Carbon dioxide gas energizing fracturing method

Abstract

The invention provides a carbon dioxide gas energized fracturing method, which relates to the technical field of oil-gas field development, and comprises the following steps: S1, injecting carbon dioxide gas in a supercritical state into a compact sandstone reservoir; s2, the pressure intensity of the injected carbon dioxide is adjusted, the pressure of the stratum is increased, primary fractures are promoted to expand, and new fractures are formed; s3, the pressure intensity of the injected carbon dioxide is adjusted, pressure recovery is carried out after a new crack is formed, and tight sandstone gas in the expanded primary crack and the new crack is replaced by carbon dioxide gas; and S4, stopping injecting the carbon dioxide gas into the tight sandstone reservoir after the tight sandstone gas energized fracturing and recovery, so that the carbon dioxide gas is partially sealed and stored in pores and cracks of the tight sandstone gas reservoir. The method has the advantages that release and flowing of tight sandstone gas are promoted, the natural gas production period can be prolonged, the production capacity of a natural gas reservoir is maintained, and therefore the recovery rate of the sandstone gas is increased.

Description

Technical Field

[0001] The invention relates to the technical field of oil and gas field development, and in particular to a carbon dioxide gas energized fracturing method. Background Art

[0002] Tight sandstone gas is the main type of unconventional natural gas. Due to the poor physical properties of tight sandstone gas reservoirs, the overall characteristics are "four lows, two highs, and one strong", namely low porosity, low permeability, low reserve abundance, low natural production per well, high water saturation, high development cost, and strong reservoir heterogeneity. These characteristics make the recovery rate of tight sandstone gas much lower than that of conventional gas reservoirs, and it is urgent to tackle the problem of improving the recovery rate. Traditional water-based fracturing fluids have problems such as incomplete gel breaking, incomplete flowback, and large retention in the formation, which seriously damages the formation. Therefore, it is difficult for traditional fracturing technology to effectively improve the recovery rate, and existing technologies need to be improved.

[0003] At the same time, as a greenhouse gas, the control of carbon dioxide emissions has received increasing attention. Carbon dioxide geological storage is an important technical means to reduce carbon dioxide emissions. This technology refers to capturing carbon dioxide that would otherwise be emitted into the atmosphere, injecting it into underground geological structures after treatment, and sealing it in geological bodies through a series of rock physical binding, dissolution and mineralization. This technology can achieve increased production of oil and natural gas resources while sealing carbon dioxide, and can achieve the unity of economic and environmental benefits. Summary of the invention

[0004] In view of this, in order to solve the problems of incomplete gel breaking, incomplete flowback, and large retention in the formation of traditional water-based fracturing fluids, an embodiment of the present invention provides a carbon dioxide gas energized fracturing method.

[0005] An embodiment of the present invention provides a carbon dioxide gas energized fracturing method, comprising the following steps:

[0006] S1. Injecting supercritical carbon dioxide gas into a tight sandstone reservoir;

[0007] S2. Adjust the pressure of injected carbon dioxide to increase the formation pressure, promote the expansion of primary fractures, and form new fractures;

[0008] S3. Adjust the pressure of the injected carbon dioxide, utilize the expansion characteristics of carbon dioxide gas, perform pressure recovery after the new fractures are formed, enhance the effective pressure of the tight sandstone reservoir, and replace the tight sandstone gas in the expanded primary fractures and the new fractures with carbon dioxide gas;

[0009] S4. After the tight sandstone gas energy fracturing and recovery, the injection of carbon dioxide gas into the tight sandstone reservoir is stopped, so that part of the carbon dioxide gas is sealed in the pores and cracks of the tight sandstone gas reservoir.

[0010] Furthermore, the step S3 also includes reducing the influence of residual water in the tight sandstone reservoir and lowering the resistance effect of water through the flushing and replacement effect of carbon dioxide gas.

[0011] Furthermore, the pressure of the carbon dioxide gas in the supercritical state is greater than 7.38 MPa and the temperature is greater than 31.1°C.

[0012] Furthermore, in step S2, the pressure of the supercritical carbon dioxide gas injected when the primary fractures expand and new fractures are formed is greater than the formation pressure.

[0013] Furthermore, in step S3, the tight sandstone gas is CH4.

[0014] Furthermore, the injection amount and pressure of the supercritical carbon dioxide gas injected in steps S1, S2 and S3 are obtained by simulation under laboratory conditions.

