CN113309519A - Shale anhydrous fracturing method for liquid nitrogen presplitting and nitrogen fracturing - Google Patents
Shale anhydrous fracturing method for liquid nitrogen presplitting and nitrogen fracturing Download PDFInfo
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- IJGRMHOSHXDMSA-UHFFFAOYSA-N Atomic nitrogen Chemical compound N#N IJGRMHOSHXDMSA-UHFFFAOYSA-N 0.000 title claims abstract description 612
- 229910052757 nitrogen Inorganic materials 0.000 title claims abstract description 282
- 239000007788 liquid Substances 0.000 title claims abstract description 195
- 238000000034 method Methods 0.000 title claims abstract description 35
- 239000007789 gas Substances 0.000 claims abstract description 103
- 239000012530 fluid Substances 0.000 claims abstract description 26
- 238000003860 storage Methods 0.000 claims abstract description 13
- 230000008569 process Effects 0.000 claims abstract description 10
- 238000009834 vaporization Methods 0.000 claims abstract description 5
- 230000008016 vaporization Effects 0.000 claims abstract description 5
- 238000004519 manufacturing process Methods 0.000 claims description 61
- 229910001873 dinitrogen Inorganic materials 0.000 claims description 53
- 238000011084 recovery Methods 0.000 claims description 29
- 238000002156 mixing Methods 0.000 claims description 22
- 239000000523 sample Substances 0.000 claims description 22
- 230000001105 regulatory effect Effects 0.000 claims description 16
- 230000000694 effects Effects 0.000 claims description 9
- 238000011161 development Methods 0.000 claims description 7
- 238000005553 drilling Methods 0.000 claims description 7
- 238000012544 monitoring process Methods 0.000 claims description 7
- 239000003795 chemical substances by application Substances 0.000 claims description 6
- 238000009776 industrial production Methods 0.000 claims description 6
- 238000012360 testing method Methods 0.000 claims description 6
- 230000008859 change Effects 0.000 claims description 5
- 229910001570 bauxite Inorganic materials 0.000 claims description 4
- 238000005336 cracking Methods 0.000 claims description 4
- 238000002791 soaking Methods 0.000 claims description 4
- GFQYVLUOOAAOGM-UHFFFAOYSA-N zirconium(iv) silicate Chemical compound [Zr+4].[O-][Si]([O-])([O-])[O-] GFQYVLUOOAAOGM-UHFFFAOYSA-N 0.000 claims description 4
- 238000006243 chemical reaction Methods 0.000 claims description 3
- 230000001276 controlling effect Effects 0.000 claims description 3
- 238000011065 in-situ storage Methods 0.000 claims description 3
- 239000011148 porous material Substances 0.000 claims description 3
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 abstract description 11
- 230000007613 environmental effect Effects 0.000 abstract description 3
- 230000000149 penetrating effect Effects 0.000 abstract description 3
- 238000001816 cooling Methods 0.000 abstract description 2
- 238000002347 injection Methods 0.000 description 16
- 239000007924 injection Substances 0.000 description 16
- 239000011435 rock Substances 0.000 description 14
- 238000010586 diagram Methods 0.000 description 4
- 238000007599 discharging Methods 0.000 description 4
- 238000002474 experimental method Methods 0.000 description 3
- 230000003746 surface roughness Effects 0.000 description 3
- 238000010276 construction Methods 0.000 description 2
- 238000005516 engineering process Methods 0.000 description 2
- 230000035699 permeability Effects 0.000 description 2
- 229920001938 Vegetable gum Polymers 0.000 description 1
- 230000009471 action Effects 0.000 description 1
- 238000009933 burial Methods 0.000 description 1
- 239000002734 clay mineral Substances 0.000 description 1
- 239000000110 cooling liquid Substances 0.000 description 1
- 125000004122 cyclic group Chemical group 0.000 description 1
- 230000000254 damaging effect Effects 0.000 description 1
- 239000006185 dispersion Substances 0.000 description 1
- 238000002309 gasification Methods 0.000 description 1
- 238000010438 heat treatment Methods 0.000 description 1
- 230000036571 hydration Effects 0.000 description 1
- 238000006703 hydration reaction Methods 0.000 description 1
- 230000006872 improvement Effects 0.000 description 1
- 239000002245 particle Substances 0.000 description 1
- 230000037452 priming Effects 0.000 description 1
- 230000002035 prolonged effect Effects 0.000 description 1
- 230000001737 promoting effect Effects 0.000 description 1
- 238000004064 recycling Methods 0.000 description 1
- 230000009467 reduction Effects 0.000 description 1
- 238000011160 research Methods 0.000 description 1
- 230000009967 tasteless effect Effects 0.000 description 1
- 239000002562 thickening agent Substances 0.000 description 1
