CN113309519B - An anhydrous fracturing method for shale with liquid nitrogen pre-fracturing followed by nitrogen fracturing - Google Patents

Abstract

The invention discloses a shale anhydrous fracturing method for liquid nitrogen presplitting and then nitrogen fracturing, and belongs to the technical field of shale gas reservoir fracturing. Firstly, performing circulating cold impact on the periphery of a drilled hole of a preset operation shale gas reservoir section by using liquid nitrogen to generate an initial damage crack on the reservoir; and the nitrogen is generated by vaporization expansion in the liquid nitrogen cold impact process and stored in a gas storage tank for later use as the post-fracturing fluid. After the low-temperature cooling stage is completed, nitrogen is converted into high-pressure nitrogen fracturing fluid through a supercharging device and is injected into the shale which has generated initial damage, the stewing operation is carried out on the target shale gas reservoir, and after the pressure tends to be stable, the nitrogen begins to be discharged. The method has the advantages that the initial damage cracks are induced by the strong liquidity and low-temperature damage characteristics of the liquid nitrogen, and then the cracks of the shale gas reservoir are expanded by the characteristic of strong penetrating power of the nitrogen, so that a complex crack network is constructed, and the environmental problems caused by the water-based fracturing fluid and the water lock problem in the exploitation process are solved.

Description

An anhydrous fracturing method for shale with liquid nitrogen pre-fracturing followed by nitrogen fracturing

technical field

The invention relates to the technical field of fracturing of shale gas reservoirs, in particular to an anhydrous fracturing method for shale gas which is suitable for increasing the production of low permeability and low porosity oil and gas reservoirs with liquid nitrogen pre-fracturing followed by nitrogen fracturing.

Background technique

Hydraulic fracturing technology can effectively improve the oil and gas production of shale reservoirs with low permeability and low porosity by affecting the development degree of shale fractures. Because water molecules cannot be efficiently injected into low-permeability shale reservoirs, the development effect cannot meet expectations. Long-term injection of water-based fracturing fluid into reservoirs will cause hydration, expansion and dispersion of downhole clay minerals, resulting in water lock damage. The thickener vegetable gum in water-based fracturing fluid cannot be completely discharged from the formation, which will cause serious reservoir pollution. Therefore, it is proposed in the industry to fracturing shale reservoirs after pressurizing liquid nitrogen. Considering that the fracturing process time is short, the thermal conductivity of the rock is low, and the influence of liquid nitrogen cooling on the reservoir is small. At the same time, because the liquid nitrogen is in a non-flowing state and the temperature of the liquid nitrogen increases after convective heat exchange with the reservoir, the resulting temperature gradient becomes smaller, resulting in a decrease in the degree of damage to the reservoir caused by temperature stress. In view of the above problems, it is proposed to treat the reservoir with liquid nitrogen as a pre-fracturing fluid, and then fracturing the reservoir with nitrogen gas. Some studies have shown that the mechanical properties of shale will change when the temperature difference is greater than 200 °C. Since the temperature of liquid nitrogen can reach minus 196 °C, the temperature difference with the shale gas reservoir can reach up to 300 °C. The coolant creates a shale fracture network, and then uses nitrogen as a post-anhydrous fracturing fluid to enrich the existing shale fracture network, so as to increase the oil and gas production of shale reservoirs in a green and environmentally friendly way.

SUMMARY OF THE INVENTION

Technical problem: The purpose of the present invention is to solve the environmental problems caused by water-based fracturing fluid and the water lock phenomenon in the process of shale gas exploitation, and to provide anhydrous fracturing of shale with liquid nitrogen pre-fracturing followed by nitrogen fracturing The method uses the low temperature characteristics of liquid nitrogen to cause damage to fractures around the reservoir borehole, and then generates a complex fracture network in the shale reservoir near the borehole through the penetrating ability of nitrogen gas, so as to ensure the construction scale and improve the anhydrous pressure at the same time. The problem of crack propagation in cracked reservoirs is increased, and the production of shale gas is increased.