[0015] Furthermore, the injection amount and pressure of the supercritical carbon dioxide gas injected in steps S1, S2 and S3 are obtained according to the following method:

[0016] S100, prepare representative tight sandstone samples, the tight sandstone samples have the same physical properties as tight sandstone reservoirs;

[0017] S200, placing the tight sandstone sample into a container that can simulate underground conditions of a tight sandstone reservoir;

[0018] S300, respectively controlling the pressure and water content in the dense sandstone sample, gradually injecting different amounts of supercritical carbon dioxide into the dense sandstone sample, and simultaneously monitoring changes in carbon dioxide pressure and fluid flow;

[0019] S400, Evaluating Gas Recovery: Measuring CO2 Injection into Tight Sandstone Samples

[0020] The change in gas volume before and after is used to evaluate the gas recovery rate;

[0021] S500, evaluating carbon dioxide storage efficiency: evaluating the storage efficiency by monitoring the difference between the amount of injected carbon dioxide and the amount of original carbon dioxide in the tight sandstone sample;

[0022] S600, analyzing the monitoring experimental data to determine the relationship between the carbon dioxide injection amount, pressure and gas recovery rate and storage efficiency under different conditions.

[0023] Furthermore, the physical properties include porosity and permeability.

[0024] Furthermore, the container is an autoclave.

[0025] The beneficial effects brought about by the technical solution provided by the embodiment of the present invention are:

[0026] 1. A carbon dioxide gas energized fracturing method of the present invention first injects carbon dioxide gas in a supercritical state into a dense sandstone reservoir, and utilizes the physical properties of low viscosity and high diffusivity of the carbon dioxide gas in a supercritical state to make it penetrate into tiny cracks, so that the original cracks expand and new cracks are formed at the same time, thereby forming a complex crack network, increasing the channels for gas flow, and thus improving the gas recovery rate; then, using the expansion characteristics of the carbon dioxide gas, the pressure is restored after the crack network is formed, the effective pressure of the dense sandstone reservoir is enhanced, and the release and flow of the dense sandstone gas are further promoted, which can extend the natural gas production cycle, maintain the production capacity of the natural gas reservoir, and thus improve the recovery rate of the sandstone gas.

[0027] 2. In the carbon dioxide gas energized fracturing method of the present invention, after carbon dioxide is injected, it is adsorbed on the rock surface or dissolved in formation water due to its adsorption and dissolution effects, which contributes to the long-term geological storage of carbon dioxide. At the same time, it can improve the sealing of gas reservoirs, prevent gas leakage, and extend the production cycle; at the same time, the stored carbon dioxide can reduce greenhouse gas emissions and has good environmental benefits.

[0028] 3. In the carbon dioxide gas energized fracturing method of the present invention, the viscosity of carbon dioxide is lower than that of water, so it can flow more easily in the pores. Under the same conditions, it is not easy to form water locks at the pore throats. Compared with traditional water-based fracturing fluids, carbon dioxide gas reduces reservoir damage, avoids water lock effects, and further improves gas recovery rates. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 This is a working schematic diagram of a carbon dioxide gas energized fracturing method of the present invention;

[0030] Figure 2 It is a working principle diagram of a carbon dioxide gas energized fracturing method of the present invention. DETAILED DESCRIPTION

[0031] In order to make the purpose, technical solution and advantages of the present invention clearer, the embodiments of the present invention will be further described below in conjunction with the accompanying drawings. The following is a preferred embodiment of the present invention, which is intended to provide a basic understanding of the present invention, but is not intended to confirm the key or decisive elements of the present invention or to limit the scope of protection.

[0032] In all examples shown and discussed herein, any specific values ​​should be interpreted as merely exemplary and not as limiting. Therefore, other examples of the exemplary embodiments may have different values.

[0033] Technologies, methods, and equipment known to ordinary technicians in the relevant art may not be discussed in detail, but where appropriate, the technologies, methods, and equipment should be considered part of the specification.

[0034] It should be noted that similar reference numerals and letters represent similar items in the following drawings, so once an item is defined in one drawing, it does not need to be further discussed in the subsequent drawings. At the same time, it should be understood that for ease of description, the sizes of the various parts shown in the drawings are not drawn according to the actual proportional relationship.

[0035] It should be noted that, unless otherwise clearly specified and limited, the terms "installation" and "connection" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection or an indirect connection through an intermediate medium, or it can be the internal communication of two components. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0036] Please refer to Figure 1 and 2 The embodiment of the present invention provides a carbon dioxide gas energized fracturing method, which can improve the recovery rate of low-pressure water-containing tight sandstone gas and realize the effective storage of carbon dioxide, and specifically includes the following steps:

[0037] S1. Inject supercritical carbon dioxide gas into tight sandstone reservoirs.

