Waterless Fracturing Research Articles

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.

SiO2 synergistic modification of siloxane thickener to improve the viscosity of supercritical CO2 fracturing fluid

ABSTRACT Supercritical carbon dioxide (SC-CO2) fracturing technology has the advantages of CO2 storage and waterless fracturing and has great application potential in the development of unconventional oil and gas resources. However, the low viscosity of SC-CO2 limits the development of this technology. This paper investigates the synergistic effect of nanomaterials on a developed thickener (HBD-2, high branched D4H). The research results show that among the five micro/nanomaterials Cu-BTC, SiO2, MCC, MWCNTs, and TiO2, only SiO2 has a good synergistic effect on the HBD-2/SC-CO2 system, and significantly improves its thickening, temperature resistance, temperature resistance and shear resistance. At 60s−1, 305.15 K and 10 MPa, 5 wt.% HBD-2 + 1 wt.% SiO2 increased the apparent viscosity of SC-CO2 to 5.82 mPa·s(The viscosity of 5 wt.% HBD-2 under the same conditions is 4.48 mPa·s). The apparent viscosity of SC-CO2 is positively correlated with pressure and SiO2 dosage and negatively correlated with temperature and shear rate. The SiO2/HBD-2 system can effectively avoid clay swelling and can change the water wetness on the core surface to neutral wettability. In addition, we also studied the mechanism of SiO2 in HBD-2/SC-CO2 thickening system by RDF(radial distribution function). The results show that the introduction to SiO2 shifts the peak position of the system to the left and enhances the peak intensity. The macroscopic manifestation is the enhancement of the apparent viscosity of HBD-2/SC-CO2.This paper proves the feasibility of nanomaterials to synergize SC-CO2, and provides a new thread for the research and development of SC-CO2 thickeners in the future.

Exploration of pore structure evolution and damage mechanism of coal under liquid nitrogen freeze-thaw cycles

Liquid nitrogen (LN2) freeze-thaw cycle, a new waterless fracturing method, has a significant potential in coalbed methane (CBM) production. The ultra-low temperature of LN2 causes continuous damage to coal's pore structure and mechanical properties during the freeze-thaw cycles. A series of related experiments were conducted to investigate the damage mechanism of freeze-thaw cycles. The same coal samples were tested by ultrasonic tester, a nuclear magnetic resonance (NMR) instrument, and a comprehensive mechanics test system after freeze-thaw cycles. The experimental results show that the fracture gradually expanded with an increased width under the action of freeze-thaw cycles. Some secondary fractures were formed along the original fracture and connected to form a fracture network. As the number of freeze-thaw cycles increased, the micropores transformed to mesopores and macropores. The proportion of micropores gradually decreased, while mesopores and macropores continuously increased. The porosity of coal samples was positively correlated, while the wave velocity was negatively correlated with the number of freeze-thaw cycles.The uniaxial compressive strength and elastic modulus of coal samples showed a slight and then rapid decrease with increasing freeze-thaw cycles. Based on rock damage mechanics, wave velocity, elastic modulus, and two-parameter coupling models were chosen to define the damage of coal samples by freeze-thaw cycles. It was found that the predicted values of the coupled model were closest to the experimental values. The two-parameter coupling model has a good prediction of the uniaxial compressive strength of coal samples in freeze-thaw cycles without destroying the coal samples. The results would guide LN2 fracturing technology.

Cryogenic fracturing of synthetic coal specimens under true-triaxial loadings-An experimental study

Cryogenic fracturing with liquid nitrogen (LN2) has been recognized as a potential and promising waterless fracturing technique for coalbed methane extraction, owing to the environmental harmlessness and low-temperature feature. Though it showed spirited performances in some field pilots in the late 20th century, it remains poorly understood, especially when LN2 is used directly as the fracturing fluid. In this work, we evaluated the performance of LN2 fracturing on cubic synthetic coal specimens under true-triaxial loadings. The study also included two subtasks that examined the necessity of a pre-cooling step and the effect of water saturation. Hence, the specimens are classified into dry and wet specimens, and the LN2 treatments are divided into two modes based on whether being pre-cooled. The results showed that the pre-cooling improved the fracturing performance of LN2 such that the breakdown pressures of the pre-cooled specimen are about 40% lower than the non-precooled ones. Based on the semilog pressure–time analysis, the pre-cooled specimens showed higher permeability enhancement (2.9- to 7.9-fold) than the non-precooled ones (1.4- to 1.7-fold). The dry specimens after LN2 fracturing gave a fracture along the direction perpendicular to the minimum horizontal stress, whereas fractures of the wet ones deviated from the expected direction. It indicates that the expansion pressure caused by the frost heaving may change the fracture orientation. In addition, small thermal cracks were observed around the borehole in the open-hole section from the cross-section of the wet specimen, which agrees with the widely accepted thermal damage theory.