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- E—FIXED CONSTRUCTIONS
- E21—EARTH OR ROCK DRILLING; MINING
- E21C—MINING OR QUARRYING
- E21C37/00—Other methods or devices for dislodging with or without loading
- E21C37/06—Other methods or devices for dislodging with or without loading by making use of hydraulic or pneumatic pressure in a borehole
- E21C37/14—Other methods or devices for dislodging with or without loading by making use of hydraulic or pneumatic pressure in a borehole by compressed air; by gas blast; by gasifying liquids
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- E—FIXED CONSTRUCTIONS
- E21—EARTH OR ROCK DRILLING; MINING
- E21F—SAFETY DEVICES, TRANSPORT, FILLING-UP, RESCUE, VENTILATION, OR DRAINING IN OR OF MINES OR TUNNELS
- E21F17/00—Methods or devices for use in mines or tunnels, not covered elsewhere
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- E—FIXED CONSTRUCTIONS
- E21—EARTH OR ROCK DRILLING; MINING
- E21F—SAFETY DEVICES, TRANSPORT, FILLING-UP, RESCUE, VENTILATION, OR DRAINING IN OR OF MINES OR TUNNELS
- E21F17/00—Methods or devices for use in mines or tunnels, not covered elsewhere
- E21F17/18—Special adaptations of signalling or alarm devices
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- Life Sciences & Earth Sciences (AREA)
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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
Technical Field
The invention relates to the technical field of shale gas reservoir fracturing, in particular to a shale anhydrous fracturing method suitable for improving the yield of a low-permeability and low-porosity oil and gas reservoir by performing nitrogen fracturing after liquid nitrogen prefracturing.
Background
The hydraulic fracturing technology can effectively improve the oil and gas yield of the shale reservoir with low permeability and low porosity by influencing the development degree of shale fractures. Because water molecules cannot be injected into the low-permeability shale reservoir efficiently, the development effect cannot reach the expectation. After the water-based fracturing fluid is injected into a reservoir for a long time, the water-lock damage is caused by hydration expansion and dispersion of underground clay minerals. The thickening agent vegetable gum in the water-based fracturing fluid cannot be completely discharged out of the stratum, and serious reservoir pollution is caused. Therefore, the industry proposes fracturing shale reservoirs after pressurizing with liquid nitrogen. Considering that the fracturing process time is short, the heat conduction coefficient of the rock is low, and the influence range of the liquid nitrogen cooling effect on the reservoir is small. Meanwhile, because the liquid nitrogen is in a non-flowing state and the temperature of the liquid nitrogen rises after heat convection with the reservoir, the formed temperature gradient is reduced, and the damage degree of the reservoir caused by temperature stress is reduced. In order to solve the problems, the method comprises the steps of treating a reservoir by using liquid nitrogen as a pre-fracturing fluid and then fracturing the reservoir by using nitrogen. Research shows that the mechanical property of shale is changed when the temperature difference is more than 200 ℃, and the temperature difference between the shale gas reservoir and liquid nitrogen can reach 196 ℃ below zero, the temperature difference between the shale gas reservoir and the reservoir can reach 300 ℃ at most, the liquid nitrogen is used as a front-mounted cooling liquid to manufacture a shale fracture network, and then nitrogen is used as a rear-mounted anhydrous fracturing liquid to enrich the existing shale fracture network, so that the oil and gas yield of the shale reservoir is improved in an environment-friendly manner.
Disclosure of Invention
The technical problem is as follows: the invention aims to solve the environmental problem caused by water-based fracturing fluid and the water lock phenomenon in the shale gas exploitation process, and provides a shale anhydrous fracturing method for fracturing by connecting liquid nitrogen after liquid nitrogen is pre-fractured, which utilizes the low-temperature characteristic of liquid nitrogen to cause damage cracks around a drilled hole of a reservoir, and then enables the shale reservoir near the drilled hole to generate a complex fracture network through the penetrating power of the nitrogen, thereby ensuring the construction scale, simultaneously improving the difficult problem of crack propagation of the reservoir subjected to anhydrous fracturing and increasing the yield of the shale gas.