Technical scheme: In order to achieve the above purpose, a method for anhydrous shale fracturing of liquid nitrogen pre-fracturing followed by nitrogen fracturing of the present invention includes the use of a liquid nitrogen pre-fracturing damage circulation system and a nitrogen pressure boosting system. The nitrogen pre-fracture damage circulation system includes a nitrogen liquefaction production device, a liquid nitrogen storage container and a liquid nitrogen circulation pre-fracture pipeline, wherein the liquid nitrogen circulation pre-fracture pipeline includes a horizontal well fracturing pipeline, a shale gas horizontal production well, a production Well exhaust valve, gas recovery device, nitrogen discharge pipeline, nitrogen intake valve, gas-liquid transportation pipeline, liquid nitrogen intake valve, liquid nitrogen discharge valve, fracturing fluid sand-carrying mixing device and fracturing outlet fluid control valve The nitrogen pressurization system includes a pressurization device, a high-pressure nitrogen container and a shut-in fracturing pipeline; the well shut-in and fracturing pipeline includes a horizontal well fracturing pipeline, a shale gas horizontal production well, and a production well row Gas valve, nitrogen gas inlet valve, fracturing fluid sand-carrying mixing device and fracturing outlet fluid control valve; the shale water-free fracturing method includes the following steps:

Step 1. After the drilling of the shale gas horizontal production well is completed, the horizontal well fracturing pipeline is laid from the wellhead to the position of the predetermined shale gas interval in the downhole, and the wellhead of the horizontal well fracturing pipeline is fracturing with the ground through the pipeline. The outlet of the liquid-carrying sand mixing device is connected, and the wellhead of the shale gas horizontal production well is connected with the gas recovery device through the gas conveying pipeline. The inlet of the gas recovery device is provided with a production well exhaust valve, and the outlet is provided with an evacuation valve;

Step 2: Open the production well vent valve, vent valve, nitrogen gas inlet valve and fracturing outlet fluid control valve, and use dry nitrogen to evacuate the horizontal well fracturing pipeline, shale gas horizontal production well and gas recovery device To remove the moisture and air inside, when the nitrogen concentration in the gas recovery device is higher than 99%, stop the exhaust, close the exhaust valve and the nitrogen intake valve;

Step 3. Open the nitrogen inlet valve 1. The nitrogen pressure regulating valve, the liquid nitrogen inlet valve and the liquid nitrogen discharge valve, provide pressure P ₁ through the high-pressure nitrogen container, and the liquid nitrogen in the liquid nitrogen storage container is carried by the fracturing fluid. The mixing device is injected into the scheduled shale gas interval from the horizontal well fracturing pipeline, and uses the low temperature effect of liquid nitrogen to damage and pre-fracture around the scheduled shale gas interval, which will occur during the liquid nitrogen damage process under normal temperature and pressure. The vaporization reaction produces nitrogen, which is transported to the gas recovery device on the ground through the shale gas horizontal production well, and then enters the nitrogen liquefaction production device through the nitrogen discharge pipeline connected to the gas recovery device, and the nitrogen liquefaction is adjusted through the integrated control system. The production unit continuously generates liquid nitrogen and flows into the liquid nitrogen storage container;

Step 4. Stop injecting liquid nitrogen when the downhole temperature monitoring probe detects that the surface temperature of the shale gas interval of the scheduled operation no longer changes, close the nitrogen pressure regulating valve, the liquid nitrogen inlet valve and the liquid nitrogen discharge valve, and use the integrated control The system suspends the work of the nitrogen liquefaction production unit, opens the nitrogen inlet valve 2 and the nitrogen control valve, and stores the nitrogen stored in the gas recovery device in the high-pressure nitrogen container through the booster device;

Step 5. Close the exhaust valve of the production well, the second nitrogen gas inlet valve and the nitrogen control valve, open the third nitrogen gas inlet valve, and transport the high-pressure nitrogen in the fracturing fluid sand-carrying mixing device to the liquid nitrogen damage through the horizontal fracturing pipeline. In the subsequent shale gas interval, the nitrogen pressure P2 in the fracturing pipeline of the horizontal well is determined according to the fracturing pressure of the shale reservoir _on site;

Step 6. When the pressure in the fracturing pipeline of the horizontal well is greater than the maximum nitrogen pressure P ₂ that can be withstood, close the nitrogen inlet valve 3 and the fracturing outlet fluid control valve of the fracturing fluid and sand-carrying mixing device, and check the scheduled operation page. Carry out soaking operation in the rock gas interval, push nitrogen molecules into smaller nano-scale shale pores, and increase the development of fracture network. The shut-in time is 2-4 hours, and the shut-in pressure value is recorded;

