Method of nitrogen hydraulic fracturing

Abstract

FIELD: hydraulic fracturing.SUBSTANCE: in the method of nitrogen hydraulic fracturing of a formation, after drilling a horizontal well for the extraction of shale hydrocarbons, a line for transporting the mixture (1) is laid from the wellhead to the target section of the formation (2), the wellhead is connected to a device for mixing the mixture (3), in which a process mixture of liquid nitrogen, guar gum and proppant is prepared at a proppant concentration in the range of 4.5-5.5 kg / m3. During the process of preparing the process mixture, the released gas is fed to a gas utilization device (4), where it is purified from impurities and cooled to a temperature of minus 120 °C. Before feeding the process mixture, the mixture transport line (1) is purged with dry nitrogen. After the liquid nitrogen concentration reaches at least 99%, the control system (23) gives a command to complete the purge. Then liquid nitrogen from the liquid nitrogen storage device (9) is fed into a high-pressure nitrogen tank (24), where the pressure P1 is maintained in the range from 0.1 to 1 MPa. Next, liquid nitrogen is directed through the mixture transport line (1) into the contact zone with the rock (10) in the section of the formation (2), which leads to the development of an artificial network of cracks (12). The gaseous nitrogen formed during evaporation is transported through the nitrogen discharge pipeline (13) to the gas utilization device (4), and then through the hydrocarbon release valve (14) is fed to the nitrogen liquefaction device (11) for its return to the cycle. The process mixture is pumped into the horizontal section of the well (16) using a device for creating high pressure (17). The nitrogen supply is regulated through the gas supply valve (21). When the pressure in the formation P2 reaches 50 MPa and the temperature in the treatment zone stabilizes, the supply of liquid nitrogen is stopped. After completion of injection, the process is soaked for 2 to 4 hours, sealing the well.EFFECT: increase in hydrocarbon recovery from a low-permeability and low-porosity oil and gas formation, and environmentally friendly production of hydrocarbon raw materials.1 cl, 4 dwg, 3 tbl, 3 ex

Description

[0001] The invention relates to the technical field of hydraulic fracturing of shale hydrocarbon formations, in particular, to a method of anhydrous hydraulic fracturing of a formation, suitable for increasing the hydrocarbon recovery of a low-permeability and low-porosity oil and gas formation by carrying out nitrogen hydraulic fracturing after preliminary treatment with liquid nitrogen.

[0002] Foam-nitrogen hydraulic fracturing is known (Maslakov P.S., Vakorin E.V., Kustyshev D.A. Evaluation of the feasibility of foam-nitrogen hydraulic fracturing in the Cenomanian reservoir / / Geology, Geography and Global Energy. - 2010. - No. 3 (38). - Pp. 88-93), which involves pumping into the well under high pressure not only a gel with proppant of a certain concentration, but also an inert gas - nitrogen. Due to this, high conductivity of the formed fracture is achieved. At the same time, nitrogen significantly facilitates the subsequent development of the well after hydraulic fracturing, since after several hours all components of the fracturing fluid disintegrate into their components, and thus the near-wellbore zone of the formation receives a minimal polluting effect.

[0003] The method demonstrated low efficiency when applied to the Berezovskaya Formation, located in the upper part of the section above the Cenomanian reservoir. The Berezovskaya Formation has no commercial gas reserves; the gas reserves here are classified as hard-to-recover; the rocks composing the formation are characterized by extremely low permeability and consist of homogeneous argillite-like clays. Given that the Cenomanian reservoir currently has abnormally low formation pressure and high water cut, the effect of foam-nitrogen fracturing did not meet expectations. Furthermore, the well could not be developed due to the removal of carbonated fluid. This is due to the fact that the fracturing process artificially created a zone with a higher permeability in the near-wellbore zone than in the formation itself.For this reason, water and gas contained in the underlying Cenomanian reservoir rushed into this zone due to the hydrodynamic connection created in the impermeable clay seal, rather than toward the fracture. Furthermore, analysis of the results of foam-nitrogen hydraulic fracturing in the Berezovskaya Formation shows that the fracture developed vertically, rather than horizontally as originally planned. Consequently, it was not possible to achieve a higher degree of reliability for the projected fracture profile.

[0004] A known method of hydraulic fracturing (RU Patent No. 2507389, published February 20, 2014) involves injecting a mixture of fracturing fluid and a proppant into the formation. Gas crystal hydrates are used as the proppant, and injection is performed under the thermobaric conditions of their existence. After fracturing, the gas crystal hydrates decompose, releasing a gas phase that further props the macro- and microfractures in the formation. In this case, the injection of the fracturing fluid and proppant mixture, the formation fracturing, and the decomposition of the crystal hydrates are performed single or multiple times.

