Waterless Fracturing Technology Research Articles

Mechanism and Model Analysis of Ultralow-Temperature Fluid Fracturing in Low-Permeability Reservoir: Insights from Liquid Nitrogen Fracturing

Ultralow-temperature fluids (such as liquid nitrogen, liquid CO2) are novel waterless fracturing technologies designed for dry, water-sensitive reservoirs. Due to their ultralow temperatures, high compression ratios, strong frost heaving forces, and low viscosities, they offer a solution for enhancing the fracturing and permeability of low-permeability reservoirs. In this study, we focus on the combined effects of high-pressure fluid rock breaking, low-temperature freeze-thaw fracturing, and liquid-gas phase transformation expansion on coal-rock in low-permeability reservoirs during liquid nitrogen fracturing (LNF). We systematically analyze the factors that limit the LNF effectiveness, and we discuss the pore fracture process induced by low-temperature fracturing in coal-rock and its impact on the permeability. Based on this analysis, we propose a model and flow for fracturing low-permeability reservoirs with low-temperature fluids. The analysis suggests that the Leidenfrost effect and phase change after ultralow-temperature fluids enter the coal support the theoretical feasibility of high-pressure fluid rock breaking. The thermal impact and temperature exchange rate between the fluid and coal determine the temperature difference gradient, which directly affects the mismatch deformation and fracture development scale of different coal-rock structures. The low-temperature phase change coupling fracturing of ultralow-temperature fluids is the key to the formation of reservoir fracture networks. The coal-rock components, natural fissures, temperature difference gradients, and number of cycles are the key factors in low-temperature fracturing. In contrast to those in conventional hydraulic fracturing, the propagation and interaction of fractures under low-temperature conditions involve multifield coupling and synergistic temperature, fluid flow, fracture development, and stress distribution processes. The key factors determining the feasibility of the large-scale application of ultralow-temperature fluid fracturing in the future are the reconstruction of fracture networks and the enhancement of the permeability response in low-permeability reservoirs. Based on these considerations, we propose a model and process for LNF in low-permeability reservoirs. The research findings presented herein provide theoretical insights and practical guidance for understanding waterless fracturing mechanisms in deep reservoirs.

In-situ laboratory study on influencing factors of pre-SC-CO2 hybrid fracturing effect in shale oil reservoirs

Supercritical CO2 (SC-CO2) fracturing, being a waterless fracturing technology, has garnered increasing attention in the shale oil reservoir exploitation industry. Recently, a novel pre-SC-CO2 hybrid fracturing method has been proposed, which combines the advantages of SC-CO2 fracturing and hydraulic fracturing. However, the specific impacts of different pre-SC-CO2 injection conditions on the physical parameters, mechanical properties, and crack propagation behavior of shale reservoirs remain unclear. In this study, we utilize a newly developed "pre-SC-CO2 injection → water-based fracturing" integrated experimental device. Through experimentation under in-situ conditions, the impact of pre-SC-CO2 injection displacement and volume on the shale mineral composition, mechanical parameters, and fracture propagation behavior are investigated. The findings of the study demonstrate that the pre-injection SC-CO2 leads to a reduction in clay and carbonate mineral content, while increasing the quartz content. The correlation between quartz content and SC-CO2 injection volume is positive, while a negative correlation is observed with injection displacement. The elastic modulus and compressive strength exhibit a declining trend, while Poisson's ratio shows an increasing trend. The weakening of shale mechanics caused by pre-injection of SC-CO2 is positively correlated with the injection displacement and volume. Additionally, pre-injection of SC-CO2 enhances the plastic deformation behavior of shale, and its breakdown pressure is 16.6% lower than that of hydraulic fracturing. The breakdown pressure demonstrates a non-linear downward trend with the gradual increase of pre-SC-CO2 injection parameters. Unlike hydraulic fracturing, which typically generates primary fractures along the direction of the maximum principal stress, pre-SC-CO2 hybrid fracturing leads to a more complex fracture network. With increasing pre-SC-CO2 injection displacement, intersecting double Y-shaped complex fractures are formed along the vertical axis. On the other hand, increasing the injection rate generates secondary fractures along the direction of non-principal stress. The insights gained from this study are valuable for guiding the design of pre-SC-CO2 hybrid fracturing in shale oil reservoirs.