Step 7. After the well soaking, open the production well exhaust valve, nitrogen inlet valve 1, nitrogen pressure regulating valve, liquid nitrogen inlet valve, liquid nitrogen discharge valve and fracturing outlet fluid control valve, and the liquid nitrogen flows through the pressure. The low temperature resistant proppant is added to the fracturing fluid and sand-carrying mixing device, which is transported to the artificial fracture network through the horizontal well fracturing pipeline, and the artificial fracture network is again damaged at low temperature by liquid nitrogen, which further promotes the development of the fracture network;

Step 8. When the downhole temperature monitoring probe detects that the temperature of the artificial fracture network no longer changes, stop injecting liquid nitrogen, close the nitrogen inlet valve 1, nitrogen pressure regulating valve, liquid nitrogen inlet valve and liquid nitrogen discharge valve and fracturing The outlet liquid control valve opens the shale gas production inlet valve for shale gas extraction;

Step 9. When the daily gas production of the shale gas single well test does not meet the industrial production requirements, it is necessary to check and debug all the instruments and pipelines in the liquid nitrogen pre-splitting damage circulation system and the nitrogen booster system, and repeat steps 3 to 8 until The gas production reaches the industrial production requirements.

The pressure P1 provided by the high _- pressure nitrogen container is 0.1-1MPa,

The maximum nitrogen pressure P ₂ that the horizontal well fracturing pipeline can bear is 150MPa.

The low temperature resistant proppant is 30-50 mesh high-strength low temperature resistant bauxite, or 40-70 mesh zirconium silicate;

The concentration of the low temperature resistant proppant is 5kg/m ³ , and the proppant added to each horizontal fracturing well section is 100kg.

Beneficial effects: due to the adoption of the above technical solutions, the present invention makes full use of the low temperature characteristics of liquid nitrogen, organically combines nitrogen fracturing and enhanced oil recovery, and uses the low temperature effect of liquid nitrogen to cold shock the rock and then connect to nitrogen pressure First, the liquid nitrogen is injected to the shale gas reservoir to perform cold shock, which causes initial damage to the shale and reduces the fracture pressure around the horizontal well borehole. The nitrogen produced by the gasification of the fracturing fluid is recycled and used as the post-fracturing fluid to fracturing the reservoir, realizing the sustainable recycling of resources; the nitrogen used as the post-fracturing fluid The shale gas reservoir is modified to increase the permeability. Nitrogen is a colorless, odorless and non-corrosive inert liquid, which can prevent the reservoir from being damaged. Using the extremely strong fluidity and low temperature damage characteristics of liquid nitrogen to induce the formation of initial damage cracks, and then using the strong penetrating power of nitrogen to expand the cracks in shale gas reservoirs, construct a complex crack network, and solve the problem of water-based fracturing. Environmental problems caused by liquids and water lock problems in the mining process. The use of liquid nitrogen to carry proppant causes secondary damage to fractures, maintains and expands the fracture network of the reservoir, not only improves the recovery rate of shale gas reservoirs, but also does not cause reservoir pollution in nitrogen fracturing after liquid nitrogen pre-injection damage, as well as the phenomenon of water lock, greatly prolong the service life of horizontal wells. The nitrogen fracturing method after liquid nitrogen pre-injection not only ensures the construction scale, but also reduces the reservoir pollution caused by hydraulic fracturing, which has important practical significance.

Description of drawings

FIG. 1 is a schematic diagram of the anhydrous fracturing method of shale with liquid nitrogen pre-fracturing followed by nitrogen fracturing according to the present invention.

In Figure 1: 1-shale gas interval for scheduled operation; 2-artificial fracture network; 3-downhole temperature monitoring probe; 4-horizontal fracturing pipeline; 5-shale gas horizontal production well; 6-production well exhaust Valve; 7- gas recovery device; 8- vent valve; 9- shale gas production inlet valve; 10- nitrogen discharge pipeline; 11- nitrogen inlet valve one; 12- nitrogen liquefaction production device; 13- gas-liquid Transport pipeline; 14-nitrogen inlet valve two; 15-pressurization device; 16-nitrogen control valve; 17-high pressure nitrogen container; 18-pressure regulating valve; 19-liquid nitrogen storage container; 20-liquid nitrogen inlet valve ; 21- liquid nitrogen discharge valve; 22- nitrogen inlet valve three; 23- fracturing fluid sand-carrying mixing device; 24- fracturing outlet fluid control valve; 25- integrated control system; 26- formation.