The technical scheme is as follows: in order to achieve the purpose, the invention discloses a shale anhydrous fracturing method for fracturing liquid nitrogen by connecting the liquid nitrogen with nitrogen after pre-fracturing, which comprises a liquid nitrogen pre-fracturing damage circulating system and a nitrogen pressurizing system, wherein the liquid nitrogen pre-fracturing damage system comprises a nitrogen liquefaction production device, a liquid nitrogen storage device and a liquid nitrogen circulating pre-fracturing pipeline, and the liquid nitrogen circulating pre-fracturing pipeline comprises a horizontal well fracturing pipeline, a shale gas horizontal production well, a production well exhaust valve, a gas recovery device, a nitrogen gas inlet valve of a nitrogen gas discharge pipeline, a gas-liquid transportation pipeline, a liquid nitrogen inlet valve, a liquid nitrogen discharge valve, a fracturing liquid sand-carrying mixing device and a fracturing outlet liquid control valve; the nitrogen pressurization fracturing system comprises a pressurization device, a high-pressure nitrogen container and a well closing fracturing pipeline; the well closing fracturing pipeline comprises a horizontal well fracturing pipeline, a shale gas horizontal production well, a production well exhaust valve, a nitrogen gas inlet valve, a fracturing fluid sand-carrying mixing device and a fracturing outlet fluid control valve; the shale anhydrous fracturing method comprises the following steps:
after drilling of a shale gas horizontal production well, laying a horizontal well fracturing pipeline to the position of a shale gas layer section with preset operation underground from a well mouth, wherein the well mouth of the horizontal well fracturing pipeline is connected with the outlet of a fracturing fluid sand-carrying mixing device on the ground through a pipeline, the well mouth of the shale gas horizontal production well is connected with a ground gas recovery device through a gas conveying pipeline, a production well exhaust valve is arranged at the inlet of the ground gas recovery device, and an exhaust valve is arranged at the outlet of the ground gas recovery device;
opening an exhaust valve of the production well, an exhaust valve, a nitrogen gas inlet valve and a fracturing outlet liquid control valve, using dry nitrogen to perform evacuation treatment on the horizontal fracturing pipeline, the shale gas horizontal production well and the gas recovery device so as to expel moisture and air in the horizontal fracturing pipeline, stopping exhausting when the concentration of nitrogen in the gas recovery device is higher than 99%, and closing the exhaust valve and the nitrogen gas inlet valve;
opening a nitrogen gas inlet valve, a nitrogen gas pressure regulating valve, a liquid nitrogen inlet valve and a liquid nitrogen drain valve, and providing pressure P through a high-pressure nitrogen container1Liquid nitrogen in a liquid nitrogen storage container is injected into a preset operation shale gas layer section from a horizontal fracturing pipeline through a fracturing fluid sand-carrying mixing device, damage and pre-cracking are carried out on the periphery of the shale gas layer section by utilizing the low-temperature effect of the liquid nitrogen, a vaporization reaction can be generated in the liquid nitrogen damage process at normal temperature and normal pressure to generate nitrogen, and the generated nitrogenThe nitrogen gas is lifted and conveyed to a gas recovery device on the ground through a horizontal production well, enters a nitrogen gas liquefaction production device through a nitrogen gas discharge pipeline connected with the gas recovery device, and is regulated by an integrated control system to continuously generate liquid nitrogen which flows into a liquid nitrogen storage container;
stopping injecting liquid nitrogen when the underground temperature monitoring probe monitors that the surface temperature of the shale gas layer section in the preset operation is not changed any more, closing a nitrogen pressure regulating valve, a liquid nitrogen inlet valve and a liquid nitrogen discharge valve, suspending the operation of the nitrogen liquefaction production device through the integrated control system, opening a nitrogen gas inlet valve and a nitrogen gas control valve, and storing the nitrogen gas stored in the gas recovery device in a high-pressure nitrogen container through a pressurizing device;
step five, closing an exhaust valve of the production well, a nitrogen gas inlet valve and a nitrogen gas control valve, opening the nitrogen gas inlet valve, conveying high-pressure nitrogen in the fracturing fluid sand-carrying mixing device into the shale gas layer section damaged by liquid nitrogen through a horizontal fracturing pipeline, and controlling the pressure P of the nitrogen in the horizontal fracturing pipeline2Determining according to the fracture pressure condition of the in-situ shale reservoir;
step six, when the pressure in the horizontal fracturing pipeline is larger than the highest bearable nitrogen pressure P2Closing a nitrogen gas inlet valve and a fracturing outlet liquid control valve of a fracturing liquid sand-carrying mixing device, carrying out soaking operation on the shale gas layer section, pushing nitrogen molecules into smaller nano-scale shale pores, increasing the development of a fracture network, keeping the shut-in time for 2-4h, and recording the shut-in pressure value;
seventhly, after the well is stewed, opening a production well exhaust valve, a nitrogen gas inlet valve, a nitrogen gas pressure regulating valve, a liquid nitrogen inlet valve, a liquid nitrogen discharge valve and a fracturing outlet liquid control valve, adding a low-temperature-resistant propping agent when liquid nitrogen flows through a fracturing liquid sand-carrying mixing device, conveying the liquid nitrogen to an artificial fracture network through a horizontal fracturing pipeline, and performing low-temperature damage on the artificial fracture network again by using the liquid nitrogen to further promote the development of the fracture network;
step eight, stopping injecting liquid nitrogen when the underground temperature monitoring probe monitors that the temperature of the artificial fracture network does not change any more, closing a nitrogen gas inlet valve, a nitrogen gas pressure regulating valve, a liquid nitrogen inlet valve, a liquid nitrogen liquid discharging valve and a fracturing outlet liquid control valve, and opening a shale gas production inlet valve to extract shale gas;
step nine, when the daily gas yield of the shale gas single well test does not meet the industrial production requirement, all instruments and pipelines in the liquid nitrogen pre-splitting damage circulating system and the nitrogen pressurization system need to be checked and debugged, and the steps three to eight are repeated until the gas yield meets the industrial production requirement.