[0005] The disadvantage of this method is its low productivity and is only applicable at depths greater than 700 m. When the pressure decreases, gas hydrate decomposes from its fragile crystalline structure into gas and water. At low temperatures, the released water reverts to ice (freezes) and prevents the gas from escaping. Heating is the most suitable method for decomposing gas hydrate into gas and water. However, the enthalpy of fusion of gas hydrate (in combination with methane) is estimated at 3.06 kJ / g, while water ice melts (for comparison) with a heat consumption of 2.83 kJ / g. This requires high energy expenditure for the gas hydrate decomposition process.

[0006] A known method of hydraulic fracturing with supercritical carbon dioxide (Galimov D.I., Savenok O.V. Hydraulic fracturing with supercritical carbon dioxide in a low-permeability reservoir / / SCIENCE. TECHNOLOGY. TECHNOLOGIES (Polytechnic Bulletin). - 2022. - No. 4. - Pp. 289-295), which involves replacing traditional hydraulic fracturing fluid with supercritical carbon dioxide in order to increase the efficiency of the event in a low-permeability oil and gas reservoir. When using ScCO2 as a hydraulic fracturing fluid, its supercritical state under reservoir conditions is not disturbed due to the fact that the critical values ​​​​of pressure and temperature of carbon dioxide in most cases are lower than the reservoir ones.Hydraulic fracturing with supercritical carbon dioxide creates multiple, irregularly shaped fractures that are likely to cause secondary fractures in the rock, connecting natural and artificial fractures to form a complex network.

[0007] The disadvantages of this method are: 1) the method has not been sufficiently studied in real conditions; the effectiveness of using ScCO2 is mainly determined by the results of laboratory studies; 2) additional costs that reduce the economic attractiveness of the project; 3) the risk of groundwater pollution and environmental damage to the work area.

[0008] A fracturing method using high-pressure nitrogen and low-temperature liquid nitrogen to enhance oil and gas production is known (CN109707360 High-pressure nitrogen-low-temperature liquid nitrogen composite fracturing method for increasing oil and gas production, published May 3, 2019). This method involves alternately and repeatedly injecting high-temperature, high-pressure nitrogen and low-temperature liquid nitrogen into the formation during fracturing. This approach eliminates the drawback of traditional hydraulic fracturing, which typically produces only a single main fracture. Instead, a complex, large-volume fracture system is formed in the formation, significantly increasing the efficiency of enhanced oil and gas recovery.

[0009] The disadvantage of this method is that, due to its low viscosity, nitrogen cannot transport sufficient amounts of proppant. As a result, fractures quickly collapse and production declines. This is a key limitation for large-scale application of the technology. Therefore, it is critical to improve the ability of fractures to remain open after nitrogen injection is complete.

[0010] A method for anhydrous fracturing using pre-injection of liquid nitrogen and subsequent fracturing with nitrogen for hydrocarbon production from low-permeability formations is known (CN Patent No. 113309519, published August 27, 2021). This method, adopted as a prototype, creates cracks in the rock around the well using low temperatures, and then the nitrogen penetrates deeper, creating a complex fracture network. This solves the problem of fracture development during anhydrous fracturing and increases hydrocarbon production.

[0011] The disadvantage of this method is its limited effectiveness at high reservoir pressures and unstable thermodynamic conditions, which can lead to incomplete evaporation of nitrogen and a reduction in its displacement effect. Furthermore, the use of liquid nitrogen requires additional costs for cryogenic equipment and transportation, increasing the cost of the operation in remote and hard-to-reach regions.

[0012] The technical result is to increase the efficiency of anhydrous hydraulic fracturing and the environmental friendliness of production and the completeness of hydrocarbon raw material extraction.