Figure 2 is a schematic diagram of a water lock in the existing hydraulic fracturing technology.

3 is a schematic diagram of the liquid nitrogen pre-injection of the present invention.

Fig. 4 is a schematic diagram of nitrogen fracturing of the present invention.

FIG. 5 is a three-dimensional view of a shale sample according to an embodiment of the present invention.

FIG. 6( a ) is a data diagram of the fracturing pressure at different liquid nitrogen pre-injection times in an embodiment of the liquid nitrogen pre-injection nitrogen fracturing of the present invention.

Fig. 6(b) is a data diagram of surface roughness and fracture inclination angle after fracturing with different liquid nitrogen pre-injection time in the liquid nitrogen pre-injection nitrogen fracturing embodiment of the present invention.

Fig. 6(c) is a data diagram of fracture pressure at different reservoir temperatures in the liquid nitrogen pre-injection nitrogen fracturing embodiment of the present invention.

Fig. 6(d) is a data diagram of surface roughness and fracture inclination angle after fracturing at different reservoir temperatures according to the liquid nitrogen pre-injection nitrogen fracturing embodiment of the present invention.

Fig. 6(e) is a data diagram of the fracture pressure at different confining pressures according to the liquid nitrogen pre-injection nitrogen fracturing embodiment of the present invention.

Fig. 6(f) is a data diagram of surface roughness and fracture inclination angle after different confining fracturing in the liquid nitrogen pre-injection nitrogen fracturing embodiment of the present invention.

Fig. 7 is a flow chart of the liquid nitrogen pre-injection nitrogen fracturing of the present invention.

Detailed ways

Embodiments of the present invention will be further described below in conjunction with the accompanying drawings:

As shown in FIG. 1, the anhydrous fracturing method for shale with liquid nitrogen pre-fracturing followed by nitrogen fracturing of the present invention includes the use of a liquid nitrogen pre-fracturing damage circulation system and a nitrogen pressure boosting system. The liquid nitrogen pre-fracturing damage The circulation system includes a nitrogen liquefaction production device 12, a liquid nitrogen storage container 19 and a liquid nitrogen circulation pre-fracture pipeline, the liquid nitrogen circulation pre-fracture pipeline includes a horizontal well fracturing pipeline 4, a shale gas horizontal production well 5, a production Well exhaust valve 6, gas recovery device 7, nitrogen discharge pipeline 10 nitrogen inlet valve-11, gas-liquid transport pipeline 13, liquid nitrogen inlet valve 20, liquid nitrogen discharge valve 21, fracturing fluid sand-carrying mixing device 23 and fracturing outlet fluid control valve 24; the nitrogen pressurization system includes a pressurization device 15, a high-pressure nitrogen container 17 and a shut-in fracturing pipeline; the well shut-in and fracturing pipeline includes a horizontal well fracturing pipe Road 4, shale gas horizontal production well 5, production well exhaust valve 6, nitrogen gas inlet valve 3 22, fracturing fluid sand-carrying mixing device 23 and fracturing outlet fluid control valve 24; fracturing liquid nitrogen through horizontal drilling Pipeline 4 is continuously injected around the borehole in the perforated well section of the shale reservoir. First, the low temperature effect of liquid nitrogen is used to induce the shrinkage and deformation of different organic matters in the shale around the borehole to generate micro-cracks, and then nitrogen is injected to fracturing the shale, so that the The shale cracks are fully expanded and developed. After the shale fracturing is completed, the low temperature-resistant bauxite and zirconium silicate particle proppant are injected into the shale fractures by using the fluid properties of liquid nitrogen, causing secondary damage to the shale and preventing the Stress causes shale fractures to close; the specific steps are as follows:

Step 1. After the drilling of the shale gas horizontal production well 5 is completed, the horizontal well fracturing pipeline 4 is laid from the wellhead to the position of the shale gas interval 1 of the predetermined operation downhole, and the wellhead of the horizontal well fracturing pipeline 4 is connected with the pipeline through the pipeline. The outlet of the fracturing fluid-carrying sand mixing device 23 on the ground is connected, and the wellhead of the shale gas horizontal production well 5 is connected to the surface gas recovery device 7 through a gas delivery pipeline, and the inlet of the gas recovery device 7 is provided with a production well exhaust valve 6. There is an emptying valve 8 at the outlet;