Effect of proppant pumping schedule on the proppant placement for supercritical CO2 fracturing

Supercritical CO2 fracturing is a potential waterless fracturing technique which shows great merits in eliminating reservoir damage, improving shale gas recovery and storing CO2 underground. Deep insight into the proppant-transport behavior of CO2 is required to better apply this technique in the engineering field. In the present paper, we adopted a coupled Computational Fluid Dynamics and Discrete Element Method (CFD-DEM) approach to simulate the proppant transport in a fracking fracture with multiple perforation tunnels. Previous experiments were first simulated to benchmark the CFD-EDM approach, and then various pumping schedules and injection parameters (injection location, multi-concentration injection order, multi-density injection order and injection temperature) were investigated to determine the placement characteristics of proppant. Results indicate that the swirling vortex below the injection tunnels dominates the proppant diffusion in the fracture. The velocity of fluid flow across the proppant bank surface in multi-concentration injection shows a positive correlation with the proppant concentration. Injecting high-density proppant first can promote the transportation of low-density proppant injected later in the fracture to a certain extent. Decreasing the initial injection temperature of supercritical CO2 slurry helps enhance the particle-driving effect of fluid and improve the performance of supercritical CO2 in carrying proppant.

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.

Strategy Targets Risk Management for CO2 Waterless Fracturing

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper OTC 30437, “Risk Management and Control for CO2 Waterless Fracturing,” by Siwei Meng, Qinghai Yang, SPE, and Yongwei Duan, PetroChina, et al., prepared for the 2020 Offshore Technology Conference Asia, originally scheduled to be held in Kuala Lumpur, 2–6 November. The paper has not been peer reviewed. Copyright 2020 Offshore Technology Conference. Reproduced by permission. Given shortages and uneven distribution of water resources in China, efforts must be made to develop waterless fracturing techniques. The fluid experiences high pressures and low temperatures during carbon dioxide (CO2) waterless fracturing operations, which can lead to accidents and environmental pollution. In the complete paper, a safety-management approach and a contingency plan for such operations are developed. At the time of writing, this CO2 waterless fracturing methodology has been completed successfully more than 20 times. Surface Process Work Flow of CO2 Waterless Fracturing The basic process of a CO2 waterless fracturing operation is shown in Fig. 1. First, several CO2 storage tanks are connected in parallel. The booster, sealed blender, fracturing pump (all mounted on trunks), and wellhead equipment are connected. The measuring trunk communicates with each vehicle to monitor operation status. Proppant is put into the sealed blender, into which liquid CO2 is injected for pre-cooling. Pump testing is conducted on the high-pressure line and the wellhead and the low-pressure liquid supply line is pressure-tested. Operation does not proceed until pressure-testing results are positive. Afterward, liquid CO2 is injected into formations to fracture them and, moreover, extend created fractures. The sealed blender is enabled to inject prop-pants, and displacement begins after the end of proppant injection. Finally, a series of tasks, including well shut-in for soaking and flowback, is carried out successively.

Technology Focus: EOR Operations (November 2021)

As the discovery rate of new hydrocarbon resources decreases, the need for more-efficient enhanced-oil-recovery (EOR) processes increases. Unlike in the past, however, when the efficiency was defined in terms of maximizing the recovery factor (RF), the new interpretation of efficiency is based on optimizing the balance between RF and the reduction of carbon footprint. This is done through an integrated approach in which both surface and subsurface elements of the oil-production systems are used to determine energy efficiency and carbon footprint of a unit volume of oil produced by EOR methods. When choosing traditional EOR methods, new innovations may be needed to arrive at new injectant composition to reduce emissions or make the process more efficient. Adding chemicals to the injectant gas to improve the mobility ratio and increase the sweep efficiency is desirable. One example is the use of hydrogels. These are hydrophilic structures that swell when hydrated. Hydrogels are of interest in EOR because of their ability to respond to stimuli such as pH, temperature, light, and ionic strength. EOR methods that involve use of fresh water are also switching to alternative methods that reduce or remove its usage as part of water sustainability. The produced water could be treated properly to make it suitable for injection. Alternatively, polymers that are effective under high salinity or temperature could be used to deal with injecting saline water. For unconventional reservoirs, waterless fracturing techniques are progressing. Paper SPE 201609 discusses the application of a reversible hydrogel that can be added to the injected carbon dioxide (CO2) stream in order to make it a more-efficient injectant for EOR and, hence, create more opportunity for CO2 storage. Paper SPE 202809 deals with utility of new polymers that are suitable for injection into carbonate reservoirs under high-temperature and ultrahigh-salinity conditions. Finally, paper OTC 30437 discusses ways of mitigating safety risks associated with CO2 waterless fracturing in unconventional reservoirs as part of water sustainability as well as prevention of environmental pollution. Recommended additional reading at OnePetro: www.onepetro.org. SPE 200357 - Fundamental Investigation of Auto-Emulsification of Water in Crude Oil: An Interfacial Phenomenon and Its Pertinence for Low-Salinity EOR by Duboué Jennifer, TotalEnergies, et al. SPE 205118 - Experimental Design and Evaluation of Surfactant Polymer for a Heavy-Oil Field in South of Sultanate of Oman by Ali Reham Al-Jabri, Petroleum Development of Oman, et al. SPE 200256 - Chemical Enhanced Oil Recovery and the Dilemma of More and Cleaner Energy by Rouhi Farajzadeh, Delft University of Technology, et al.