[0013] The technical result is achieved in that after drilling a horizontal well for the production of shale hydrocarbons, a mixture transportation line (1) is laid from the wellhead to the target section of the formation (2), the wellhead is connected to a device for mixing the mixture (3), in which a process mixture of liquid nitrogen, guar gum and proppant is prepared at a proppant concentration in the range of 4.5-5.5 kg / m3, during the preparation of the process mixture, the released gas is sent to a gas utilization device (4), where it is purified from impurities and cooled to a temperature of minus 120 ° C, before feeding the process mixture, the mixture transportation line (1) is purged with dry nitrogen, for this purpose the gas discharge valve of the production well (5), the nitrogen inlet valve (6), the nitrogen gas inlet valve (7) and the bypass valve for feeding the hydraulic fracturing fluid (8) are opened in stages, forming a circulation contour,the temperature and pressure are monitored by reservoir temperature and pressure monitoring sensors (27), which transmit data to the control system (23), after the liquid nitrogen concentration reaches at least 99%, the control system (23) gives a command to terminate the blowdown, then liquid nitrogen is fed from the liquid nitrogen storage device (9) to the high-pressure nitrogen tank (24), where the pressure P1 is maintained in the range from 0.1 to 1 MPa, then the liquid nitrogen is directed through the mixture transport line (1) to the contact zone with the rock (10) in the reservoir section (2), which leads to the development of an artificial crack network (12), the gaseous nitrogen formed during evaporation is transported through the nitrogen discharge pipeline (13) to the gas utilization device (4), and then through the hydrocarbon release valve (14) is directed to the nitrogen liquefaction device (11) for its return to the cycle,the process mixture is pumped into the horizontal section of the well (16) by a device for creating high pressure (17) through the hydraulic fracturing fluid supply bypass valve (8), the liquid supply valve (18) and the liquid nitrogen release valve (19) for regulating the supply, the nitrogen supply is regulated through the gas supply valve (21), when the pressure in the formation P2 reaches 50 MPa and the temperature in the treatment zone, recorded by the temperature and pressure control sensors under formation conditions (27), stabilizes, the supply of liquid nitrogen is stopped and the valves are closed: the nitrogen inlet valve (6), the nitrogen gas inlet valve (7), the liquid supply valve (18), the liquid nitrogen release valve (19) and the gas pressure regulating valve (22), the gaseous nitrogen remaining in the system is directed to the nitrogen liquefaction device (11) and then to the high-pressure nitrogen tank (24), after the injection is completed, a technological holding period of 2 to 4 hours is carried out, sealing the well,after completion of the holding period, the production well gas discharge valve (5), the nitrogen inlet valve (6), the nitrogen gas inlet valve (7), the liquid nitrogen outlet valve (19), the gas pressure control valve (22) and the gas supply control valve (25) are opened; after the temperature conditions have stabilized and the changes recorded by the reservoir temperature and pressure control sensors (27) have been completed, the liquid nitrogen supply is finally stopped and the nitrogen inlet valve (6), the nitrogen gas inlet valve (7), the liquid nitrogen outlet valve (19), the gas pressure control valve (22) and the gas supply control valve (25) are closed; if the required fracturing and permeability parameters are not achieved during hydraulic fracturing, the process steps from purging until the injection is completed are repeated to obtain the required result, then the nitrogen gas inlet valve (26) is opened and gas is supplied to the well to begin industrial production of shale hydrocarbons.

[0014] The method is explained by the following figures:

[0015] Fig. 1 - general diagram of the method;

[0016] Fig. 2 - graph of the dependence of the depth of the fracture zone on pressure;

[0017] Fig. 3 - graph of the effect of proppant concentration on permeability;

[0018] Fig. 4 - graph of the efficiency of nitrogen phase transitions, where:

[0019] 1 - mixture transportation line;

[0020] 2 - section of the formation;

[0021] 3 - device for mixing the mixture;

[0022] 4 - gas disposal device;

[0023] 5 - gas relief valve, production well;

[0024] 6 - nitrogen inlet valve;

[0025] 7 - nitrogen gas inlet valve;

[0026] 8 - bypass valve for supplying hydraulic fracturing fluid;

[0027] 9 - Liquid nitrogen storage device;

[0028] 10 - rock;

[0029] 11- device for liquefying nitrogen;

[0030] 12 - artificial crack network;

[0031] 13 - nitrogen discharge pipeline;

[0032] 14 - hydrocarbon release valve;

[0033] 15 - dispenser for adding guar gum;

[0034] 16 - horizontal section of the well;

[0035] 17 - high pressure generating device;

[0036] 18 - liquid supply valve;

[0037] 19 - liquid nitrogen release valve;

[0038] 20 - gas and liquid transportation pipeline;

[0039] 21 - gas supply valve;

[0040] 22 - gas pressure regulating valve;

[0041] 23 - control system;

[0042] 24 - high pressure nitrogen tank;

[0043] 25 - gas supply control valve;

[0044] 26 - Nitrogen gas inlet valve;

[0045] 27 - reservoir temperature and pressure monitoring sensor.

[0046] After drilling a horizontal well for shale hydrocarbon production, the pipeline for the mixture transport line 1 (Fig. 1) is laid from the wellhead to the target section of the formation 2. The wellhead is connected to a mixture mixing device 3, where the process mixture is prepared. The process mixture is a composition of liquid nitrogen, guar gum, and proppant.