Step 2. Open the production well exhaust valve 6, the emptying valve 8, the nitrogen gas inlet valve 22 and the fracturing outlet fluid control valve 24 at the same time, and use dry nitrogen to fracturing the pipeline 4 of the horizontal well and the shale gas horizontal production well 5 Carry out emptying treatment with the gas recovery device 7, that is, use the nitrogen stored in the high-pressure nitrogen container 17 to transport the nitrogen stored in the high-pressure nitrogen container 17 to the horizontal fracturing pipeline 4, the shale gas horizontal production well 5 and the gas recovery device 7 to remove excess water in the device. And air is discharged through the emptying valve 8 to drive off the moisture and air inside it, when the nitrogen concentration in the gas recovery device 7 is higher than 99%, stop exhausting, close the emptying valve 8 and the nitrogen inlet valve 22;

Step 3: Open the nitrogen inlet valve 11, the nitrogen pressure regulating valve 18, the liquid nitrogen inlet valve 20 and the liquid nitrogen discharge valve 21, and the pressure P ₁ is provided through the high-pressure nitrogen container 17, and the liquid nitrogen is injected into the pressure of the shale drilling hole P ₁ is 0.1-1 MPa, and the liquid nitrogen in the liquid nitrogen storage container 19 is injected from the horizontal well fracturing pipeline 4 into the scheduled operation shale gas interval 1 through the fracturing fluid sand-carrying mixing device 23, and the low temperature effect of liquid nitrogen is used. The damage pre-fracture is carried out around the shale gas interval 1 of the scheduled operation. During the damage process of liquid nitrogen under normal temperature and pressure, nitrogen gas will be generated by vaporization reaction, and the generated nitrogen will be transported to the gas recovery device on the ground through the shale gas horizontal production well 5. In 7, enter the nitrogen liquefaction production device 12 through the nitrogen discharge pipeline 10 connected to the gas recovery device 7, and adjust the nitrogen liquefaction production device 12 through the integrated control system 25 to continuously generate liquid nitrogen and flow into the liquid nitrogen storage container 19;

Step 4. When the downhole temperature monitoring probe 3 detects that the surface temperature of the shale gas interval 1 of the scheduled operation no longer changes, the injection of liquid nitrogen is stopped, and the nitrogen pressure regulating valve 18, the liquid nitrogen inlet valve 20 and the liquid nitrogen discharge valve are closed. 21. Suspend the work of the nitrogen liquefaction production device 12 through the integrated control system 25, open the nitrogen inlet valve 14 and the nitrogen control valve 16, and store the nitrogen stored in the gas recovery device 7 in the high-pressure nitrogen container 17 through the booster device 15; Nitrogen flows out from the storage container 17 through the horizontal fracturing pipeline 4 to reach the shale gas interval 1 for the intended operation, while closing the production well exhaust valve 6, the nitrogen gas inlet valve 22 and the fracturing outlet fluid control valve 24 to the shale reservoir. fracturing. The production well vent valve 6 and the vent valve 8 are opened to discharge the high-pressure nitrogen gas in the production well 5 .

Step 5. Close the production well exhaust valve 6, the nitrogen gas inlet valve 2 14 and the nitrogen control valve 16, open the nitrogen gas inlet valve 22, and pass the high-pressure nitrogen in the fracturing fluid sand-carrying mixing device 23 through the horizontal well fracturing pipeline 4. The nitrogen gas pressure P2 in the horizontal well fracturing pipeline ₄ is determined according to the fracture pressure of the shale reservoir on site;

Step 6. When the pressure in the horizontal well fracturing pipeline 4 is greater than the maximum nitrogen pressure P ₂ that can be withstood, the maximum nitrogen pressure P ₂ that the horizontal well fracturing pipeline 4 can withstand is 150MPa; shut off the fracturing fluid The nitrogen gas inlet valve 3 22 of the sand-carrying mixing device 23 and the fracturing outlet fluid control valve 24 are used to perform the well soaking operation in the scheduled shale gas interval 1, and push nitrogen molecules into smaller nanoscale shale pores. Increase the development of the fracture network, the shut-in time is 2-4h, and the shut-in pressure value is recorded;

