Supercritical CO2 Fracturing Research Articles

EVALUATION OF SUPERCRITICAL CO2 FRACTURING EFFECT IN DA'AN RESERVOIR OF SONGLIAO BASIN

Carbon dioxide (CO<sub>2</sub>) fracturing technology is introduced to improve the development effect of low permeability reservoirs. To understand the advantages of CO<sub>2</sub> fracturing and the mechanism of CO<sub>2</sub> assisting oil displacement efficiency, laboratory core displacement experiments and nuclear magnetic resonance (NMR) tests are conducted. For core displacement experiments, a water flooding, a water flooding-CO<sub>2</sub> huff and puff experiment are conducted. Water flooding simulates the oil displacement after hydraulic fracturing and the latter simulates the effect after CO<sub>2</sub> fracturing. Furthermore, an NMR test is conducted in each stage of the experiments to quantify the recovery degree of crude oil in different pore throats for a low-permeability reservoir. The results show that the water flooding recovery of the two cores is 31.26% and 30.14%, respectively. The CO<sub>2</sub> huff and puff enhanced oil recovery and subsequent water flooding are 11.48% and 5.19%, respectively. Compared with conventional hydraulic fracturing, CO<sub>2</sub> fracturing enhanced oil recovery by 15.55%. Furthermore, the crude oil recovery degree of water flooding in macro-pores is 75.71%, while that in mesopores is 62.17% and that in small pores is 7.49%. After CO<sub>2</sub> huff and puff, the recovery degrees in macropores, mesopores, and small pores are 99.10%, 67.51%, and 23.21%, respectively. It demonstrates CO<sub>2</sub> can penetrate deeply into the tiny pores that cannot be affected by water flooding for oil displacement, effectively increasing the swept volume and improving the oil recovery. Research shows the advantages of CO<sub>2</sub> fracturing and may help to enlarge its application in low-permeability reservoirs.

Numerical Simulation of the Proppant Settlement in SC-CO2 Sand-Carrying Fluid in Fracturing Fractures

Supercritical CO2 fracturing has unique advantages for improving unconventional reservoir recovery. Supercritical CO2 can penetrate deep into the reservoir and increase reservoir reform volume, and it is less damaging to reservoir and easy to flow back. However, when the supercritical CO2 flows as the sand-carrying fluid in the fracture, the settlement of the proppant is still worth studying. Based on the study of supercritical CO2 density and viscosity properties, assuming that the reservoir has been pressed out of the vertical crack by injecting prepad fluid, the proppant characteristics in sand-carrying fluid under different conditions were studied by numerical simulation. After the analysis, the proppant accumulation and backflow will occur at the end of the crack. Large sand diameters, high fluid flow rates, high sand concentrations, high reservoir temperatures, and low reservoir pressures can help to shorten deposition time, and the small particle size, high fluid flow rate, low sand concentration, low reservoir temperature, and high reservoir pressure can help increase the uniformity of sand deposition. Shortening the sand deposition time can help to complete the fracturing efficiently, and increasing the deposition uniformity can improve the fracture conductivity. This article has studied the proppant settling and crack formation characteristics. It is hoped that this study can provide theoretical support for field fracturing and provide theoretical assistance to relevant researchers.

Fundamental study and utilization on supercritical CO2 fracturing developing unconventional resources: Current status, challenge and future perspectives

Under the fact that considerable exploration and production of unconventional resources and worsening global climate, reducing carbon emission and rationally utilizing carbon resources have been drawn increasing attention. Supercritical CO2 (SC-CO2) has been proposed as anhydrous fracturing fluid to develop unconventional reservoirs, since its advantages of reducing water consumption, reservoir contamination etc. Well understanding of SC-CO2 fracturing mechanism and key influencing factors will exert significant impact on the application of this technology in the field. In this paper, the fundamental studies on SC-CO2 fracturing from the aspects of laboratory experiment and simulation are reviewed. The fracturing experimental setups, fracture monitoring and characterizing methods, unconventional formation categories, numerical simulation approaches, fracturing mechanism and field application etc., are analyzed. The fundamental study results indicate that compared with conventional hydraulic fracturing, SC-CO2 fracturing can reduce fracture initiation pressure and easily induce complex fracture networks with multiple branches. The field test further verifies the application prospect and the possibility of carbon storage. However, due to the limitation of reservoir complexity and attributes of SC-CO2, massive challenges will be encountered in SC-CO2 fracturing. According to the current research status, the limitations in basic research and field application are summarized, and the future development direction of this technology and relevant suggestions are proposed.

