invention is to propose a natural gas-assisted oil shale autothermal in-situ conversion development method, which greatly improves the applicability of oil shale autothermal in-situ conversion technology in different oil shale reservoirs by mixing with natural gas.

[0005] To achieve the above-mentioned purpose, the present invention adopts the following technical scheme: a natural gas-assisted oil shale autothermal in-situ conversion development method, comprising the following steps: Step 1: Determine the target area for natural gas-assisted oil shale autothermal in-situ conversion development, where the reservoir condition of the target area for natural gas-assisted oil shale autothermal in-situ conversion development is that the oil content of the oil shale is 2.9% to 9.8%; Step 2: arranging a development well pattern in the target area for natural gas-assisted oil shale autothermal in-situ conversion development, wherein the development well pattern adopts an inverted seven-spot well pattern, and the ratio of injection wells to production wells in the development well pattern is 1:6; Step 3: Reservoir transformation is carried out in the target area for natural gas-assisted oil shale autothermal in-situ conversion development to form a fracture network with a matrix permeability of 0.1 md and a fracture permeability of 100 md; Step 4: After the reservoir is transformed, high-temperature nitrogen at 500°C is injected into the injection well for 90 days to preheat the area near the injection well. Step 5: injecting room temperature air into the injection well to cool the bottom temperature of the injection well to 300°C; Step 6: Use double-layer tubing as the injection pipeline to separately inject room temperature air and natural gas into the bottom of the injection well to prevent the natural gas from mixing with air and causing explosion in the wellbore of the injection well. At the bottom of the injection well, the air and natural gas are mixed to form an air and natural gas mixture, and the proportion of natural gas in the total amount of the air and natural gas mixture is less than or equal to 4%. The injection volume of the air and natural gas mixture is 200m 3 / (h•m)~500m 3 / (h•m), after the introduction of natural gas and air, the oil shale triggers a self-heating reaction. With the assistance of natural gas, the residues produced after the thermal cracking of kerogen undergo an oxidation reaction to release heat, thereby realizing convection heating of the oil shale reservoir. The oil and gas products produced by the thermal cracking of kerogen are produced from production wells, so that the self-heating reaction can proceed smoothly in the low-oil-content oil shale reservoir and improve the energy return rate.

[0006] Furthermore, the development well pattern is an inverted seven-point well pattern consisting of seven vertical wells, and the inverted seven-point well pattern includes an injection well located at the center of the hexagon and production wells located at the six vertices of the hexagon.

[0007] Furthermore, the well spacing of the reverse seven-spot well pattern is 50m, so that the preheating effect is optimal.

[0008] Furthermore, in step 3, the volume fracturing method and the shock wave fracturing method are used in turn to transform the oil shale reservoir to form a fracture network. The volume fracturing method and the shock wave fracturing method are widely used in oil and natural gas extraction, and the specific method steps will not be described in detail here.

[0009] Preferably, in step 6, during the autothermal reaction stage, according to the experimental design, natural gas is mixed into the air at room temperature, and the proportion of natural gas in the total amount of the mixture of air and natural gas is 2%.

[0010] Furthermore, the gas injection method of the injection well is constant pressure injection, and the injection pressure is set to 10MPa. In order to ensure efficient production of oil and gas products, the wellhead pressure of the production well is set to 0MPa, and the bottom hole pressure of the production well is set to 200kPa.

[0011] Furthermore, in step 6, the amount of air and natural gas mixture injected is 200m 3 / (h•m)~500m 3 / (h•m), as oil production increases, the gas injection volume increases from 200m 3 / (h•m) gradually increases to 500m 3 / (h•m), as the oil production decreases, the gas injection volume increases from 500m 3 / (h•m) gradually decreases to 200m 3 / (h•m) to improve gas utilization and enable the self-heating reaction to proceed smoothly.

[0012] Furthermore, in step 6, double-layer oil pipes are used for injection when injecting air and natural gas. The double-layer oil pipes are used as injection pipes, and the outer diameter is 150 mm and the inner diameter is 125 mm. Natural gas accounts for a small amount of injected gas, so it is injected through the outer pipe, and air is injected through the inner pipe to prevent the two from mixing in the double-layer oil pipe, thereby reducing the risk of explosion.