Step 7. After the well soaking operation is completed, open the production well exhaust valve 6 and the emptying valve 8 to discharge the high-pressure nitrogen in the production well 5, then close the emptying valve 8, carry out the second liquid nitrogen pre-injection, and turn on the nitrogen gas. Inlet valve 11, nitrogen pressure regulating valve 18, liquid nitrogen inlet valve 20, liquid nitrogen discharge valve 21 and fracturing outlet fluid control valve 24, when liquid nitrogen flows through fracturing fluid sand-carrying mixing device 23, add low temperature resistant The proppant of 100kg is injected into the reservoir with liquid nitrogen carrying the proppant with a concentration of 5kg/m ³ , which will cause secondary damage to the shale reservoir, maintain and promote the expansion of cracks. The low temperature resistant proppant is 30-50 mesh high-strength low temperature resistant bauxite, or 40-70 mesh zirconium silicate. It is transported to the artificial fracture network 2 through the horizontal fracturing pipeline 4, and the artificial fracture network 2 is again damaged at low temperature by liquid nitrogen, which further promotes the development of the fracture network;

Step 8. When the downhole temperature monitoring probe 3 detects that the temperature of the artificial fracture network 2 no longer changes, the injection of liquid nitrogen is stopped, and the nitrogen gas inlet valve 11, the nitrogen pressure regulating valve 18, the liquid nitrogen inlet valve 20 and the liquid nitrogen drain are closed. valve 21 and fracturing outlet fluid control valve 24, open the shale gas production inlet valve 9 for shale gas production and extraction;

Step 9. When the daily gas production of the shale gas single well test does not meet the industrial production requirements, it is necessary to check and debug all the instruments and pipelines in the liquid nitrogen pre-splitting damage circulation system and the nitrogen booster system, and repeat steps 3 to 8 until The gas production reaches the industrial production requirements.

Fig. 3 is a schematic diagram showing the damage and fracturing of the scheduled shale gas interval by liquid nitrogen pre-injection.

Figure 4 shows a schematic diagram of fracturing the reservoir with nitrogen in the damaged artificial fracture network.

Figure 5 shows the size of the standard sample for rock mechanics test, and its mechanical properties are representative. Experimental study on nitrogen fracturing of shale after liquid nitrogen pre-injection. The test plan is shown in Table 1.

Table 1 Control variable table of experimental program

In scheme 1: inject liquid nitrogen for different times, and compare the fracture pressure changes of shale; with the increase of liquid nitrogen injection time, the average fracture pressure of rock samples in nitrogen fracturing shows a decreasing trend, and the maximum decrease is compared with that when no liquid nitrogen is injected. up to 65%.

In scheme 2, the shale samples are heated to different temperatures and then pre-injected with liquid nitrogen; for the samples without liquid nitrogen injection, the fracture pressure first increases and then decreases with the increase of rock mass temperature. , which is mainly due to the effect of temperature on the organic matter inside the rock sample. For the samples that were subjected to nitrogen fracturing after injection of liquid nitrogen, the fracture pressure showed a decreasing trend with the increase of rock mass temperature. This indicates that a higher temperature difference leads to more adequate development of cracks inside the specimen.

Option 3: Change the confining pressure of the test device to study the effect of liquid nitrogen pre-injection on shale fracture pressure under different confining pressures. The rupture pressure increases linearly with the increase of confining pressure, and the rupture pressure under all confining pressures after liquid nitrogen injection is lower than that of the samples without liquid nitrogen injection.

Through the above three embodiments, it can be concluded that after the liquid nitrogen pre-injection, a large temperature gradient is generated around the shale borehole, and the resulting temperature stress causes crack initiation and expansion, and the generation of these microcracks can be greatly reduced The strength of the rock promotes the expansion of cracks after nitrogen injection, thereby reducing the fracture pressure of the rock.