The pressure P of the liquid nitrogen injected into the shale borehole1Is 0.1 to 1MPa,
the highest nitrogen pressure P which can be borne by the horizontal fracturing pipeline (4)2Is 150 MPa.
The low-temperature resistant proppant is high-strength low-temperature resistant bauxite of 30-50 meshes or zirconium silicate of 40-70 meshes;
the concentration of the added low temperature resistant proppant is 5kg/m3100kg of proppant was added per horizontal fracturing interval.
Has the advantages that: by adopting the technical scheme, the low-temperature characteristic of liquid nitrogen is fully utilized, nitrogen fracturing and recovery efficiency improvement are organically combined, the rock is subjected to cold impact by utilizing the low-temperature effect of the liquid nitrogen and then is subjected to nitrogen fracturing, the liquid nitrogen is injected firstly to perform cold impact on a shale gas reservoir, so that the shale is promoted to generate initial damage, the fracture pressure around a horizontal well drilling hole is reduced, the liquid nitrogen is vaporized and expanded into nitrogen, a high-pressure gas storage tank is utilized to recover nitrogen generated by gasification of a front fracturing liquid, the nitrogen released by vaporization and expansion of the liquid nitrogen is recycled and used as a rear fracturing liquid to fracture a reservoir, and the sustainable cyclic utilization of resources is realized; the nitrogen is used as a post-fracturing fluid to modify and increase the permeability of a shale gas reservoir, and the nitrogen is colorless, tasteless and corrosion-free inert liquid and can prevent a reservoir from being damaged. 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. The liquid nitrogen is used for carrying the propping agent to cause secondary damage to the cracks, the fracture network of the reservoir is maintained and expanded, the recovery ratio of the shale gas reservoir is improved, the nitrogen fracturing after the liquid nitrogen is pre-injected has no reservoir pollution damage and water lock phenomenon, and the service life of the horizontal well is greatly prolonged. The method for fracturing nitrogen after pre-injecting liquid nitrogen not only ensures the construction scale, but also reduces reservoir pollution caused by hydraulic fracturing, and has important practical significance.
Drawings
FIG. 1 is a schematic diagram of a shale anhydrous fracturing method of the invention, which is performed by liquid nitrogen pre-fracturing and then nitrogen fracturing.
In fig. 1: 1-a predetermined working shale gas interval; 2-artificial fracture network; 3-underground temperature monitoring probe; 4-horizontal fracturing of the pipeline; 5-a shale gas horizontal production well; 6-production well exhaust valve; 7-a gas recovery unit; 8-an evacuation valve; 9-shale gas production air inlet valve; 10-nitrogen gas discharge line; 11-nitrogen inlet valve; 12-nitrogen liquefaction production units; 13-gas-liquid transport pipeline; 14-nitrogen inlet valve; 15-a pressure boosting device; 16-a nitrogen control valve; 17-high pressure nitrogen vessel; 18-a pressure regulating valve; 19-liquid nitrogen storage container; 20-liquid nitrogen liquid inlet valve; 21-liquid nitrogen drain valve; 22-nitrogen inlet valve; 23-a fracturing fluid sand-carrying mixing device; 24-fracturing outlet fluid control valve; 25-an integrated control system; 26-formation.
Fig. 2 is a schematic diagram of water lock in the prior art of hydraulic fracturing.
FIG. 3 is a schematic of the liquid nitrogen priming of the present invention.
Figure 4 is a schematic of a nitrogen fracture of the present invention.
FIG. 5 is a three-dimensional view of a shale sample in accordance with an embodiment of the present invention.
FIG. 6(a) is a graph of the burst pressure data for different liquid nitrogen pre-injection times for the liquid nitrogen pre-injection nitrogen fracturing examples of the present invention.
FIG. 6(b) is a graph of surface roughness and fracture dip angle data after fracturing at different times of liquid nitrogen pre-injection for embodiments of liquid nitrogen pre-injection nitrogen fracturing of the present invention.
FIG. 6(c) is a graph of fracture pressure data for different reservoir temperatures for embodiments of the liquid nitrogen pre-injection nitrogen fracturing of the present invention.
FIG. 6(d) is a graph of surface roughness and fracture dip angle data after fracturing at different reservoir temperatures for embodiments of the liquid nitrogen pre-injection nitrogen fracturing of the present invention.
FIG. 6(e) is a graph of the burst pressure data for different confined pressures for the embodiments of the present invention where nitrogen is pre-injected into the liquid nitrogen fracturing.
FIG. 6(f) is a graph of surface roughness and fracture dip angle data after different confined fractures for a liquid nitrogen pre-injection nitrogen fracturing example of the present invention.
FIG. 7 is a flow chart of the liquid nitrogen pre-injection nitrogen fracturing of the present invention.