[0013] Furthermore, in step 6, a check valve should be installed at the bottom of the injection well on the double-layer oil pipe to prevent the backflow of air and natural gas, ensure their mixing in the oil shale reservoir, and reduce the risk of explosion.

[0014] Through the above-mentioned design scheme, the present invention can bring the following beneficial effects: a natural gas-assisted oil shale autothermal in-situ conversion technology suitable for the development of low-oil-content oil shale is proposed, that is, a certain proportion of natural gas is mixed in the process of injecting air during the autothermal in-situ conversion process to provide external energy for the air injection autothermal in-situ conversion, thereby ensuring the successful triggering and efficient operation of the autothermal reaction. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The accompanying drawings herein are used to provide a further understanding of the present invention and constitute a part of the present application. The exemplary embodiments of the present invention and their descriptions are used to understand the present invention and do not constitute improper limitations of the present invention. In the accompanying drawings: Figure 1 This is the principle diagram of natural gas-assisted in-situ conversion of oil shale autogenous heat; Figure 2 This is a diagram of the well layout for natural gas-assisted in-situ conversion of oil shale autothermal energy; Figure 3 It is the design drawing of the injection pipeline for natural gas-assisted oil shale autothermal in-situ conversion; Figure 4 This is a cross-sectional view of the injection pipeline for natural gas-assisted autothermal in-situ conversion of oil shale; Figure 5 Develop a process flow chart for natural gas-assisted in-situ conversion of oil shale autothermal energy; Figure 6 This is a graph showing the change in production of low oil content oil shale mixed with different proportions of natural gas and oil over time; Figure 7 This is a graph showing the energy return rate of low oil content oil shale mixed with different proportions of natural gas over time; Figure 8 This is a graph showing the change in oil production over time for medium oil content oil shale mixed with different proportions of natural gas and oil; Fig. 9 This is a graph showing the energy return rate of medium oil content oil shale mixed with different proportions of natural gas over time; Fig.10 This is a graph showing the change in production of high oil content oil shale mixed with different proportions of natural gas and oil over time; Fig.11 This is a graph showing the energy return rate of high oil content oil shale mixed with different proportions of natural gas over time; Fig.12 This is a graph showing the variation of the injection volume of natural gas and air mixture over time.

[0016] In the figure: 1-injection well; 2-residue zone; 3-autogenous heat zone; 4-thermal cracking zone; 5-preheating zone; 6-production well; 7-natural gas channel; 8-air channel; 9-check valve. DETAILED DESCRIPTION

[0017] In order to make the purpose, features and advantages of the present invention more obvious and easy to understand, the technical scheme of the present invention is clearly and completely described below in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the present invention is not limited to the following embodiments, and the specific implementation method can be determined according to the technical scheme of the present invention and the actual situation. In order to avoid confusing the essence of the present invention, the known methods, processes, procedures, components and circuits are not described in detail.

[0018] Figure 1 The schematic diagram of natural gas-assisted oil shale autogenous thermal in-situ conversion is shown. During the development of natural gas-assisted oil shale autogenous thermal in-situ conversion, the first step is to Figure 1Injection well 1 injects nitrogen at 500°C to preheat the oil shale reservoir, and then injects air at room temperature to cool the bottom temperature of the injection well to 300°C. After cooling to 300°C, air and natural gas are injected through a double-layer oil pipe. According to the temperature profile and oxygen concentration differences in the process of advancing the reaction zone, a chemical reaction zone consisting of a residue zone 2, an autogenous heat zone 3, a thermal cracking zone 4 and a preheating zone 5 is established in the oil shale reservoir between injection well 1 and production well 6. Finally, oil and gas are produced through production well 6. Figure 2 The development well pattern shown is an inverted seven-point well pattern consisting of seven vertical wells, with injection well 1 in the center and production wells 6 on the periphery. Double-layer oil pipe is used as injection pipeline, such as Figure 3 and Figure 4 As shown, the double-layer oil pipe includes an inner pipe and an outer pipe coaxially arranged outside the inner pipe. The annular cavity formed by the outer pipe and the inner pipe serves as a natural gas channel 7 for injecting natural gas, and the inner cavity of the inner pipe serves as an air channel 8 for injecting air. The double-layer oil pipe is used as an injection pipeline with an outer diameter of 150 mm and an inner diameter of 125 mm. Natural gas accounts for a small proportion of the injected gas, so it is injected through the outer pipe, and air is injected through the inner pipe to prevent the two from mixing in the double-layer oil pipe and reduce the risk of explosion. Located at the bottom of the injection well 1, a check valve 9 is installed at the end of the double-layer oil pipe to prevent natural gas and air from mixing in the wellbore of the injection well 1.