Numerical Simulation of the Fracture Propagation Mechanism during Supercritical Carbon Dioxide Fracturing in Shale Reservoirs

Abstract To investigate the fracture propagation mechanism during supercritical CO2 fracturing in shale reservoirs, a numerical model was proposed based on the displacement discontinuity method. The Peng–Robinson equation was introduced to determine the variations in CO2 properties during the fracturing process. Considering natural fracture distribution in shale reservoirs, the fracture propagation mechanisms during supercritical CO2 fracturing in shale reservoirs under different horizontal stress differences and matrix permeabilities were analyzed. The influence of the proportion of CO2 preenergizing on fracture morphology was discussed. The results obtained via numerical simulation show that supercritical CO2 is beneficial to create a more complex fracture network by activating natural fractures under the same horizontal stress difference. CO2 easily penetrates into the matrix near the fracture surfaces, increasing reservoir energy. However, when the permeability of shale reservoirs exceeds 0.04×10−3 μm2, substantial filtration of CO2 into the reservoir matrix occurs near the well bore, limiting the activation of natural fractures around the fracture tip. A higher proportion of CO2 preenergizing during fracturing is conducive to improve the fracture complexity while reducing the fracture aperture.

Influence of supercritical CO2 on the physical property of tight sandstone

In low-pressure gas reservoirs, water-based fracture fluid is difficult to flowback, which is unfavorable for several tight sandstone gas reservoirs in the Sichuan Basin with low pressure and high permeability geological characteristics. Supercritical CO2 possesses a number of remarkable physical and chemical features, including a density near to water, a viscosity close to gas, and high diffusion. Supercritical CO2 fracturing is a new type of non-aqueous fracturing method that is favorable to fracturing flowback in low-pressure tight sandstone and has a wide range of applications. To discuss on whether supercritical CO2 fracturing with low pressure tight sandstone is feasible. Tight sandstone cores from the Jinqiu gas field in the Sichuan Basin were used to study the influence of supercritical CO2 on the physical properties of sandstone reservoirs. Supercritical CO2 was used to interact with tight sandstone samples, and then the changes in porosity, permeability, and rock microstructure of tight sandstone were observed under various time, pressure, and temperature conditions. After the interaction between tight sandstone and supercritical CO2, new dissolution pores will appear, or the original pores will be increased, and the width of some natural fractures will also be increased, and the porosity will increase by 1.09%–8.85%, and the permeability will increase by 2.34%–21.26%, quantifying the influence of supercritical CO2 on physical properties of tight sandstone, and further improving the interaction mechanism between supercritical CO2 and tight sandstone. This study improves in the understanding of the tight sandstone-supercritical CO2 interaction mechanism, as well as providing an experimental foundation and technological guarantee for field testing and use of supercritical CO2 in low-pressure tight sandstone gas reservoirs.

Proppant Transport Characteristics in Tortuous Fractures Induced by Supercritical CO2 Fracturing