Detailed Description
Embodiments of the invention will be further described with reference to the accompanying drawings in which:
as shown in fig. 1, the shale anhydrous fracturing method of the invention comprises a liquid nitrogen presplitting damage circulation system and a nitrogen pressurization system, wherein the liquid nitrogen presplitting damage system comprises a nitrogen liquefaction production device 12, a liquid nitrogen storage device 19 and a liquid nitrogen circulation presplitting pipeline, and the liquid nitrogen circulation presplitting pipeline comprises a horizontal well fracturing pipeline 4, a shale gas horizontal production well 5, a production well exhaust valve 6, a gas recovery device 7, a nitrogen exhaust pipeline 10, a nitrogen gas inlet valve 11, a gas-liquid transportation pipeline 13, a liquid nitrogen inlet valve 20, a liquid nitrogen discharge valve 21, a fracturing liquid sand-carrying mixing device 23 and a fracturing outlet liquid control valve 24; the nitrogen pressurization fracturing system comprises a pressurization device 15, a high-pressure nitrogen container 17 and a well shut-in fracturing pipeline; the well closing fracturing pipeline comprises a horizontal well fracturing pipeline 4, a shale gas horizontal production well 5, a production well exhaust valve 6, a nitrogen gas inlet valve 22, a fracturing fluid sand-carrying mixing device 23 and a fracturing outlet fluid control valve 24; liquid nitrogen is continuously injected into the periphery of a drilling hole of a perforation well section of a shale reservoir through a horizontal drilling fracturing pipeline 4, the low-temperature effect of the liquid nitrogen is firstly utilized to promote different organic matters in shale around the drilling hole to shrink and deform to generate microcracks, then nitrogen is injected to fracture the shale, the shale cracks are fully expanded and developed, after the fracturing of the shale is finished, low-temperature-resistant bauxite and zirconium silicate particle propping agents are injected into the shale cracks by utilizing the fluid characteristic of the liquid nitrogen, so that the shale is secondarily damaged, and the shale cracks are prevented from being closed under the action of ground stress; the method comprises the following specific steps:
after drilling of a horizontal shale gas production well 5 is completed, laying a horizontal well fracturing pipeline 4 to a position of a shale gas layer section 1 with preset operation underground from a well mouth, connecting the well mouth of the horizontal well fracturing pipeline 4 with an outlet of a fracturing fluid sand-carrying mixing device 23 on the ground through a pipeline, connecting the well mouth of the horizontal shale gas production well 5 with a ground gas recovery device 7 through a gas conveying pipeline, arranging a production well exhaust valve 6 at an inlet of the ground gas recovery device 7, and arranging an exhaust valve 8 at the outlet;
step two, simultaneously opening a production well exhaust valve 6, an exhaust valve 8, a nitrogen gas inlet valve 22 and a fracturing outlet liquid control valve 24, and using dry nitrogen to perform evacuation treatment on the horizontal fracturing pipeline 4, the shale gas horizontal production well 5 and the gas recovery device 7, namely conveying nitrogen stored in a high-pressure nitrogen container 17 into the horizontal fracturing pipeline 4, the shale gas horizontal production well 5 and the gas recovery device 7, discharging redundant moisture and air in the device through the exhaust valve 8 to expel moisture and air in the device, stopping discharging when the concentration of the nitrogen in the gas recovery device 7 is higher than 99%, and closing the exhaust valve 8 and the nitrogen gas inlet valve 22;
step three, opening a nitrogen gas inlet valve 11, a nitrogen gas pressure regulating valve 18, a liquid nitrogen inlet valve 20 and a liquid nitrogen discharge valve 21, and providing pressure P through a high-pressure nitrogen container 171The pressure P of the liquid nitrogen injected into the shale borehole1The pressure of the liquid nitrogen is 0.1-1MPa, liquid nitrogen in a liquid nitrogen storage container 19 is injected into a preset operation shale gas layer section 1 from a horizontal fracturing pipeline 4 through a fracturing liquid sand-carrying mixing device 23, the damage and pre-cracking are carried out on the periphery of the shale gas layer section 1 by utilizing the low-temperature effect of the liquid nitrogen, a vaporization reaction can be generated in the liquid nitrogen damage process at normal temperature and normal pressure to generate nitrogen, the generated nitrogen is lifted and conveyed to a gas recovery device 7 on the ground through a horizontal production well 5, and then enters a nitrogen liquefaction production device 12 through a nitrogen discharge pipeline 10 connected with the gas recovery device 7, and the nitrogen liquefaction production device 12 is regulated by an integrated control system 25 to continuously generate liquid nitrogen which flows into the liquid nitrogen storage container 19;
stopping injecting liquid nitrogen when the underground temperature monitoring probe 3 monitors that the surface temperature of the preset operation shale gas layer section 1 does not change any more, closing a nitrogen pressure regulating valve 18, a liquid nitrogen inlet valve 20 and a liquid nitrogen discharge valve 21, suspending the operation of the nitrogen liquefaction production device 12 through an integrated control system 25, opening a nitrogen inlet valve 14 and a nitrogen control valve 16, and storing the nitrogen stored in the gas recovery device 7 in a high-pressure nitrogen container 17 through a pressurizing device 15; nitrogen flows out of the storage container 17 through the horizontal fracturing pipeline 4 to reach the preset operation shale gas layer section 1, and meanwhile, the production well exhaust valve 6, the nitrogen gas inlet valve 22 and the fracturing outlet liquid control valve 24 are closed to fracture the shale reservoir. And opening the exhaust valve 6 and the exhaust valve 8 of the production well, and exhausting the high-pressure nitrogen in the production well 5.