Investigation of Rock Mechanical Properties under Liquid Nitrogen Environment

In order to promote sustainable energy development and reduce the impact of fossil fuels on the environment, it is crucial to strengthen the development and utilization of clean and renewable geothermal energy. Liquid nitrogen fracturing, as an emerging waterless fracturing technology, has outstanding advantages in rock fracturing effect and thermal exchange ability with hot dry rock and is more environmentally friendly. In order to evaluate the influence of liquid nitrogen on the mechanical properties, acoustic emission characteristics, and cross-sectional crack propagation characteristics of granite at different initial temperatures, this paper carried out three-point bending tests and acoustic emission detection on granite treated by high-temperature heating and liquid nitrogen cooling. Finally, based on the cross-sectional scanning test, the expansion characteristics of microcracks in granite were analyzed. The results show that the higher the initial temperature of granite, the stronger the cold impact of liquid nitrogen on granite, and the faster the rock’s mechanical performance declines. The acoustic emission ringing count is closely related to the development of microcracks in granite, and as the initial temperature of granite increases, the more ringing counts there are, indicating that the huge temperature difference induces more microcracks inside the rock. In addition, the cold impact of liquid nitrogen can effectively promote the fracturing of granite. After liquid nitrogen treatment, the fractal dimension of the granite cross-section increases, the shape of the cross-section becomes rough, and many micropores appear. This study can provide a scientific basis for the engineering application of liquid nitrogen fracturing technology.

Damage characteristics of pore and fracture structures of coal with liquid nitrogen freeze thaw

Liquid nitrogen (LN2) fracturing technology is a novel waterless fracturing technology that has significant potential for application in the development of coalbed methane. However, the changes in the microstructure after coal samples are treated with LN2 freeze thaw are poorly understood. Therefore, a combination of mercury intrusion porosimetry and micro-computed tomography (micro CT) was employed to investigate the evolution of pore and fracture structure of coal samples treated with LN2. The experimental results showed that the pore volume and average pore size of coal samples increase after LN2 freeze thaw. After 12 freeze thaw cycles, the change in pore volume of micropores and minipores of coal samples was not significant, while the pore volume of mesopores and macropores increased significantly before LN2 freeze thaw. The specific surface area of the pores in different size ranges of coal samples increases with the increase in the number of LN2 freeze thaw cycles; the structure of micropores and miniopores were damaged by thermal stress and frost heave force during LN2 freeze thaw; and the pore size gradually increases to form mesopores and macropores. Micro-CT images of coal samples after LN2 freeze thaw indicated the primary fractures of coal sample expanded and generated a large number of secondary fractures. The primary and secondary fractures are interconnected and ultimately form penetrated fracture enhancing the connectivity of fractures, enhancing the connectivity of the fracture structure. The key finding study is expected to provide a theoretical basis for LN2 fracturing.

Numerical simulations of supercritical carbon dioxide fracturing: A review

As an emerging waterless fracturing technology, supercritical carbon dioxide (SC-CO 2 ) fracturing can reduce reservoir damage and dependence on water resources, and can also promote the reservoir stimulation and geological storage of carbon dioxide (CO 2 ). It is vital to figure out the laws in SC-CO 2 fracturing for the large-scale field implementation of this technology. This paper reviews the numerical simulations of wellbore flow and heat transfer, fracture initiation and propagation, and proppant transport in SC-CO 2 fracturing, including the numerical approaches and the obtained findings. It shows that the variations of wellbore temperature and pressure are complex and strongly transient. The wellhead pressure can be reduced by tubing and annulus co-injection or adding drag reducers into the fracturing fluid. Increasing the temperature of CO 2 with wellhead heating can promote CO 2 to reach the well bottom in the supercritical state. Compared with hydraulic fracturing, SC-CO 2 fracturing has a lower fracture initiation pressure and can form a more complex fracture network, but the fracture width is narrower. The technology of SC-CO 2 fracturing followed by thickened SC-CO 2 fracturing, which combines with high injection rates and ultra-light proppants, can improve the placement effect of proppants while improving the complexity and width of fractures. The follow-up research is required to get a deeper insight into the SC-CO 2 fracturing mechanisms and develop cost-effective drag reducers, thickeners, and ultra-light proppants. This paper can guide further research and promote the field application of SC-CO 2 fracturing technology.