Supercritical CO2 fracturing is an important development trend to reach the goal of "dual carbon" and avoid the problem that hydraulic fracturing is influenced by water resource. In order to clarify the transport characteristics of proppant in the fractures induced by supercritical CO2 fracturing, this paper reconstructs the fracture surface of rock samples after supercritical CO2 fracturing using the laser morphological scanning technology, and establishes a model of proppant carrying and transport of supercritical CO2 in tortuous fractures on the basis of CFD-DEM method. In addition, the transport and placement characteristics of proppant in tortuous fractures are analyzed by comparing with flat fractures, and the effects of proppant density, injection rate of proppant carrying liquid, proppant concentration and other key parameters on proppant transport and distribution in fractures are investigated. And the following research results are obtained. First, compared with those in flat fractures, the flow paths of the proppant carrying supercritical CO2 liquid in tortuous fractures are tortuous and diverse, and the proppant presents stronger fluctuations and jumps laterally and vertically during its transport. Second, the proppant placement in tortuous fractures morphologically presents a wavy or even clustered non-uniform distribution. Third, low-density proppant has a better pass-ability in tortuous fractures, and the high injection rate can reduce the influence of tortuous fracture structure on proppant blocking. Fourth, if the concentration of injected proppant in the tortuous fracture is too low, good fracturing support effect cannot be achieved, and the optimal value under the simulation conditions in this paper is around 3%. In conclusion, the simulation results are of important theoretical and engineering significance to understanding the mechanism of proppant blocking in the pumping process of proppant carrying liquid for supercritical CO2 fracturing and optimizing the field fracturing design.

The enhanced geothermal system heat mining prediction based on fracture propagation simulation of thermo-hydro-mechanical-damage coupling: Insight from the integrated research of heat mining and supercritical CO2 fracturing

• Integrated research of HDR fracturing and EGS heat mining in field-scale is realized. • Heat mining based on fracture propagation simulation of THM-D coupling is launched. • Heat mining of SCO 2 fractured EGS and hydraulic fractured EGS is explored. • Synchronous fracturing and asynchronous fracturing in doublet-well EGS is compared. Due to the two key stages involving the enhanced geothermal system (EGS) development: the HDR fracturing stage and the EGS heat mining stage after fracturing, it is necessary to conduct the integrated research of heat mining and EGS fracturing for the accurate prediction of EGS productivity. Based on the THM-D coupling, we carry out the field-scale fracture propagation of HDR, and then launch the research on heat mining evaluation based on the obtained fracture morphology. Firstly, the fracture morphology and the corresponding heat mining performance obtained by hydraulic and SCO 2 fracturing are compared by synchronous fracturing of doublet-well. Subsequently, the impact of fracturing procedure, duration, velocity and in-situ stress on fracture propagation of SCO 2 fracturing and the EGS heat mining performance after fracturing is studied. The results show that the SCO 2 fracturing can reduce the fracture connection time by 77.01% and increase the damage area of 16.1% compared with hydraulic fracturing. Compared with asynchronous fracturing, the synchronous fracturing can save the fracturing time by 51.2% and increase the fracture surface area by 5.99%. The increase of 10 MPa in horizontal stress difference enhances the fracture surface area by 1.37%, the corresponding heat mining rate increase by 7.01%.

Impact of Supercritical CO2 on Shale Reservoirs and Its Implication for CO2 Sequestration