Step five, closing the exhaust valve 6, the nitrogen gas inlet valve 14 and the nitrogen gas control valve 16 of the production well, opening the nitrogen gas inlet valve 22, conveying high-pressure nitrogen in the fracturing fluid sand-carrying mixing device 23 into the shale gas layer section 1 damaged by liquid nitrogen through the horizontal fracturing pipeline 4, and controlling the nitrogen pressure P in the horizontal fracturing pipeline 42Determining according to the fracture pressure condition of the in-situ shale reservoir;
step six, when the pressure in the horizontal fracturing pipeline 4 is larger than the highest bearable nitrogen pressure P2The highest nitrogen pressure P that the horizontal fracturing pipeline 4 can bear2Is 150 MPa; closing a nitrogen gas inlet valve 22 and a fracturing outlet liquid control valve 24 of a fracturing liquid sand-carrying mixing device 23, carrying out soaking operation on the shale gas layer section 1, pushing nitrogen molecules into smaller nano-scale shale pores, increasing the development of a fracture network, keeping the shut-in time for 2-4h, and recording the shut-in pressure value;
seventhly, after the soaking operation is finished, opening a production well exhaust valve 6 and an exhaust valve 8, discharging high-pressure nitrogen in a production well 5, then closing the exhaust valve 8, performing secondary liquid nitrogen pre-injection, opening a nitrogen gas inlet valve 11, a nitrogen gas pressure regulating valve 18, a liquid nitrogen inlet valve 20, a liquid nitrogen discharge valve 21 and a fracturing outlet liquid control valve 24, adding 100kg of low-temperature-resistant propping agent when the liquid nitrogen flows through a fracturing liquid sand-carrying mixing device 23, and carrying the liquid nitrogen with the concentration of 5kg/m3And injecting the propping agent into the reservoir to perform secondary damage on the shale reservoir, and maintaining and promoting the crack to expand. The low temperature resistant proppant is high-strength low temperature resistant bauxite of 30-50 meshes or zirconium silicate of 40-70 meshes. The artificial crack network 2 is conveyed to the artificial crack network 2 through the horizontal fracturing pipeline 4, and the artificial crack network 2 is subjected to low-temperature damage again by using liquid nitrogen, so that the further promotion is realizedThe crack network is developed;
step eight, stopping injecting liquid nitrogen when the underground temperature monitoring probe 3 monitors that the temperature of the artificial fracture network 2 does not change any more, closing the nitrogen gas inlet valve 11, the nitrogen gas pressure regulating valve 18, the liquid nitrogen inlet valve 20, the liquid nitrogen discharge valve 21 and the fracturing outlet liquid control valve 24, and opening the shale gas production inlet valve 9 to extract shale gas;
step nine, when the daily gas yield of the shale gas single well test does not meet the industrial production requirement, all instruments and pipelines in the liquid nitrogen pre-splitting damage circulating system and the nitrogen pressurization system need to be checked and debugged, and the steps three to eight are repeated until the gas yield meets the industrial production requirement.
FIG. 3 is a schematic diagram illustrating damage and fracturing to a predetermined shale gas interval by liquid nitrogen pre-injection.
FIG. 4 is a schematic diagram illustrating fracturing of a reservoir with nitrogen in an artificial fracture network after damage.
FIG. 5 shows the size of a standard sample for rock mechanical test, the mechanical property of the standard sample is representative, and the diameter D of a rock sample, the height H of the rock sample, the diameter phi of a rock sample central hole and the height H of the rock sample central hole in the figure are used for carrying out nitrogen fracturing experimental study on shale subjected to liquid nitrogen pre-injection on the sample. The protocol is shown in Table 1.
Table 1 experimental protocol control variable table
In the first scheme: injecting liquid nitrogen at different time, and comparing the fracture pressure change of the shale; the average fracture pressure of the rock sample nitrogen fracturing is reduced along with the increase of the liquid nitrogen injection time, and the maximum reduction amplitude is 65% compared with that of the rock sample nitrogen fracturing without the liquid nitrogen.