[0038] The supercritical carbon dioxide gas generally has a pressure greater than 7.38MPa and a temperature greater than 31.1℃. Supercritical carbon dioxide gas is injected into a tight sandstone gas reservoir, and its physical and chemical properties are used to penetrate into the micro-cracks of the reservoir under high pressure conditions. Supercritical carbon dioxide gas has special physical properties such as low viscosity and high diffusivity, and can penetrate into the cracks in tight sandstone reservoirs.

[0039] S2. Adjust the pressure of injected carbon dioxide to increase the formation pressure, promote the expansion of primary fractures, and form new fractures.

[0040] The pressure of the injected carbon dioxide is adjusted so that the pressure of the injected supercritical carbon dioxide gas is greater than the formation pressure when the original fractures expand and new fractures are formed. The injection of carbon dioxide gas will increase the pressure of the formation, and the expansion effect of carbon dioxide gas will promote the expansion of fractures. It will not only penetrate into the original fractures to expand the original fractures, but also form new fractures in the tight sandstone reservoir, forming a complex fracture network, thereby increasing the flow path of gas and the recovery rate.

[0041] S3. Adjust the pressure of the injected carbon dioxide, utilize the expansion characteristics of carbon dioxide gas, restore the pressure after the new fractures are formed, enhance the effective pressure of the tight sandstone reservoir, and replace the tight sandstone gas in the expanded primary fractures and the new fractures with carbon dioxide gas.

[0042] By utilizing the expansion characteristics of carbon dioxide gas, pressure recovery is carried out after the fractures expand to form a complex fracture network, thereby enhancing the effective pressure of the reservoir and further promoting the release and flow of tight sandstone gas.

[0043] Tight sandstone gas is CH4. In tight sandstone gas reservoirs, since the seepage of fluids in sandstone micropores is different from that of fluids in conventional reservoirs, the interaction between CO2 and CH4 will make the seepage mechanism more complicated when CO2 is injected into the tight pores of sandstone. Studies have shown that when CO2 exists, CO2 can replace about half of CH4. After competitive adsorption, the total adsorption of CO2 is greater than that of CH4. Therefore, the injection of CO2 can promote the desorption of CH4, thereby improving the recovery rate of sandstone gas.

[0044] In addition, the impact of residual water in tight sandstone reservoirs can be reduced through the flushing and replacement of carbon dioxide gas, reducing the resistance effect of water. Carbon dioxide has a lower viscosity than water, so it can flow more easily in pores. Under the same conditions, it is not easy to form water locks at the pore throats. Compared with traditional water-based fracturing fluids, carbon dioxide gas reduces reservoir damage, avoids water lock effects, and further improves gas recovery.

[0045] S4. After the tight sandstone gas energy fracturing and recovery, the injection of carbon dioxide gas into the tight sandstone reservoir is stopped, so that part of the carbon dioxide gas is sealed in the pores and cracks of the tight sandstone gas reservoir.

[0046] After gas energized fracturing and recovery, part of the injected carbon dioxide is sealed in the pores and cracks of the tight sandstone gas reservoir, forming a long-term and stable carbon sequestration effect. Part of the carbon dioxide is adsorbed on the rock surface, and part of the carbon dioxide is dissolved in the formation water. These effects can improve the sealing of the gas reservoir and achieve long-term storage of carbon dioxide. At the same time, the stored carbon dioxide can reduce greenhouse gas emissions and has good environmental benefits.

[0047] It should be noted that the injection conditions of supercritical carbon dioxide gas in steps S1, S2 and S3 need to be adapted to local conditions and determined according to the actual situation of the tight sandstone reservoir to achieve the effect of promoting carbon dioxide diffusion and gas replacement, so as to achieve high natural gas production.

[0048] The injection amount and pressure of the supercritical carbon dioxide gas injected in steps S1, S2 and S3 can be simulated under laboratory conditions, and the specific method is as follows:

[0049] S100, preparing a representative tight sandstone sample, wherein the tight sandstone sample has physical properties consistent with the tight sandstone reservoir, including porosity and permeability.

[0050] S200, placing the dense sandstone sample into a container that can simulate underground conditions of the dense sandstone reservoir. The container can be an autoclave to simulate underground conditions, including temperature, pressure, and fluid flow.