Calculation of fracture number and length formed by methane deflagration fracturing technology

The innovative Methane in-situ deflagration fracturing technology offers several benefits. It can prevent formation pollution and eliminates the need for complex treatment procedures for backflow materials. Additionally, it reduces ground transportation expenses and is poised to become a highly effective and eco-friendly waterless fracturing technology. This paper suggests methods for determining the number and length of fractures in oil and gas well stimulation, considering the main influencing factors. The fracture number is calculated using mass conservation principles, while the fracture length is determined using the crack propagation model. The fracturing experiments with large-sized samples were conducted to verify the technical feasibility of the new fracturing technology and fill the gap in large-scale physical model experimental research for deflagration fracturing technology. Following the experiment, compare experimental data with calculated data, and analyze the factors that affect the fracture number and length. The results indicate that both methods are highly feasible and possess strong predictive accuracy. It is advised that to enhance the fracturing effect on formations with high crustal stress, it is beneficial to appropriately increase the peak pressure, reduce the pressure rate, and increase the fracturing time while ensuring that the wellbore remains undamaged. Moreover, these proposed methods are versatile and adaptable, making them suitable for explosive fracturing within hydraulically fractured formations and high-energy gas fracturing technology.

Coupled TMD modeling of cryogenic treatment of borehole in coal under confinement

Cryogenic fracturing using liquid nitrogen (LN2) is a waterless fracturing technology that is under extensive research nowadays given the high demand of environmental-friendly development of unconventional oil and gas resources. Excluding the exerted hydraulic pressure, the initiation and propagation of cryogenic fractures in rock materials largely depend on thermal stress generated by LN2 and the rock properties. So far, there are quite a few experimental studies elaborating on the cryogenic fracturing process of coal rocks; however, modeling studies on the topic are still very preliminary with no damage quantification. This study first set up a coupled thermo-mechanical (TM) modeling procedure by solving the heat conduction and rock mechanics equations in a serial manner and incorporating the maximum principal stress failure criterion. Then the numerical stability of this modeling procedure was examined through time step and grid resolution tests, and the model accuracy was corroborated by analytical solutions. Furthermore, the damage criterion was quantitatively validated against our experimental observations. Using this thermo-mechanical damage (TMD) modeling procedure, a systematic examination of the effects of key factors on cryogenic circulation treatment of coal rock borehole was carried out using realistic parameters. Trends and patterns of the cryogenic damage upon key factors, including realistic temperature boundary with the Leidenfrost effect, Young's modulus, thermal expansion coefficient, Poisson's ratio, specific heat capacity, density, thermal conductivity, in-situ stresses, and initial temperature of the coal rocks have been revealed and discussed. The modeling results herein are of great significance to understand the mechanisms of cryogenic fracturing of coal rocks, and this TMD modeling procedure could be further expanded to evaluate and optimize cryogenic treatment process of coal seams in field pilot tests.

Experimental Investigation on Cracking Characteristics of Dry and Saturated Shales in Nitrogen Fracturing after Liquid Nitrogen (LN2) Injection

Cryogenic LN2 fracturing is one of the environmentally friendly waterless fracturing technologies that promote the fracture complexity of shale gas reservoir. The water-ice phase transition under freezing condition causes frost heave in saturated shale. The effect of moisture in shale should be taken into account during cryogenic damage process. Therefore, the differences of cracking characteristics between dry and saturated shales were studied in this paper. A laboratory triaxial and high temperature fracturing system was developed for nitrogen fracturing dry and saturated shale after LN2 injection. The influence of moisture on breakdown pressure was studied under different confining pressures (3 MPa, 6 MPa, 9 MPa, and 12 MPa) and bedding directions (parallel bedding and vertical bedding). The experimental results demonstrated that when the confining pressure increased from 3 MPa to 12 MPa, the breakdown pressure of dry parallel bedding after LN2 preconditioning decreased 7.12 MPa, 6.06 MPa, 4.58 MPa, and 3.11 MPa, respectively. Therefore, LN2 preconditioning could damage shale resulting in a lower breakdown pressure, but the effect of cryogenic damage decreased with the confining pressure increasing. The moisture in shale had little impact on nitrogen fracturing without LN2 injection because the breakdown pressure difference between dry and saturated shales was small. However, the breakdown pressure of saturated shale after LN2 preconditioning was always lower than that of dry shale. The breakdown pressure of saturated parallel bedding shale after LN2 injection decreased 8.62 MPa, 7.67 MPa, 6.08 MPa, and 4.63 MPa, respectively, with the confining pressure increasing from 3 MPa to 12 MPa. The breakdown pressure difference between dry and saturated shales was impacted by the migration of unfrozen water and frost heave. In addition, the extent of cryogenic damage varied substantially between different bedding directions. When the confining pressure was 3 MPa, the breakdown pressure of saturated parallel bedding shale reduced by 69.18% after LN2 preconditioning, but that of saturated vertical bedding shale only decreased by 22.49%. The tensile strength of shale had a greater influence on the breakdown pressure. According to the Brazilian disc test results, the tensile strength of matrix was much higher than that of bedding planes. As a result, it is useful to wet the shale in order to reduce the breakdown pressure. The fracturing direction of horizontal drilling should be consistent with the bedding direction for better cryogenic fracturing effect.