Three sets of experiments can prove the effect of liquid nitrogen pre-fracturing followed by nitrogen fracturing shale water-free fracturing method. The results show that the method of liquid nitrogen pre-fracturing followed by nitrogen fracturing has advantages over traditional nitrogen water-free fracturing technology. Secondly, due to the constraints of various factors in the horizontal fracturing process, different reservoir temperatures, different burial depths and the length of liquid nitrogen pre-fracturing time will all affect the fracturing effect. Three sets of experiments also proved that this method can adapt to Different geological conditions have a broad scope of application. Each protocol involved pre-injection of liquid nitrogen, and each set of trials demonstrated the damaging effects of liquid nitrogen. The invention utilizes the low temperature property of liquid nitrogen to damage the shale. In the process, the nitrogen gas after the vaporization and expansion of the liquid nitrogen is discharged to the surface for storage and pressurization and then re-made into liquid nitrogen to achieve the purpose of recycling. Compared with liquid nitrogen fracturing, The requirements for the low temperature pressure resistance performance of the fracturing pipeline are reduced, and the damage of liquid nitrogen is increased compared with nitrogen fracturing.

Claims (5)

1. An anhydrous fracturing method for shale followed by nitrogen fracturing after liquid nitrogen pre-fracturing, comprising adopting a liquid nitrogen pre-fracture damage circulation system and a nitrogen booster system, and the liquid nitrogen pre-fracture damage circulation system comprises a nitrogen liquefaction system. A production device (12), a liquid nitrogen storage container (19) and a liquid nitrogen circulating pre-fracturing pipeline, the liquid nitrogen circulating pre-fracturing pipeline comprising a horizontal well fracturing pipeline (4), a shale gas horizontal production well (5) ), production well exhaust valve (6), gas recovery device (7), nitrogen discharge pipeline (10), nitrogen gas inlet valve one (11), gas-liquid transport pipeline (13), liquid nitrogen inlet valve (20) ), a liquid nitrogen discharge valve (21), a fracturing fluid sand-carrying mixing device (23) and a fracturing outlet fluid control valve (24); the nitrogen boosting system includes a boosting device (15), a high-pressure nitrogen container (17) and shut-in fracturing pipeline; the well shut-in and fracturing pipeline includes horizontal well fracturing pipeline (4), shale gas horizontal production well (5), production well exhaust valve (6), nitrogen gas The three (22) inlet valve, the fracturing fluid sand-carrying mixing device (23), and the fracturing outlet fluid control valve (24) are characterized in that: the water-free fracturing method for shale comprises the following steps: Step 1. After the drilling of the shale gas horizontal production well (5) is completed, the horizontal well fracturing pipeline (4) is laid from the wellhead to the position of the predetermined shale gas interval (1) in the downhole, and the horizontal well fracturing pipeline The wellhead of (4) is connected to the outlet of the fracturing fluid and sand-carrying mixing device (23) on the ground through a pipeline, and the wellhead of the shale gas horizontal production well (5) is connected to the gas recovery device (7) through a gas conveying pipeline. A production well exhaust valve (6) is arranged at the inlet of the recovery device (7), and an emptying valve (8) is arranged at the outlet; Step 2: Open the production well exhaust valve (6), the evacuation valve (8), the nitrogen gas inlet valve three (22) and the fracturing outlet fluid control valve (24), and use dry nitrogen to irrigate the horizontal well fracturing pipeline ( 4), the shale gas horizontal production well (5) and the gas recovery device (7) are evacuated to remove the moisture and air inside, and when the nitrogen concentration in the gas recovery device (7) is higher than 99%, stop Exhaust, close the exhaust valve (8) and nitrogen intake valve three (22); Step 3: Open the nitrogen inlet valve one (11), the nitrogen pressure regulating valve (18), the liquid nitrogen inlet valve (20) and the liquid nitrogen discharge valve (21), and provide the pressure P1 through the high-pressure nitrogen container ( ₁₇ ). , the liquid nitrogen in the liquid nitrogen storage container (19) is injected into the predetermined operation shale gas interval (1) from the horizontal well fracturing pipeline (4) through the fracturing fluid sand-carrying mixing device (23). The low temperature effect damages and pre-cracks around the shale gas interval (1) of the scheduled operation. During the damage process of liquid nitrogen under normal temperature and pressure, a vaporization reaction will occur to generate nitrogen, and the generated nitrogen will be transported upward through the shale gas horizontal production well (5). To the gas recovery device (7) on the ground, and then enter the nitrogen liquefaction production device (12) through the nitrogen discharge pipeline (10) connected to the gas recovery device (7), and adjust the nitrogen liquefaction production device through the integrated control system (25). (12) Continuously generate liquid nitrogen and flow into the liquid nitrogen storage container (19); Step 4. When the downhole temperature monitoring probe (3) detects that the surface temperature of the shale gas interval (1) for the scheduled operation no longer changes, stop the injection of liquid nitrogen, close the nitrogen pressure regulating valve (18), and the liquid nitrogen inlet valve (18). 