[0051] S300, respectively controlling the pressure and water content in the dense sandstone sample, gradually injecting different amounts of supercritical carbon dioxide into the dense sandstone sample, and simultaneously monitoring changes in carbon dioxide pressure and fluid flow;

[0052] S400, assessing gas recovery rate: assessing gas recovery rate by measuring the change in gas volume before and after carbon dioxide is injected into the dense sandstone sample. The dense sandstone sample is decompressed to release the adsorbed gas, and the amount of released gas is measured, so that the gas recovery rate can be calculated.

[0053] S500, evaluate the CO2 storage efficiency: evaluate the storage efficiency by monitoring the difference between the amount of injected CO2 and the amount of original CO2 in the tight sandstone sample. Use gas analysis technology to determine the amount of CO2 that is not stored, so that the CO2 storage efficiency can be calculated.

[0054] S600. Analyze the monitoring experimental data to determine the relationship between the carbon dioxide injection amount, pressure, gas recovery rate and storage efficiency under different conditions. Use statistics and mathematical models to analyze the experimental data to determine the relationship between the carbon dioxide injection amount, gas recovery rate and storage efficiency under different conditions, so as to select the injection conditions of supercritical carbon dioxide gas during energy fracturing of actual tight sandstone reservoirs.

[0055] Under appropriate pressure and water content conditions, there is a certain relationship between the injection amount of carbon dioxide gas and the gas recovery rate and storage efficiency. By controlling the pressure, water content, carbon dioxide gas injection amount and other conditions, the gas recovery rate and carbon dioxide gas storage efficiency can be improved to a certain extent.

[0056] A carbon dioxide gas energized fracturing method of the present invention has been applied to the Sulige gas field in Ordos, Inner Mongolia. The physical characteristics of the tight sandstone reservoir of the gas field are as follows: the middle depth of the gas layer is 3370m, the thickness of the producing gas layer + the gas-bearing layer is 8.4m, the average porosity is 9.5%, the average permeability is 2.7mD, the average gas saturation is 67.7%, the average movable water saturation is 14.1%, the mud content is 9.2%, the predicted formation pressure is 28.6MPa, and the reservoir temperature is 110.4℃. The specific implementation steps are as follows: (1) Through high temperature and high pressure experiments, test the influence of confining pressure and pore pressure at different temperatures on the elastic modulus, compressive strength, Poisson's ratio and brittleness index of tight sandstone, and determine the key coefficients such as the fracture pressure of the area (the fracture pressure of the area is 24MPa); (2) Inject supercritical carbon dioxide gas into the reservoir (pressure greater than 7.38MPa, temperature greater than 31.1℃); (3) Adjust the pressure so that the pressure is greater than the test fracture initiation pressure (that is, greater than 24MPa) to achieve crack expansion and new crack formation; (4) Wait until the formation pressure state returns to stability (the pressure is 35MPa at this time), stop injecting carbon dioxide, and the replacement of tight sandstone gas and partial storage of carbon dioxide can be achieved. The maximum gas production after field compression is 2.4×104m 3 / d, and the gas production at the end of the first year remained at 1.0×104m 3 / d, achieving good results in increasing and stabilizing production.

[0057] Field tests showed that this method reduced water consumption by 80% compared with water-based fracturing, and the backflow rate after fracturing was 50-70%. Application results showed that this method significantly reduced water consumption, achieved environmental benefits, and saved water resources; at the same time, the backflow rate was 10 percentage points higher, with significant results.

[0058] On the other hand, the embodiments of the present invention also use an indoor experimental device to verify the carbon dioxide gas energized fracturing method, as follows:

[0059] The maximum temperature of the currently used fracturing experiment is 300℃, and the maximum applied stress is 52.5MPa. Through certain indoor experimental devices, we can feel the role of this technology in achieving gas recovery and carbon dioxide geological storage. The indoor experiment was carried out under the conditions of confining pressure of 38MPa and temperature of 85℃, and the tight sandstone core of Sugeli gas field was selected for the experiment. The experiment used a rock triaxial test device and a carbon dioxide booster device to monitor the carbon dioxide injection pressure and the stress of the rock after carbon dioxide injection. The specific experimental steps are as follows: (1) The confining pressure and temperature are adjusted to the condition value (confining pressure 38MPa, temperature 85℃) through the experimental device; (2) Liquid carbon dioxide is converted into a supercritical state (pressure greater than 7.38MPa, temperature greater than 31.1℃) through the booster device and injected into the core; (3) The injection pressure is continuously increased to reach the set pore pressure value; (4) The injection is stopped until the core is damaged; (5) In this stage, the stress-strain data is collected through the detection software to obtain the stress-strain curve. The experiment showed that the brittleness index of tight sandstone showed a trend of first increasing and then decreasing with the increase of injection pressure, indicating that after the carbon dioxide fully contacted the tight sandstone in the early stage, the brittleness of the tight sandstone could be enhanced. The enhanced brittleness of the rock indicates that the rock is easy to break, realizing the expansion of the original pores and the formation of new cracks, establishing new channels for the flow of tight sandstone gas, and also providing space for the storage of carbon dioxide, indicating that the application of this method in tight sandstone reservoirs is feasible and can achieve the expected effect.