Effects of Liquid CO2 Phase Transition Fracturing on Methane Adsorption of Coal

Liquid CO2 phase transition fracturing (LCPTF) is a kind of novel waterless fracturing technology for enhancing the coalbed methane (CBM) recovery. Relevant current studies are focused on exploring the transformed effect of LCPTF on the nanopore structure in coal. However, its influence on gas adsorption capacity has been rarely reported. This study addresses coal induced with LCPTF. The structure alterations of mesopores (2–50 nm) and macropores (>50 nm) and their effects on the gas adsorption capacity are evaluated with comprehensive measurements of a mercury intrusion porosimetry, low-temperature N2 adsorption, and isothermal adsorption. The results indicate that LCPTF has an enlarged effect on macropores, resulting in an increase in pore size and volume and a reduction of the pore specific surface area. The pore-enlarged-transformed effects of LCPTF cause an increase in pore size and a reduction of the pore volume and pore specific surface area of mesopores. The variations of the pore structure after LCPTF cause a reduction of the adsorption constant a and the increase in the adsorption constant b, indicating LCPTF’s reductive effect on adsorption capacity as well as its enhanced effect on desorption capacity. A novel effect evaluation method of LCPTF for improving CBM recoverability is proposed based on the variations of gas saturation and critical desorption pressure. This study examines, from the perspective of adsorption and desorption, the mechanism of LCPTF for enhancing CBM recovery, which provides theoretical guidance for LCPTF’s technical improvement and optimization of field application so as to secure a more reliable and efficient CBM recovery.

Experimental study on the fissure structure and permeability evolution characteristics of coal under liquid nitrogen freezing and freeze–thaw

Liquid nitrogen (LN2) fracturing technology as a kind of waterless fracturing technology has received extensive attention in recent years. In order to investigate the evolution law of fissure structure and seepage characteristics of coal samples under LN2 freezing and freeze–thaw, the evolution of fissure of coal samples before and after LN2 freeze–thaw was monitored by micro-computed tomography, the change of permeability of coal samples under different LN2 freezing time and freeze–thaw cycles was measured, and the damage mechanism of LN2 freezing and freeze–thaw to coal was discussed. The experimental results show that (1) LN2 freeze–thaw can cause the shrinkage of the coal matrix, resulting in damage to the fissure structure of the coal sample, which promotes the initiation, expansion, and extension of fissure of coal sample to form new fissure and, finally, forms a fracture network. (2) The permeability and the increment of permeability of coal samples increase with increase of LN2 freezing time and the number of freeze–thaw cycles under different gas pressure and confining pressure condition. (3) Under the same freezing time, the permeability growth rate of coal samples under LN2 freeze–thaw condition is significantly greater than that of coal samples under LN2 freezing condition. (4) The frost-heave force and thermal stress are the main factors leading to the damage, promoting fissure formation and increasing permeability of coal samples LN2 freezing and freeze–thaw. This study provides a theoretical basis for the understanding of fracturing technology with LN2.

Simulation of wellbore temperature and pressure field in supercritical carbon dioxide fracturing

As unconventional oil and gas receive more and more attention, scholars pay more and more attention to waterless fracturing technology. Among them, carbon dioxide fracturing technology, as the main representative of waterless fracturing technology, has unique advantages and broad application prospects in the development of unconventional oil and gas reservoirs. However, when using carbon dioxide fracturing fluid for construction, carbon dioxide is greatly affected by temperature and pressure, so it is necessary to simulate the temperature and pressure in the wellbore during carbon dioxide fracturing construction. In this article, the basic physical properties of carbon dioxide under different temperatures and pressures are analyzed, and the changes of wellbore temperature and pressure during carbon dioxide fracturing are studied based on the generalized heat conduction theory. The results show that the faster the injection rate of CO2 fracturing fluid at the wellhead, the lower the bottom hole temperature and the higher the bottom hole pressure. However, the higher the wellhead injection temperature, the higher the bottom hole temperature, and the lower the bottom hole pressure.