20) and the liquid nitrogen discharge valve (21), suspend the work of the nitrogen liquefaction production device (12) through the integrated control system (25), open the second nitrogen gas inlet valve (14) and the nitrogen control valve (16), and store the gas in the gas. The nitrogen in the recovery device (7) is stored in the high-pressure nitrogen container (17) through the pressurizing device (15); Step 5. Close the production well exhaust valve (6), the second nitrogen gas inlet valve (14) and the nitrogen control valve (16), open the third nitrogen gas inlet valve (22), and add the fracturing fluid to the sand-carrying mixing device (23). The high-pressure nitrogen in the horizontal well fracturing pipeline (4) is transported to the predetermined operation shale gas interval (1) after the liquid nitrogen damage, and the nitrogen pressure P2 in the horizontal well fracturing pipeline ( ₄ ) is based on the site Determination of the fracture pressure of shale reservoirs; Step 6. When the pressure in the horizontal well fracturing pipeline ( ₄ ) is greater than the maximum nitrogen pressure P2 that can be withstood, close the nitrogen gas inlet valve three (22) of the fracturing fluid sand-carrying mixing device (23) and fracturing The outlet fluid control valve (24) is used to perform the soaking operation on the scheduled shale gas interval (1) to push nitrogen molecules into smaller nano-scale shale pores to increase the development of the fracture network, and the shut-in time is 2 -4h, record the shut-in pressure value; Step 7. After the well soaking, open the production well exhaust valve (6), the nitrogen gas inlet valve one (11), the nitrogen pressure regulating valve (18), the liquid nitrogen inlet valve (20), and the liquid nitrogen discharge valve ( 21) and the fracturing outlet fluid control valve (24), when the liquid nitrogen flows through the fracturing fluid sand-carrying mixing device (23), low temperature-resistant proppant is added, and is transported through the horizontal well fracturing pipeline (4) to the artificial fracture network ( 2), using liquid nitrogen to damage the artificial fracture network (2) again at low temperature to further promote the development of the fracture network; Step 8. When the downhole temperature monitoring probe (3) detects that the temperature of the artificial fracture network (2) no longer changes, the injection of liquid nitrogen is stopped, and the nitrogen inlet valve one (11), the nitrogen pressure regulating valve (18), and the liquid nitrogen are closed. a liquid inlet valve (20), a liquid nitrogen discharge valve (21) and a fracturing outlet liquid control valve (24), and the shale gas production inlet valve (9) is opened to perform shale gas extraction; Step 9. When the daily gas production of the shale gas single well test does not meet the industrial production requirements, it is necessary to check and debug all the instruments and pipelines in the liquid nitrogen pre-splitting damage circulation system and the nitrogen booster system, and repeat steps 3 to 8 until The gas production reaches the industrial production requirements. 2. The anhydrous fracturing method for shale followed by nitrogen fracturing after liquid nitrogen pre-fracturing according to claim 1, characterized in that: the pressure P ₁ provided by the high-pressure nitrogen container (17) is 0.1-1MPa . 3. The anhydrous fracturing method for shale followed by nitrogen fracturing after liquid nitrogen pre-fracturing according to claim 1, characterized in that: the maximum nitrogen gas that the horizontal well fracturing pipeline (4) can withstand _The pressure P2 was 150MPa. 4. the anhydrous fracturing method of shale followed by nitrogen fracturing after a kind of liquid nitrogen pre-cracking according to claim 1, it is characterized in that: described low temperature resistant proppant is 30-50 purpose high strength low temperature resistant aluminum Bauxite, or 40-70 mesh zirconium silicate. 5. The anhydrous fracturing method for shale followed by nitrogen fracturing after a liquid nitrogen pre-fracturing according to claim 1 or 4, wherein the concentration of the low-temperature resistant proppant is 5kg/m ³ , and each The proppant added to each horizontal fracturing well section is 100kg.

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