Hydraulic fracturing has transformed the international energy landscape by becoming the go-to method for the exploitation of natural gas from unconventional shale reservoirs. However, in the recent years, the search for an alternative method of shale-gas exploration has intensified, because of various problems (e.g., contamination of ground and surface water, overexploitation of precious water resources, air pollution, etc.) associated with the usage of water-based fracturing techniques. The use of CO2 for shale gas exploitation has emerged as a better alternative to aqueous-based gas exploration techniques. CO2 when injected into deep shale reservoirs, transitions into supercritical CO2 (SC-CO2) when temperature and pressure condition exceeds the critical point, i.e., 31.1 °C and 7.38 MPa. In this paper, we comprehensively review the impact of SC-CO2 on shale gas reservoirs during the different stages of shale-gas exploration, i.e., (i) drilling, which involves the superiority of SC-CO2 over water-based drilling fluids, in terms of achieving under-balanced well condition, higher rates of penetration, and resistance to formation damage; (ii) fracturing, which involves factors affecting the tortuosity of fractures created by SC-CO2 fracturing, breakdown pressure, and proppant-carrying capacity; and (iii) injection, which involves the twin-headed benefit of enhanced recovery due to CO2/CH4 competitive adsorption and geological sequestration, CO2 vs CH4 excess sorption as a function of pressure, etc. Several research works have indicated discrepancies on how SC-CO2 impacts different shale properties. Some studies show low-pressure N2-gas-adsorption-derived surface area and total pore volume to be increasing with SC-CO2 imbibition, while others show a decreasing trend for the same. Similarly, for some shales, the quartz content, along with the clay mineral contents, decreased as the exposure to SC-CO2 increased, while in some other studies, with similar long-term exposure to SC-CO2, the quartz content was observed to increase along with the decrease in clay content and vice versa. Essentially, the increased exposure to SC-CO2 results in the dissolution of primary porous structures and fractures, and reformation of newer porous structure and conduits in shales. Nonetheless, these changes in the mineralogy weaken the microstructure of the rock bringing significant changes in the mechanical properties of the shales with implications on the wellbore stability and fracturing efficiency. The mechanical properties such as uniaxial compressive strength (UCS), Young’s modulus, and tensile strength decrease as the SC-CO2 saturation period increases. However, some studies have shown factors like bedding angle and phase-state of CO2 having varying effect on the strength behavior of the shales. Moreover, changes in the structure of shales caused by the creation of fractures and the reduction of their strength can also pose major risks, because of potential leakage of CO2 through these created pathways. How these processes would interact at field scale would control the sealing capacity, especially at field-scale for addressing long-term seepage of CO2.

Experimental investigation of proppant transport in hydraulically fractured wells using supercritical CO2

Supercritical CO2 fracturing is a promising waterless stimulation technology, and pumping proppant during fracturing is an essential part of the frac job design to obtain a high-conductivity flow channel after the artificial fracture is created. To date, only a few laboratory experiments have been conducted using supercritical CO2 directly to carry the proppant. In this study, an experimental system was designed to allow investigation of supercritical CO2 transport of proppants within a fracture. To determine the supercritical CO2 transport proppant mechanism and the impact of key operational variables, a series of laboratory experiments were performed considering various pumping schemes. The results demonstrate that the mechanism of supercritical CO2 transport proppant includes various flow patterns such as scouring, throwing and fluidization, among which fluidization is the key to the further migration of proppant. Continued injection of clear-fluid after the addition of proppant is complete, will help the dunes extend deeper into the fracture. Fluid velocity has a significant effect on supercritical CO2 transport proppant. While under the premise of constant mass flow rate, increasing the injection temperature, decreasing the injection pressure or reducing the proppant concentration will make the dune equilibrium height lower and the dune laying distance longer. Meanwhile, small-sized proppant particles can still be easily lifted and suspended in supercritical CO2, and reducing proppant size significantly increases the transportability. Furthermore, dune equilibrium height correlates significantly with the variation of the Reynolds number of the fluid flow. Therefore, from a practical field operation point of view, using Reynolds number as a controlling parameter seems to be a viable option for selection of optimum pumping operational variables and prediction of proppant migration within the fracture. The results of this study are expected to provide guidance for field operators to better design operational variables (e.g. CO2 injection rates, proppant load and pumping schedule, etc.) used for supercritical CO2 fracturing.

An experimental apparatus for supercritical CO2 fracturing of shale: System design and application tests