Effect of liquid nitrogen cooling on mechanical characteristics and fracture morphology of layer coal under Brazilian splitting test

Liquid nitrogen (LN 2 ) fracturing, as an environmentally friendly waterless fracturing technology, is given more and more attention in coalbed methane (CBM) exploitation. In this paper, a series of microscopic tests and a thermal damage simulation were combined to evaluate the changes in the internal structure of the coal caused by the LN 2 cooling, firstly. Then, Brazilian splitting tests were carried out on the bedding coal, and influences of cryogenic cooling fracturing and bedding orientation on mechanical properties and fracture morphology of the bedding coal were analyzed. The results show that the micro-structure of the coal is obviously damaged under the LN 2 cooling due to the thermal tensile stress. The P-wave velocity, tensile strength , Brazilian splitting modulus, and brittleness index of the coal sample have significant anisotropic characteristics and their values are reduced after the LN 2 cooling. The weakening degree in mechanical characteristics of the coal induced by the LN 2 cooling is also closely related to the bedding direction of the coal. After the LN 2 cooling treatment, the maximum and minimum reduction of the tensile strength is at the bedding angle 0° and 45°, respectively, with the reduction of 55.81% and 6.98%, respectively. The maximum and minimum changes of the Brazilian splitting modulus and brittleness index before and after the LN 2 cooling locate at 0° and 60°, respectively. The LN 2 cooling treatment can increase the ductility of the coal. The length and surface complexity of the induced fracture increase after the LN 2 cooling. The increase in the fracture length is particularly evident at the high bedding angle, while the significant increase of the crack surface complexity is more likely to occur at the low bedding angle. The results contribute to the design of LN 2 fracturing for coal beds.

Changes in mechanical properties and fracture behaviors of heated marble subjected to liquid nitrogen cooling

Liquid nitrogen (LN2) fracturing is a novel waterless fracturing technology that has significant potential for application in the development of geothermal resources. In this study, we performed physical and mechanical experiments on marble samples to determine the influence of LN2 cooling on the damage behavior of hot dry rock (HDR). Specifically, we evaluated the changes in the internal structure, mechanical properties, and fracture behavior of the marble samples after subjecting them to thermal shock. The results revealed that the porosity of the heated marble increased significantly after LN2 cooling, whereas the P-wave velocity, compression strength, and elastic modulus decreased. The complexity of the fracture propagation process and the induced fracture morphology of the marble samples subjected to thermal shock increased under uniaxial compression. Furthermore, the changes in these properties increased with the increase in the marble temperature. Compared to air cooling, LN2 cooling significantly changes the physical and mechanical properties and fracture behavior of marble. Based on the coupled thermal–mechanical simulations from macroscopic and microscopic perspectives, the thermal damage that occurs in heated marble subjected to rapid cooling is primarily due to the temperature gradient inside the rock and the difference in the thermal expansion of adjacent mineral particles.

Stability of novel cellulose-nanofiber-containing foam as environmentally friendly fracturing fluid

Foam-based fracturing is one of the waterless fracturing technologies that can reduce the environmental issues of water-based fracturing. To utilize foam as a fracturing fluid in oil/gas developments, improving foam stability is essential. To examine whether the environmentally friendly and low-cost cellulose nanofiber (CNF) can be a novel foam stabilizer, we investigated the effect of surfactant on the initial foam volume and stability of CNF-containing foam using four types of surfactants with different electrical properties. Then, we investigated the stability of CNF-containing foam under the high-pressure or high-temperature conditions as well as under the laboratory pressure and temperature condition and compared to the stability of foam containing sodium salt of CMC (NaCMC). In addition, we investigated the stability of CNF-containing foam under the presence of salts. As a result, it was concluded that the initial foam volume and stability of CNF-containing foam strongly depend on the type of surfactant because the CNF surface is negatively charged; the CNF is superior to NaCMC as a foam stabilizer, especially in the CNF concentration range above 0.10 wt%; the stability of CNF-containing foam decreases with increasing temperature and increases with increasing pressure, and it is significantly reduced with the addition of salt because the aggregation of the CNFs is formed due to the interaction between the salt cations and CNFs. Further research is expected to use CNF as a stabilizer for foam-based fracturing fluids as CNF is an attractive biomass substance as a foam stabilizer.