[0047] During the mixture preparation process, the released gas is sent to gas utilization device 4, where it is purified from impurities and cooled to a temperature of -120°C to ensure process safety and protect the system from contamination.

[0048] Before feeding the process mixture, mixture transport line 1 is purged with dry nitrogen. To do this, the production well gas discharge valve 5, nitrogen inlet valve 6, nitrogen gas inlet valve 7, and fracturing fluid bypass valve 8 are opened in stages, forming a circulation loop. Temperature and pressure are monitored by reservoir temperature and pressure sensors 27, which transmit data to control system 23. Once the liquid nitrogen concentration reaches at least 99%, control system 23 signals the end of the purging.

[0049] Then, liquid nitrogen from liquid nitrogen storage device 9 is fed to high-pressure nitrogen tank 24, where pressure P1 is maintained in the range of 0.1 to 1 MPa. Liquid nitrogen is then directed through mixture transport line 1 to the rock contact zone 10 in section 2 of formation. The effect of liquid nitrogen leads to the development of artificial fracture network 12. The depth of the fracture zone is determined by a logarithmic relationship:

[0050]

[0051] whereL тр - depth of fracturing zone, m;

[0052] k- empirical coefficient;

[0053] P1 - nitrogen injection pressure, MPa;

[0054] P пл - reservoir pressure, MPa.

[0055] The nitrogen gas generated by evaporation is transported through the nitrogen discharge pipeline 13 to the gas utilization device 4, and then through the hydrocarbon discharge valve 14 is sent to the nitrogen liquefaction device 11 to return it to the cycle.

[0056] The efficiency of the nitrogen phase transition during recirculation is determined by the formula:

[0057]

[0058] whereE фп - phase transition efficiency;

[0059] m сж - mass of newly liquefied nitrogen;

[0060] m газ - mass of evaporated nitrogen.

[0061] In the next step, guar gum is added to the process mixture using a guar gum injector 15. The mixture is pumped into the horizontal section of the well 16 using a high-pressure device 17 through a hydraulic fracturing fluid supply bypass valve 8, a fluid supply valve 18, and a liquid nitrogen release valve 19 to regulate the flow rate.

[0062] Proppant is transported with nitrogen through the gas and liquid transportation pipeline 20 at a concentration of up to 5 kg / m3 3 The mass concentration of proppant is determined by the formula:

[0063]

[0064] whereS пр- mass concentration of proppant, kg / m3 3 ;

[0065] Q тр - required volume of proppant, m 3 ;

[0066] ρ пр - proppant density, kg / m3 3 ;

[0067] V см - volume of working mixture, m 3 .

[0068] The nitrogen supply is regulated through gas supply valve 21 to maintain the required pressure and mixture homogeneity. If necessary, additional nitrogen gas is pumped through gas pressure regulating valve 22.

[0069] When the pressure P2 in the formation reaches 50 MPa and the temperature in the treatment zone, recorded by the temperature and pressure monitoring sensors under formation conditions 27, stabilizes, the supply of liquid nitrogen is stopped and the valves are closed: nitrogen inlet valve 6, nitrogen gas inlet valve 7, liquid supply valve 18, liquid nitrogen outlet valve 19 and gas pressure regulating valve 22. The gaseous nitrogen remaining in the system is directed to the nitrogen liquefaction device 11 and then to the high-pressure nitrogen tank 24.

[0070] After injection is complete, the well is held for 2 to 4 hours, sealing the well. During this time, nitrogen molecules diffuse into the rock, developing secondary fracturing. The pressure in the confined space is monitored by reservoir temperature and pressure sensors 27 and control system 23, and the resulting data is stored for subsequent analysis.

[0071] After the holding period is completed, open the production well gas discharge valve 5, the nitrogen inlet valve 6, the nitrogen gas inlet valve 7, the liquid nitrogen outlet valve 19 and the gas pressure regulating valve 22. If necessary, re-introduce the process mixture for further development of fracturing.

[0072] After the temperature conditions have stabilized and the changes recorded by the reservoir temperature and pressure monitoring sensors 27 have completed, the supply of liquid nitrogen is finally stopped and the nitrogen inlet valve 6, the nitrogen gas inlet valve 7, the liquid nitrogen outlet valve 19, the gas pressure regulating valve 22 and the gas supply control valve 25 are closed. Then, the nitrogen gas inlet valve 26 is opened and gas is supplied to the well to begin industrial production of shale hydrocarbons.