Supercritical CO2 (SC–CO2) would be a potential substitute for water-based fracturing fluid widely used in the present hydraulic fracturing of shale reservoirs. However, the field application of SC-CO2 fracturing is still in its infancy. Laboratory experiments are crucial for understanding the effects of various fracturing parameters and validating numerical simulations before conducting costly field trials. This paper first introduces the design of a self-developed experimental apparatus for fracturing cylindrical rock specimens and outlines the working principles of six functional modules. The entire experimental system is flexible because its functional modules can be used interchangeably. Subsequently, some key designs are emphasized. (1) A packer with the sealing of O and Y rings exhibits a good seal efficiency for a relatively small wellbore; moreover, this packer can be conveniently moved out from the fractured shale without damaging the specimen. (2) Downhole temperature measurement structure integrated into the packer can sensitively monitor the CO2 temperature variation at a high sampling rate during the entire SC-CO2 fracturing process. (3) The design of the triaxial core holder contributes to the realization of withstanding high axial-confining and fluid injection pressures and customized installation of acoustic emission (AE) sensors. Ultimately, some fracturing experiments with water and SC-CO2 as fracturing fluids and seepage tests with nitrogen (N2) under various test parameters are successfully conducted. The results indicate that, after the specimen fractures, the injection pressure would be approximately decreased to the confining pressure due to the confining chamber sealing design; there is a simultaneous sharp variation in the temperature at the bottom of the wellbore. SC-CO2 fracturing generates more fractures with greater bifurcation and higher tortuosity. The gas flow rate decreases with increasing confining pressure, whereas it increases significantly with increasing injection pressure. In general, these experimental results can validate the performance and feasibility of the self-developed experimental apparatus.

Study on the Alteration of Pore Parameters of Shale with Different Natural Fractures under Supercritical Carbon Dioxide Seepage

Supercritical CO2 can reduce formation fracture pressure, form more complex fractures in the near-well zone, and replace methane to complete carbon sequestration, which is an important direction for the efficient development of deep shale gas with carbon sequestration. In this paper, based on the scCO2 fracturing field test parameters and the characteristics of common shale calcite filled natural fractures, we simulated the porosity change in shale with three kinds of fractures (no fracture, named NF; axial natural fracture, named AF; and transversal natural fracture, named TF) under scCO2 seepage, and carried out the experimental verification of shale under supercritical CO2 seepage. It was found that: (1) At the same pressure, when the temperature is greater than the critical temperature, the shale porosity of three kinds of fractures gradually increases with the injection of CO2, and the higher the temperature, the more obvious the increase in porosity. (2) At the same temperature and different pressures, the effect of pressure change on the porosity of shale specimens was more obvious than that of temperature. (3) Multi-field coupling experiments of shale under supercritical CO2 seepage revealed that the porosity of all three shale specimens at the same temperature and pressure increased after CO2 injection, and the relative increase in shale porosity measured experimentally was basically consistent with the numerical simulation results. This paper reveals the mechanism of the effect of different temperatures and pressures of scCO2 and different natural fractures on the change in shale porosity, which can be used to optimize the CO2 injection in supercritical CO2 fracturing and carbon sequestration.

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.

Unconventional CO2 Storage: CO2 Mineral Trapping Predicted in Characterized Shales, Sandstones, and Coal Seam Interburden

Summary Carbon dioxide (CO2) capture from industrial sources including coal combustion, gas processing, cement or steel production, blue hydrogen, or direct air capture, and subsequent geological storage is part of the transition to reduce greenhouse gas emissions. Unconventional and conventional reservoirs provide opportunities for beneficial use such as enhanced recovery, supercritical CO2 (ScCO2) fracturing, and storage of gases such as CO2 and ancillary gases, or potentially hydrogen. The purpose of this study is to use Australian unconventional rock packages to understand the controls on CO2 reactivity and mineral trapping (the most secure form of storage) and compare the potential for CO2 storage. Characterization of core from the Surat, Eromanga, and Cooper basins, Australia, is used to populate CO2 and production water-rock reactivity models. Sensitivity to production water composition and temperature was also tested. Coal seam gas (CSG) reservoir interburden ranged from clay-rich mudstones to interlaminated sandstone and mudstone, and calcite cemented sandstones. The coal seam interburden samples contained high plagioclase and chlorite content. They were predicted to alter to carbonates calcite, ankerite, siderite, and dawsonite mineral trapping CO2. After 30 years, net mineral trapping varied from −0.1 to +0.3 kg CO2/m3, and pH was 4.6–4.9. Net mineral trapping after 1,000 years varied from 5.7 to 16.3 kg CO2/m3 and was 17.1 kg CO2/m3 with higher salinity water. The mineral content had the main control with different lithologies decreasing mineral trapping by 41 or 35% compared with a base case. Overlying plagioclase-rich sandstone trapped 17.1 kg CO2/m3 as calcite, ankerite, dawsonite, and siderite after 1,000 years with the pH increasing to 6. For the quartz-rich oil reservoir sandstone, however, only 0.3 kg CO2/m3 was trapped after 1,000 years. Gas shale and marine black oil shales contained high mica, chlorite, and feldspar content that could be converted to carbonate minerals, mineral trapping CO2. A marine black oil shale mineral trapped 8.3 and 13.9 kg CO2/m3 after 30 and 1,000 years, respectively, as siderite and ankerite. Unconventional reservoirs have a strong potential for mineral trapping during CO2 storage.