Coordinated Development Mode of Oil and Gas Resources and Water Resources in Western China

Western China is an important area for increasing oil and gas reserves and production in China, and it is also the most arid and water-deficient area in the country; there is a complex linkage between energy and water resources in this region. Hence, it is important to promote the coordinated management of energy and water resources. In this article, we review the prospects and deployment of oil and gas production in Western China, investigate the development mode and water utilization status of different oil and gas fields from seven companies of China National Petroleum Corporation in Western China, and analyze the guarantee risks of water resource and water environment during gas and oil development. The research shows that oil and gas production has steadily increased in Western China in recent years, and the consumption of fresh water in oil and gas fields has changed little, owing to the continuous improvement in sewage treatment and recycling efficiency. However, considering the progressive development of unconventional oil and gas resources, future oil and gas development in Western China is likely to significantly affect the surface and underground water systems. It is suggested to develop precise water injection and waterless fracturing technology to stabilize oil production and control water use, increase the recycling of produced water to strengthen scientific water management, and improve the comprehensive allocation guarantee system of water resources, so as to reduce the impact of oil and gas development on water system while ensuring water use security for energy industry, thereby realizing the coordinated development of energy and water resources in Western China.

Numerical simulation of dynamic interactions between two cracks originating from adjacent boreholes in the opposite directions during supercritical CO2 fracturing coal mass

As an environmentally waterless fracturing technology, supercritical carbon dioxide (SC-CO2) fracturing technology is gaining more and more attention in the process of safe mining of coal resources. Based on the phase change of CO2 in the fracturing process, a staged numerical simulation method of SC-CO2 fracturing considering the change of CO2 phase was proposed, and the fluid-solid coupling numerical model of adjacent fracturing boreholes in SC-CO2 fracturing was constructed. The dynamic interactions between two cracks originating from adjacent fracturing boreholes in the opposite directions were studied, and the influence of horizontal and vertical distances of adjacent fracturing boreholes on crack propagation was also discussed. The results indicated that the propagation process of two cracks initiated in the coal mass boreholes can be subdivided into three stages: (i) independent propagation, (ii) mutual repulsion, and (iii) mutual attraction. The crack propagation presents the characteristics of “the crack tip repels the crack tip, and the crack tip attracts the propagated crack surface”. The larger the vertical distance or horizontal distance of adjacent fracturing boreholes, the larger the crack propagation length, but the less the stress interference degree of the opposite cracks. In the design of fracturing parameters, it is necessary to ensure that the crack propagation length is large enough, and also requires the use of “stress interference” to form complex crack network. Finally, based on the research results, the fracturing borehole layout process of SC-CO2 fracturing coal mass was proposed, which can provide a basis for engineering practice of SC-CO2 fracturing.

Experiment of supercritical CO2 fracturing: Invalid experimental data analysis and enlightenment

Supercritical CO2 (SC-CO2) fracturing is considered as a new efficient and waterless fracturing technology for unconventional oil and gas stimulation. This process also can realize the CO2 sequestration and reduce the release of greenhouse gas. In order to further reveal the advantages and characteristics of SC-CO2 fracturing, laboratory fracturing experiments with shale were carried out. It was found that some rock samples could be fractured as expected while some other rock samples had higher initiation pressures that obviously deviated from normal value, furthermore several samples were not even able to be fractured. We found that when cementing the wellbore with resin glue, the additional glue would block the remaining space between the wellbore and the shale sample. Thus, the rock samples could hardly be fractured or fractured with extra high initiation pressures approaching the maximum limit of experimental equipment (55.0 MPa). In this case, dozen or more fractions with irregular shapes would be produced after the rock sample was destroyed, accompanied by strong breaking sound. Based on the results, a new method of SC-CO2 impact fracturing is proposed, which impacts and fractures the reservoir by building and instantly releasing pressure at bottom hole. This method may significantly improve the number of fractures and fracture connection in the reservoir, and realize efficient developments of unconventional oil and gas.