[0073] If the required fracturing and permeability parameters are not achieved during hydraulic fracturing, the system is reconfigured and the process steps from blowdown to completion of injection are repeated to achieve the required result.

[0074] The method is explained by the following examples.

[0075] Example 1. Determining the effective liquid nitrogen injection pressure to create stable fracturing. The effect of liquid nitrogen injection pressure on the depth and shape of microcracks was simulated in a 2800 m deep well. When nitrogen was injected at a pressure of P = 30 MPa, microcracks formed in the contact zone with the rock at a distance of up to 7 m from the borehole wall. When the pressure was increased to 50 MPa, the affected zone expanded by 40%, but the increase in fracturing depth slowed down. The optimal injection pressure is in the range from 30 to 40 MPa, since it ensures the formation of a stable fracture network with minimal liquid nitrogen consumption (Table 1). Based on the calculation data using formula (1), a graph was constructed (Fig. 2), which reflects the dependence of the fracturing depth on the injection pressure.Analysis of the graph showed that when the pressure reaches over 45 MPa, the increase in the fracture zone decreases significantly, which indicates that further increasing the pressure is inappropriate from the point of view of cost-effectiveness and resource efficiency.

[0076] Table 1 - Dependence of the depth of the fracture zone on the pressure of liquid nitrogen

[0077] Pressure, MPa Depth of fracturing zone, m 30 7,0 35 9,0 40 9,8 50 11,2

[0078] Example 2. Determining the effective concentration of low-temperature proppant. During tests in the tNavigator software package in a horizontal section of a 1000 m long well, the pumping results were compared at different proppant concentrations. At a concentration of 3 kg / m 3 Premature settling of the material in the well was observed. With an increase in concentration to 5 kg / m3 3uniform proppant distribution along the horizontal wellbore was achieved, which increased formation permeability by 22% (see Table 2). At a concentration exceeding 6 kg / m3 3 , partial plugging of microcracks and, as a result, a decrease in flow rate were observed. Thus, the optimal proppant concentration is in the range of 4.5-5.5 kg / m3. 3 .

[0079] When calculating using formula (2), a graph was plotted (Fig. 3) reflecting the effect of proppant concentration on formation permeability. Analysis of the graph confirmed the presence of an optimum in the 5 kg / m3 zone. 3 , beyond which the positive effect is reduced due to hydrodynamic resistance and aggregation of proppant particles.

[0080] Table 2 - Effect of proppant concentration on formation permeability

[0081] Proppant concentration, kg / m3 Increase in permeability, % 3 9 4 16 5 22 6 18

[0082] Example 3. Evaluating the Stability of a Liquid Nitrogen Recirculation Cycle. To confirm the viability of the recirculation system, liquid nitrogen was captured in a special device after evaporation, re-liquefied in a liquefaction unit, and then accumulated in a high-pressure tank. Over the course of three consecutive reuse cycles, the phase transition efficiency was 89-92% (Table 3), with losses not exceeding 8% of the initial volume. Using formula (3), a graph was constructed (Fig. 4) showing the dynamics of changes in the proportion of liquefied nitrogen depending on the number of cycles. The graph demonstrates that, beginning with the third cycle, the efficiency stabilizes, indicating the stability of the process during repeated use.

[0083] Table 3 - Efficiency of nitrogen phase transitions

[0084] Reuse cycle Liquefaction efficiency, % 1 91 2 89 3 92

[0085] Thus, the developed hydraulic fracturing method using liquid nitrogen (LN2) in combination with the gelling component guar gum provides a comprehensive cryogenic effect on the rock, promoting the formation of a developed and stable fracture network in shale reservoirs. Due to the sharp temperature gradient and the water-ice phase transition in saturated rocks, a localized volume increase occurs, which initiates additional opening and branching of fractures. The introduction of guar gum into the working fluid increases its viscosity, ensuring uniform proppant distribution throughout the formation and stable fracture openness. This reduces fracture pressure, decreases the required volume of working fluid, and reduces the time required to prepare the well for production.

[0086] Increasing the efficiency of anhydrous fracturing and environmental friendliness of production is achieved through the use of liquid nitrogen as the primary working fluid, eliminating water pollution. This mixture, consisting of liquid nitrogen, guar gum, and proppant, provides a balance between fluidity and proppant retention, facilitating the formation of a stable fracture network. Nitrogen recirculation and re-liquefaction minimize emissions and costs, while multi-stage temperature and pressure control enhances process accuracy and safety. All this increases hydrocarbon recovery and reduces environmental impact.

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