[0019] Figure 5 The flow chart of the natural gas-assisted oil shale autothermal in-situ conversion development method is shown. The natural gas-assisted oil shale autothermal in-situ conversion development method specifically includes the following steps: In the first step, the shale in the Songliao Basin of China was selected as the research object. Its original effective porosity is 6.40%, the reservoir is 100% water-containing in the original state, and the thermal conductivity and heat capacity of the bedrock are 7.2×10 -6 J / (m•d•℃) and 6.5×10 6 J / (m 3 / ℃), the mass fraction of carbon in the reservoir kerogen is 0.71, and the molecular weight is 14.7 g / mol. The specific parameters of the oil shale reservoir are as follows: the initial reservoir pressure is 650kPa, the initial reservoir temperature is 35℃, the initial reservoir permeability is 0.05md, the initial reservoir porosity is 6.40%, and the rock specific heat capacity is 6.5×106J / (m 3 / ℃), the thermal conductivity of rock is 1.84×10 5 J / (m•d•℃), the thermal expansion coefficient of rock is 7.2×10 -6 / ℃, the rock matrix density is 2400kg / m 3 The initial fluid saturation in the matrix is ​​100%, and the reservoir bedrock density is 2019.84 kg / m 3 The oil shale reservoir conditions are that the oil content of the oil shale is 2.9%~9.8%.

[0020] In the second step, this example uses the CMG numerical simulation software STARSTM module to numerically simulate the in-situ development process of oil shale self-heating. The CMG numerical simulation software STARSTM module belongs to the existing public technology and will not be elaborated in detail here. An anti-seven-point well network consisting of seven vertical wells was established. In order to reduce the amount of calculation, under the premise of ensuring the accuracy of model calculation and the reliability of experimental results, according to the principle of symmetry, 1 / 6 of a well network layout unit was taken, including an injection well 1 and two production wells 6. The simulated well spacing was 50 m, so that the preheating effect was optimal, and the mining area was 1082.54m 2 In this example, the heat loss of the top and bottom plates is ignored because the thicker the development layer, the smaller the impact of heat loss, which is not discussed in this example.

[0021] In the third step, due to the compactness of the oil shale reservoir, the autothermal in-situ mining must be combined with the fracturing technology. The volume fracturing method and the shock wave fracturing method are used to transform the oil shale reservoir in turn to form a fracture network. The oil shale reservoir after the reservoir transformation is assumed to be a dual-porosity and dual-permeability system, with a matrix permeability of 0.1 md and a fracture permeability of 100 md. Based on the standard Cartesian coordinate system, the finite difference discretization method is used to form a geological model with a grid size of 0.5m×0.5m×0.5m.

[0022] The fourth step is to start gas injection. Each group of experiments uniformly injects 500°C high-temperature nitrogen into the injection well for preheating. The injection time is 90 days, where d represents the time unit day; ensure that the reservoir temperature near injection well 1 exceeds the trigger temperature of the oil shale self-heating in-situ conversion technology to meet the triggering requirements of the oil shale self-heating in-situ conversion technology development. The gas injection method of injection well 1 is constant pressure injection, and the injection pressure is set to 10MPa. The wellhead pressure of production well 6 is set to 0MPa, and the bottom hole pressure of production well 6 is set to 200kPa.

[0023] The fifth step is to inject air at room temperature into the injection well 1 to cool the bottom temperature of the injection well 1 to 300°C.