Influence of cryogenic liquid nitrogen cooling and thermal shocks on petro-physical and morphological characteristics of Eagle Ford shale

Eagle Ford shale formations have great hydrocarbon potential to minimise the supply and demand problems of the overgrowing population. Whereas, due to reduced permeability and marginal porosity, it is very difficult to produce from these formations. Previously, hydraulic fracturing and acidizing techniques are used to produce the hydrocarbons from tight gas shale formations; however, due to technical difficulties and environmental problems have led to a ban on these techniques in many countries. Therefore, it is suggested to use water-less fracturing methods to avoid these problems. Recently, liquid nitrogen (LN2) cooling have shown a great potential to mitigate technical and environmental flaws, which works on the principle of providing sudden thermal shocks in the pore matrix, thus inducing micro-cracks and enhancing the petro-physical potential.In this study, we have collected tight gas shale samples from the Eagle Ford shale's Upper Late Cretaceous geologic formation in the United States. Initially, these samples were exposed to LN2 for various time intervals of 30, 60, and 90 min and were characterised based on their petro-physical, morphology, topography, and rock mechanical properties before and after LN2 exposure. To accomplish that, various techniques were used such as core flooding, porosity determination, microstructural characterisation via field emission scanning electron spectroscopy (FESEM) and energy dispersive spectroscopy (EDS), topographic characterisation via atomic force microscopy (AFM), and rock mechanical characterisation via nano-indentation technique.It is evident from our results that LN2 cooling has a substantive effect on petro-physical properties by increasing permeability from 12 nano-Darcy (nD) to 2.6 milli-Darcy (mD) and porosity from 2.32% to 4.46%. Similarly, topographic properties have shown an increase in surface roughness from 177 nm to 692 nm, and hardness (indentation moduli, GPa) was substantially reduced due to optimum exposure of LN2 at 90 min. Further, thermal shocks of LN2 cooling have induced vivid micro-cracks in the pore matrix up to 7 μm. In a nutshell, it is vital to augment the effects of LN2 cooling on microstructural, petro-physical, topography, and rock mechanical properties for better understanding in the development of hydrocarbon potential in Eagle Ford shale formations.

Stimulation and Sequestration Mechanism of CO2 Waterless Fracturing for Continental Tight Oil Reservoirs.

CO2 fracturing is a promising technology for oil field development in tight, continental deposits, with potential advantages of enhanced oil recovery (EOR), CO2 sequestration, and water conservation. Compared with CO2-EOR techniques, such as CO2 huff and puff and CO2 flooding, CO2 can interact with reservoir rock and fluid under higher pressure conditions during fracturing, resulting in CO2 stimulation and sequestration effects that differ from those that occur during conventional CO2-EOR. In this paper, the CO2 interactions between CO2 and reservoirs in continental tight oil reservoirs under fracturing conditions are systematically studied through laboratory experiments. The results show that under high pressure, CO2 effectively changes the pore structure through the extraction of hydrocarbons, dissolution of the rock matrix, and migration of minerals. CO2 dissolution of the rock matrix can significantly increase the number and complexity of fractures. Furthermore, CO2 has a higher solubility in formation fluid under high-pressure conditions. Given the higher pressures, CO2 forms a miscible phase with crude oil, diffuses more deeply into the formation, and reacts fully with the reservoir minerals and fluid during CO2 fracturing. Accordingly, CO2 can improve the permeability of the reservoir and flowability of crude oil significantly. Hence, CO2 fracturing can enhance oil recovery and CO2 sequestration more effectively. Core displacement experiments indicate that oil recovery of CO2 soaking process after CO2 fracturing is 36%, which is 12% and 9% higher than those of CO2 huff and puff and CO2 flooding with 5 pore volume, respectively. Field tests show that average oil production after CO2 fracturing is 1.42 times higher than that after CO2 flooding, which further validates the advantage of CO2 fracturing and demonstrates its huge application potential.