Calculation model of unsteady temperature–pressure fields in wellbores and fractures of supercritical CO2 fracturing

As a new potential economic and effective fracturing method, supercritical carbon dioxide (SC-CO2) waterless fracturing technology can be implemented to alleviate problems such as water sensitivity and reservoir pollution. Further, it reduces emissions, which is an advantage that is being extensively studied. In SC-CO2 fracturing, fracturing pump injection parameters, physical property changes of fracturing fluid, and the geometry size of fractures influence the temperature–pressure field. Based on an experimental regression, the calculation formula of SC-CO2 friction coefficient in fractures is obtained. Considering the physical characteristics of SC-CO2, the change in internal energy and flow work of fracturing fluid, and filtration and fracture characters, the unsteady temperature–pressure field model of SC-CO2 in wellbores and fractures is established. The model is applicable for any known fracture geometry. For a fixed position in the wellbore, with the increase of fracturing time, the temperature of the fracturing fluid rapidly drops and the pressure gradually increases; for a fixed position in the fracture, the temperature of the fracturing fluid gradually drops and the pressure gradually increases. The temperature difference between the fracturing fluid and the filtration zone gradually decreases from the fracture opening to end. As the fracturing time and fracturing fluid flow rate increases, the rock cooling distance of SC-CO2 fluid increases with fracturing time and flow rate of the fracturing fluid. By incorporating the unsteady temperature–pressure field model in wellbores and fractures with SC-CO2 fracturing, theoretical guidance for the phase state control in SC-CO2 fracturing fractures and the hydraulic parameter design of SC-CO2 fracturing is attained.

Exploration and Practice of Carbon Sequestration Realized by CO2 Waterless fracturing

Carbon sequestration is an important measure to reduce greenhouse gas emissions. This paper introduces a new method of carbon sequestration using CO2 waterless fracturing technology. Based on the supporting technology system with independent intellectual property rights, waterless fracturing field tests were carried out on 19 wells in Jilin oilfield. The on-site monitoring data shows that CO2 stays in a supercritical state for a long time in the process of waterless fracturing, soaking and flowback, which is favourable for carbon sequestration. The geological model of H87 test area was established to simulate and evaluate CO2 recovery after fracturing. The results show that the storage ratio of CO2 waterless fracturing is 76.46%. Meanwhile, the production after waterless fracturing is 2.4 times that after water-based fracturing with the same scale, and the average single well operation saves 1512 m3 water. It can be seen that CO2 waterless fracturing is a new type of carbon sequestration technology with high storage efficiency, good stimulation effect, significantly reduction of water consumption during oil and gas reservoir development and has broad market prospects.

Experimental investigation on the characteristics of fractures initiation and propagation for gas fracturing by using air as fracturing fluid under true triaxial stresses

Gas fracturing is a waterless fracturing technology, which can eliminate many disadvantages of hydraulic fracturing. Using air as fracturing fluid, large size physical model experiments of gas fracturing were performed under true triaxial stresses, the characteristics of the fracture initiation and propagation for gas fracturing under true triaxial stresses was studied, the similarities and differences between the gas fracturing and hydraulic fracturing under the same loading conditions were comparatively analyzed. The experiment results indicated that the P-T curves, acoustic emission characteristics, and fracture propagation morphology of gas fracturing are significantly different from hydraulic fracturing due to the differences in compressibility and viscosity of fracturing fluids. During fracturing by equivalent pumping rate under the same confining stress condition, the breakdown pressure of gas fracturing was larger than that of hydraulic fracturing. Both the pressure drop amplitude and rate after the fracture initiation of gas fracturing were less than that of hydraulic fracturing. The pressure fall-off curve of gas fracturing was relatively flat, and the fracture closure rate was slow during the process of measuring the pressure drop after stopping the pump. While the pressure fall-off curve of hydraulic fracturing was relatively steep, and the fracture closure rate was fast. The acoustic emission energy in the initial rupture during the gas fracturing process was significantly higher than that of hydraulic fracturing; the moisture would cause reduction of the acoustic emission energy. Because the rupture during hydraulic fracturing lasted for a longer time and released more energy, the cumulative energy of the acoustic emission during the gas fracturing was lower than that of hydraulic fracturing. When fracturing by equivalent pumping rate for same time under the same confining stress condition, the propagation range of gas fractures was smaller than hydraulic fractures. Cracks furcation tends to occur in the propagation process of gas fractures. The surface roughness of gas fractures was larger than the hydraulic fractures.