[0024] Step 6: Inject room temperature air into injection well 1 and mix it with natural gas in different proportions according to the experimental design; the injection volume of the air and natural gas mixture is 200m 3 / (h•m)~500m 3 / (h•m), as oil production increases, the gas injection volume increases from 200m 3 / (h•m) gradually increases to 500m 3 / (h•m), as the oil production decreases, the gas injection volume increases from 500m 3 / (h•m) gradually decreases to 200m 3 / (h•m) to improve gas utilization and enable the autogenous heat reaction to proceed smoothly. The oil shale triggers the autogenous heat reaction, and with the assistance of natural gas, the residue produced after kerogen thermal cracking undergoes oxidation reaction to release heat, realizing convection heating of the oil shale reservoir, and the oil and gas products produced by kerogen thermal cracking are produced from production well 6. Specific injection and production parameters: The oil content of low-oil shale is 2.9%, the oil content of medium-oil shale is 5.0%, and the oil content of high-oil shale is 9.8%. When injecting air, 0%, 2.0%, and 4.0% natural gas are mixed in respectively. The preheating temperature is 500 °C and the preheating time is 90 days. Figure 6 It shows the time variation of the production of low oil content oil shale mixed with different proportions of natural gas and oil; Figure 7 The graph shows the change of energy return rate over time for low oil content oil shale mixed with different proportions of natural gas; Figure 6 and Figure 7 L1, L2, and L3 represent low-oil-content oil shale mined with 0%, 2.0%, and 4.0% natural gas, respectively. Figure 8 It shows the time variation of oil production of medium oil content oil shale mixed with different proportions of natural gas and oil; Fig. 9 The graph shows the change of energy return rate over time for oil shale with medium oil content mixed with different proportions of natural gas; Figure 8 and Fig. 9 M1, M2, and M3 respectively represent medium-oil-content oil shale mined with 0%, 2.0%, and 4.0% natural gas mixed in. Fig.10 It shows the time variation of the production of high oil content oil shale mixed with different proportions of natural gas and oil; Fig.11 The graph shows the change of energy return rate over time for high oil content oil shale mixed with different proportions of natural gas; Fig.10 and Fig.11 H1, H2, and H3 represent high-oil-content oil shale mining mixed with 0%, 2.0%, and 4.0% natural gas, respectively; Fig.12 The graph shows the variation of the injection amount of natural gas and air mixture over time.

[0025] The experimental results are as follows: 1. For low-oil-content oil shale reservoirs, the oil shale autothermal in-situ conversion technology based on air injection cannot be successful due to insufficient heat supply from oxidation of reservoir organic matter. However, mixing a small amount of natural gas can assist the successful in-situ conversion of autothermal heat, and the energy return rate can reach 3.70. However, the higher the natural gas content, the better. There is an optimal value of 2.0%. A higher natural gas content reduces the energy return rate and increases the development time.

[0026] 2. For medium-to-high oil content oil shale reservoirs, natural gas-assisted oil shale autothermal in-situ conversion technology can significantly reduce the compression energy input during the oil production stage, thereby reducing the overall energy return rate. The energy return rate of a 5.0% oil content reservoir mixed with 2.0% natural gas reaches 8.11, which is 29.47% higher than that of unmixed natural gas. The energy return rate of a 9.8% oil content reservoir mixed with 4.0% natural gas reaches 13.04, which is 19.19% higher than that of unmixed natural gas.

[0027] 3. Mixed natural gas development will aggravate the reaction loss of oil and gas. In normal development, the natural gas-assisted oil shale autogenous heat in situ conversion technology with high oil content in reservoirs will cause an additional loss of 6.62%~8.22% of oil production compared with the oil shale autogenous heat in situ conversion technology, but the comprehensive evaluation of the energy return rate will still increase.

[0028] In summary, the present invention proposes a natural gas-assisted autogenous heat in-situ conversion technology suitable for the development of low-oil-content oil shale. The present invention arranges an inverted seven-point well pattern in the target area for the autogenous heat in-situ conversion of natural gas-assisted oil shale, and transforms the reservoir in the target area to form a fracture network. The oil shale reservoir is locally preheated by injecting 500°C high-temperature nitrogen through injection well 1, and then normal-temperature air is injected into injection well 1 to cool the bottom of injection well 1, so that the bottom temperature of the injection well is cooled to 300°C. Finally, the normal-temperature air and natural gas are separately injected into the bottom of injection well 1 by a double-layer oil pipe. With the assistance of natural gas, the residues produced after the thermal cracking of kerogen are oxidized to release heat to heat the oil shale reservoir. At the same time, natural gas oxidation can effectively supplement the lack of heat released by oxidation of the residues in the low-oil-content oil shale reservoir, ensure the successful triggering and operation of the autogenous heat reaction, and achieve efficient production of oil and gas products from the cracking of the oil shale reservoir.