Experimental investigation of shale breakdown pressure under liquid nitrogen pre-conditioning before nitrogen fracturing

Cryogenic fracturing with liquid nitrogen (LN2) offers the benefits of reducing the water consumption and adverse environmental impacts induced by water-based fracturing, as well as potentially enhancing the fracture complexity. We performed a series of laboratory experiments to explore the key mechanisms governing the breakdown pressures of shale during cryogenic fracturing. In this study, cylindrical shale samples were pre-conditioned by exposing a borehole to low-temperature LN2 for a certain time period, and then, the samples were fractured using gaseous N2 under triaxial stress and a high reservoir temperature. The effects of various key parameters on the breakdown pressure were investigated, including the duration of the low-temperature LN2 treatment, the confining pressure, the reservoir temperature, and the direction of the shale bedding relative to the borehole axis. The results demonstrate that the injection of low-temperature LN2 as a pre-fracturing fluid into a borehole can significantly reduce the breakdown pressure of the shale during subsequent nitrogen fracturing. This reduction in breakdown pressure can be further intensified by increasing the duration of the LN2 pre-conditioning. Without LN2 pre-conditioning, the breakdown pressure initially increases and then decreases with increasing reservoir temperature. When LN2 pre-conditioning is applied, the breakdown pressure keeps decreasing with increasing reservoir temperature. As the confining pressure increased, the breakdown pressure increased linearly in the tests with and without LN2 pre-conditioning. The experimental results demonstrate that LN2 pre-conditioning before N2 fracturing is a promising waterless fracturing technique that reduces the breakdown pressure and enhances the fracture complexity.

Experimental study on pore structure evolution of coal in macroscopic, mesoscopic, and microscopic scales during liquid nitrogen cyclic cold-shock fracturing

Liquid nitrogen cyclic cold shock is a new waterless fracturing method suitable for coal seams in arid areas. The pore structure of coal after cold shock plays a key role in gas drainage, but there is currently a lack of study combining macroscopic-mesoscopic-microscopic scale. Multiscale pore characterization will be helpful to clarify the damage mechanism of liquid nitrogen cold shock to coal. In this paper, the same coal samples were tested for ultrasonic inspection, stereomicroscopic imaging and nuclear magnetic resonance (NMR) after different cold shock times. The macroscopic damage were studied by ultrasonic wave; the changes of mesoscopic pores and fractures (0.1 ~ 10 mm) were studied by stereoscopic microimaging and digital image processing; the changes of microscopic pores (2 nm ~ 0.1 mm) were studied by NMR. Finally, the macroscopic - mesoscopic - microscopic pore characteristic parameters were coupled. The main results are as follows: With the increase of cold shock times, the P/S wave velocities decreased, the dynamic parameters decreased linearly, and the macroscopic damage of coal samples was obvious. The number of mesoscopic pores, surface porosity, and probability entropy increased. As can be seen from the T2 spectra of coal samples, the microscopic pores expanded and increased with cold shock times. The effective porosity increased, among which the increase of micropores contributed the most. The macroscopic wave velocity and dynamic parameters of coal samples were negatively correlated with porosities. The calculation models of T2 cutoff value were established by multiple linear regression analyses, which were in good agreement with the experimental values.

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.

Non-contaminating cryogenic fluid access to high-temperature resources: Liquid nitrogen fracturing in a lab-scale Enhanced Geothermal System

Hot dry rock (HDR) geothermal energy exploitation is essential to meet the energy demand and achieve low-carbon solutions. Creation of complex fracture networks in HDR by hydraulic fracturing to enhance the thermodynamic efficiency and heat production is significant. However, the economic and environmental burden of freshwater sourcing, transportation, and treatment raises concerns. Here, we investigate liquid nitrogen, an environmental-friendly and super-cooling fluid, as an alternative fracturing fluid for potential applications in Enhanced Geothermal System. We studied the fracturing performances of liquid nitrogen in high-temperature granites under true triaxial-confining stresses. Cryo-scanning electron microscopy and 3D X-ray micro-computed tomography were applied to illuminate the fracture-network patterns. Results show that liquid nitrogen fracturing exhibits the lowest breakdown pressure compared with water fracturing and nitrogen gas fracturing. The fracture morphology displays a 3-D volumetric pattern comprised of branched fractures and thermally-stimulated zones. Higher fluid-rock temperature difference and lower stress anisotropy cause more complex fracture networks. This study, for the first time, shows potential benefits of liquid nitrogen fracturing in high-temperature crystalline rocks, paving the road towards waterless fracturing in Enhanced Geothermal System. It is expected to provide a viable alternative for the sustainable development of deep geothermal resources in an efficient and clean way.