In the second scheme: heating the shale sample to different temperatures and then pre-injecting liquid nitrogen into the shale sample; for a sample which is not injected with liquid nitrogen, the fracture pressure is in a rule of increasing and then reducing along with the rise of the temperature of a rock body, which is mainly caused by the influence of the temperature on organic matters in the rock sample. And for the sample subjected to nitrogen fracturing after liquid nitrogen is injected, the fracture pressure tends to decrease along with the increase of the temperature of the rock body. This indicates that a higher temperature difference results in more complete development of cracks within the test specimen.
In the third scheme: the confining pressure of the test device is changed, and the influence of liquid nitrogen pre-injection on the cracking pressure of the shale under different confining pressures is researched. The burst pressure tends to increase linearly with the increase of the confining pressure, and the burst pressure at all confining pressures after the liquid nitrogen injection is reduced compared with the samples without the liquid nitrogen injection.
Through the three embodiments, a large temperature gradient can be generated around the shale drill hole after the liquid nitrogen is pre-injected, cracks are initiated and expanded due to formed temperature stress, the rock strength can be greatly reduced due to the generation of the micro cracks, the expansion of the cracks after the nitrogen is injected is promoted, and the fracture pressure of the rock is reduced.
The effect of the shale anhydrous fracturing method of fracturing with nitrogen after liquid nitrogen pre-fracturing can be proved from three groups of experiments, and the result shows that the method of fracturing with nitrogen after liquid nitrogen pre-fracturing has advantages compared with the traditional anhydrous fracturing technology; secondly, because the horizontal fracturing process is restricted by various factors, the fracturing effect can be influenced by different reservoir temperatures, different burial depths and the length of liquid nitrogen pre-fracturing time, and three groups of experiments also prove that the method can adapt to different geological conditions and has wide application range. Each protocol involved liquid nitrogen pre-injection, and each group of experiments demonstrated the damaging effects of liquid nitrogen. According to the invention, the shale is damaged by utilizing the low temperature of the liquid nitrogen, and in the process, the nitrogen after the liquid nitrogen is vaporized and expanded is discharged to the ground surface to be stored and pressurized and then is made into the liquid nitrogen again to achieve the purpose of recycling.
Claims (5)
1. A liquid nitrogen pre-splitting and then nitrogen fracturing shale anhydrous fracturing method comprises a liquid nitrogen pre-splitting damage circulation system and a nitrogen pressurization system, wherein the liquid nitrogen pre-splitting damage system comprises a nitrogen liquefaction production device (12), a liquid nitrogen storage device (19) and a liquid nitrogen circulation pre-splitting pipeline, and the liquid nitrogen circulation pre-splitting pipeline comprises a horizontal well fracturing pipeline (4), a shale gas horizontal production well (5), a production well exhaust valve (6), a gas recovery device (7), a nitrogen discharge pipeline (10), a nitrogen gas inlet valve (11), a gas-liquid transportation pipeline (13), a liquid nitrogen inlet valve (20), a liquid nitrogen discharge valve (21), a fracturing liquid sand-carrying mixing device (23) and a fracturing outlet liquid control valve (24); the nitrogen pressurization fracturing system comprises a pressurization device (15), a high-pressure nitrogen container (17) and a well shut-in fracturing pipeline; the well closing fracturing pipeline comprises a horizontal well fracturing pipeline (4), a shale gas horizontal production well (5), a production well exhaust valve (6), a nitrogen gas inlet valve (22), a fracturing fluid sand-carrying mixing device (23) and a fracturing outlet fluid control valve (24); the method is characterized in that: the shale anhydrous fracturing method comprises the following steps:
after drilling of a horizontal shale gas production well (5), paving a horizontal well fracturing pipeline (4) to the position of a shale gas layer section (1) for underground preset operation from a well mouth, connecting the well mouth of the horizontal well fracturing pipeline (4) with the outlet of a fracturing fluid sand-carrying mixing device (23) on the ground through a pipeline, connecting the well mouth of the horizontal shale gas production well (5) with a ground gas recovery device (7) through a gas conveying pipeline, arranging a production well exhaust valve (6) at the inlet of the ground gas recovery device (7), and arranging an exhaust valve (8) at the outlet;
step two, opening a production well exhaust valve (6), an exhaust valve (8), a nitrogen gas inlet valve (22) and a fracturing outlet liquid control valve (24), performing evacuation treatment on the horizontal fracturing pipeline (4), the shale gas horizontal production well (5) and the gas recovery device (7) by using dry nitrogen to expel moisture and air in the horizontal fracturing pipeline and the shale gas horizontal production well, stopping exhausting when the concentration of the nitrogen in the gas recovery device (7) is higher than 99%, and closing the exhaust valve (8) and the nitrogen gas inlet valve (22);