[0060] In this article, the directional words such as front, back, top, and bottom involved are defined by the positions of the parts in the drawings and the positions of the parts relative to each other, just for the sake of clarity and convenience in expressing the technical solution. It should be understood that they are relative concepts and can change accordingly according to different ways of use and placement, and the use of the directional words should not limit the scope of protection claimed in this application.

[0061] In the absence of conflict, the above embodiments and features in the embodiments may be combined with each other. The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A carbon dioxide gas energized fracturing method, characterized in that: The following steps are involved: S1. Injecting supercritical carbon dioxide gas into a tight sandstone reservoir; S2. Adjust the pressure of injected carbon dioxide to increase the formation pressure, promote the expansion of primary fractures, and form new fractures; S3. Adjust the pressure of the injected carbon dioxide, utilize the expansion characteristics of carbon dioxide gas, perform pressure recovery after the new fractures are formed, enhance the effective pressure of the tight sandstone reservoir, and replace the tight sandstone gas in the expanded primary fractures and the new fractures with carbon dioxide gas; S4. After the tight sandstone gas energy fracturing and recovery, the injection of carbon dioxide gas into the tight sandstone reservoir is stopped, so that part of the carbon dioxide gas is sealed in the pores and cracks of the tight sandstone gas reservoir.

2. A carbon dioxide gas energized fracturing method according to claim 1, characterized in that: The step S3 also includes reducing the influence of residual water in the tight sandstone reservoir and lowering the resistance effect of water through the flushing and replacement effect of carbon dioxide gas.

3. A carbon dioxide gas energized fracturing method according to claim 1, characterized in that: The pressure of carbon dioxide gas in the supercritical state is greater than 7.38MPa and the temperature is greater than 31.1°C.

4. A carbon dioxide gas energized fracturing method according to claim 1, characterized in that: In step S2, the pressure of the supercritical carbon dioxide gas injected when the primary fractures expand and new fractures are formed is greater than the formation pressure.

5. A carbon dioxide gas energized fracturing method according to claim 1, characterized in that: In step S3, the tight sandstone gas is CH4.

6. A carbon dioxide gas energized fracturing method according to claim 1, characterized in that: The injection amount and pressure of the supercritical carbon dioxide gas injected in steps S1, S2 and S3 are obtained by simulation under laboratory conditions.

7. A carbon dioxide gas energized fracturing method as claimed in claim 6, characterized in that: The injection amount and pressure of the supercritical carbon dioxide gas injected in steps S1, S2 and S3 are obtained as follows: S100, prepare representative tight sandstone samples, the tight sandstone samples have the same physical properties as tight sandstone reservoirs; S200, placing the tight sandstone sample into a container that can simulate underground conditions of a tight sandstone reservoir; S300, respectively controlling the pressure and water content in the dense sandstone sample, gradually injecting different amounts of supercritical carbon dioxide into the dense sandstone sample, and simultaneously monitoring changes in carbon dioxide pressure and fluid flow; S400, assessing gas recovery rate: assessing gas recovery rate by measuring the change in gas volume before and after carbon dioxide injection into tight sandstone samples; S500, evaluating carbon dioxide storage efficiency: evaluating the storage efficiency by monitoring the difference between the amount of injected carbon dioxide and the amount of original carbon dioxide in the tight sandstone sample; S600, analyzing the monitoring experimental data to determine the relationship between the carbon dioxide injection amount, pressure and gas recovery rate and storage efficiency under different conditions.

8. A carbon dioxide gas energized fracturing method as claimed in claim 7, characterized in that: The physical properties include porosity and permeability.

9. A carbon dioxide gas energized fracturing method according to claim 7, characterized in that: The container is an autoclave.

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