[0029] Of course, the above description is not a limitation of the present invention, and the present invention is not limited to the above examples. Changes, modifications, additions or substitutions made by technicians in this technical field within the essential scope of the present invention should also fall within the protection scope of the present invention.

Claims

1. A natural gas-assisted oil shale autothermal in-situ conversion development method, characterized in that: The method comprises the following steps: Step 1: Determine the target area for natural gas-assisted oil shale autothermal in-situ conversion development, where the reservoir condition of the target area for natural gas-assisted oil shale autothermal in-situ conversion development is that the oil content of the oil shale is 2.9% to 9.8%; Step 2: arranging a development well pattern in the target area for natural gas-assisted oil shale autothermal in-situ conversion development, wherein the development well pattern adopts an inverted seven-spot well pattern, and the ratio of injection wells to production wells in the development well pattern is 1:6; Step 3: Reservoir transformation is carried out in the target area for natural gas-assisted oil shale autothermal in-situ conversion development to form a fracture network with a matrix permeability of 0.1 md and a fracture permeability of 100 md; Step 4: After the reservoir is transformed, high-temperature nitrogen at 500°C is injected into the injection well for 90 days to preheat the area near the injection well. Step 5: injecting room temperature air into the injection well to cool the bottom temperature of the injection well to 300°C; Step 6: Use double-layer tubing as the injection pipeline to separately inject room temperature air and natural gas into the bottom of the injection well. At the bottom of the injection well, the air and natural gas are mixed to form an air and natural gas mixture, and the proportion of natural gas in the total amount of air and natural gas mixture is less than or equal to 4%. The injection volume of the air and natural gas mixture is 200m 3 / (h•m)~500m 3 / (h•m), after the introduction of air and natural gas, the oil shale triggers a self-heating reaction. With the assistance of natural gas, the residues produced after the thermal cracking of kerogen undergo an oxidation reaction to release heat, thereby achieving convection heating of the oil shale reservoir. The oil and gas products produced by the thermal cracking of kerogen are produced from the production wells.

2. The natural gas-assisted oil shale autothermal in-situ conversion development method according to claim 1, characterized in that: The development well network is an inverted seven-point well network consisting of seven vertical wells, and the inverted seven-point well network includes an injection well located at the center of the hexagon and production wells located at the six vertices of the hexagon.

3. The natural gas-assisted oil shale autothermal in-situ conversion development method according to claim 1, characterized in that: The well spacing of the reverse seven-spot well pattern is 50m.

4. The natural gas-assisted oil shale autothermal in-situ conversion development method according to claim 1, characterized in that: In step 3, the volume fracturing method and the shock wave fracturing method are used in sequence to transform the oil shale reservoir to form a fracture network.

5. The natural gas-assisted oil shale autothermal in-situ conversion development method according to claim 1, characterized in that: In step 6, natural gas is mixed into the air at room temperature, and the proportion of natural gas in the total amount of the mixture of air and natural gas is 2%.

6. The natural gas-assisted oil shale autothermal in-situ conversion development method according to claim 1, characterized in that: The gas injection method of the injection well is constant pressure injection, the injection pressure is set to 10MPa, the wellhead pressure of the production well is set to 0MPa, and the bottom hole pressure of the production well is set to 200kPa.

7. The natural gas-assisted oil shale autothermal in-situ conversion development method according to claim 1, characterized in that: In step 6, as the oil production increases, the gas injection volume increases from 200m 3 / (h•m) gradually increases to 500m 3 / (h•m), as the oil production decreases, the gas injection volume increases from 500m 3 / (h•m) gradually decreases to 200m 3 / (h•m).

8. The natural gas-assisted oil shale autothermal in-situ conversion development method according to claim 1, characterized in that: The outer diameter of the double-layer oil pipe is 150 mm, and the inner diameter is 125 mm.

9. The natural gas-assisted oil shale autothermal in-situ conversion development method according to claim 1, characterized in that: A check valve is installed on the double-layer oil pipe at the bottom of the injection well.

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