step three, opening a nitrogen gas inlet valve (11), a nitrogen gas pressure regulating valve (18), a liquid nitrogen inlet valve (20) and a liquid nitrogen discharge valve (21), and providing pressure P through a high-pressure nitrogen container (17)1Liquid nitrogen in a liquid nitrogen storage container (19) is injected into the shale gas layer section (1) in the preset operation from the horizontal fracturing pipeline (4) through a fracturing fluid sand-carrying mixing device (23), the damage and pre-cracking are carried out on the periphery of the shale gas layer section (1) by utilizing the low-temperature effect of the liquid nitrogen, and the vaporization reaction can be generated in the liquid nitrogen damage process at normal temperature and normal pressureThe method comprises the steps that nitrogen is generated, the generated nitrogen rises through a horizontal production well (5) and is conveyed into a gas recovery device (7) on the ground, then enters a nitrogen liquefaction production device (12) through a nitrogen discharge pipeline (10) connected with the gas recovery device (7), and the nitrogen liquefaction production device (12) is regulated through an integrated control system (25) to continuously generate liquid nitrogen which flows into a liquid nitrogen storage container (19);
stopping injecting liquid nitrogen when the underground temperature monitoring probe (3) monitors that the surface temperature of the shale gas layer section (1) with the preset operation is not changed any more, closing a nitrogen pressure regulating valve (18), a liquid nitrogen inlet valve (20) and a liquid nitrogen drain valve (21), suspending the operation of a nitrogen liquefaction production device (12) through an integrated control system (25), opening a nitrogen gas inlet valve (14) and a nitrogen gas control valve (16), and storing nitrogen stored in a gas recovery device (7) in a high-pressure nitrogen container (17) through a pressurizing device (15);
step five, closing an exhaust valve (6), a nitrogen gas inlet valve (14) and a nitrogen gas control valve (16) of the production well, opening a nitrogen gas inlet valve (22), conveying high-pressure nitrogen in a fracturing fluid sand-carrying mixing device (23) into the shale gas layer section (1) damaged by liquid nitrogen through a horizontal fracturing pipeline (4), and controlling the nitrogen pressure P in the horizontal fracturing pipeline (4)2Determining according to the fracture pressure condition of the in-situ shale reservoir;
step six, when the pressure in the horizontal fracturing pipeline (4) is larger than the bearable highest nitrogen pressure P2Closing a nitrogen gas inlet valve (22) and a fracturing outlet liquid control valve (24) of a fracturing liquid sand-carrying mixing device (23), carrying out soaking operation on the shale gas layer section (1), pushing nitrogen molecules into smaller nano-scale shale pores, increasing the development of a fracture network, setting the shut-in time to be 2-4h, and recording the shut-in pressure value;
seventhly, after the well is stewed, opening a production well exhaust valve (6), a nitrogen gas inlet valve (11), a nitrogen gas pressure regulating valve (18), a liquid nitrogen inlet valve (20), a liquid nitrogen discharge valve (21) and a fracturing outlet liquid control valve (24), adding a low-temperature-resistant propping agent when liquid nitrogen flows through a fracturing liquid sand-carrying mixing device (23), conveying the liquid nitrogen to the artificial fracture network (2) through a horizontal fracturing pipeline (4), and performing low-temperature damage on the artificial fracture network (2) again by using the liquid nitrogen to further promote the fracture network to develop;
step eight, stopping injecting liquid nitrogen when the underground temperature monitoring probe (3) monitors that the temperature of the artificial fracture network (2) does not change any more, closing a nitrogen gas inlet valve (11), a nitrogen gas pressure regulating valve (18), a liquid nitrogen inlet valve (20), a liquid nitrogen discharge valve (21) and a fracturing outlet liquid control valve (24), and opening a shale gas production inlet valve (9) to extract shale gas;
step nine, when the daily gas yield of the shale gas single well test does not meet the industrial production requirement, all instruments and pipelines in the liquid nitrogen pre-splitting damage circulating system and the nitrogen pressurization system need to be checked and debugged, and the steps three to eight are repeated until the gas yield meets the industrial production requirement.
2. The shale anhydrous fracturing method of claim 1, which is performed by liquid nitrogen pre-fracturing and then nitrogen fracturing, and is characterized in that: the pressure P of the liquid nitrogen injected into the shale borehole1Is 0.1-1 MPa.
3. The shale anhydrous fracturing method of claim 1, which is performed by liquid nitrogen pre-fracturing and then nitrogen fracturing, and is characterized in that: the highest nitrogen pressure P which can be borne by the horizontal fracturing pipeline (4)2Is 150 MPa.
4. The shale anhydrous fracturing method of claim 1, which is performed by liquid nitrogen pre-fracturing and then nitrogen fracturing, and is characterized in that: the low temperature resistant proppant is high-strength low temperature resistant bauxite of 30-50 meshes or zirconium silicate of 40-70 meshes.
5. The shale anhydrous fracturing method of claim 1 or 4, which is performed by liquid nitrogen pre-fracturing and then nitrogen fracturing, and is characterized in that: the concentration of the added low temperature resistant proppant is 5kg/m3100kg of proppant was added per horizontal fracturing interval.
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