Modeling study on supercritical CO2 fracturing applicability and capacity to stimulate reservoirs with different permeabilities

Hydraulic fracturing (HF) using water-based fluids is an effective approach for reservoir stimulation in the exploitation of unconventional resources. Recently, supercritical carbon dioxide (S–CO2) has been proposed as a prospective fracturing fluid in reservoir stimulation because it exhibits higher fracturing capacity compared with water-based fracturing fluid. However, S–CO2 appears to have a distinct drawback of intensifying leak-off when used in reservoirs with relatively high rock matrix permeability (RMP). In this work, a modeling study on water fracturing and S–CO2 fracturing (SCF) was implemented in reservoirs with varied RMPs to assess their applicability. In the modeling, the natural fracture system in the reservoir was considered through the discrete fracture network method for simulating different reservoirs. The lower viscosity and density of S–CO2 compared with those of water enable the easier fracturing of reservoir rocks and allow the generation of more complex fracture networks. However, the intensifying leak-off of S–CO2 due to fracture propagation can hamper the build-up of hydraulic pressure and affect fracture propagation, especially in reservoirs with relatively high RMP. The inflection of fracture length development during HF can also reflect the impact of fluid leak-off. Permeability determines the final fracture network of SCF, and the injection rate increase can offset the leak-off to a certain extent; however, the improvement in fracturing results becomes limited. The modeling results clearly demonstrate the importance of RMP, which can directly determine the applicability and capacity of SCF.

A review of experimental apparatus for supercritical CO2 fracturing of shale

Supercritical carbon dioxide (SC–CO 2 ) is a promising fracturing fluid with unique properties, making it well-suited for unconventional reservoir stimulation. Laboratory experiments are essential for determining hydraulic parameters prior to field trials. The fracturing apparatus thus plays a primary and indispensable role in evaluating SC-CO 2 fracturing potential. In this review article, a simple overview of water-based and waterless fracturing fluids is first provided from the perspective of their application effects and limitations. It is concluded that, compared with hydraulic fracturing, CO 2 fracturing shows tremendous potential, the research of which has the need to conduct laboratory experiments with new or improved fracturing apparatuses. Second, many multifunctional apparatuses have been designed to investigate the SC-CO 2 fracturing of reservoir rocks. In this regard, the currently developed SC-CO 2 fracturing apparatuses are reviewed with respect to their function units and related existing shortcomings. Different apparatuses correspond to various test schemes and results, among which the influences of the apparatus design on the experimental results are discussed. In particular, some remarks are made regarding the control of the initial injected CO 2 phase and the temperature–pressure measurement of CO 2 during fracturing. Third, inspired by the literature review, a self-designed experimental apparatus is developed to conduct SC-CO 2 fracturing tests. In particular, an integrated structure for the temperature–pressure measurement associated with CO 2 injection is proposed. Moreover, a triaxial core holder is customized to assist in measuring specimen strains and acoustic emission (AE) signals while ensuring the sealing performance of loading and injection systems. Ultimately, the challenges faced in industrial applications, such as fracturing equipment, CO 2 –rock interactions, and CO 2 phase control, have impeded the field-scale application of SC-CO 2 fracturing for shale gas development. Hereby, future apparatus design concepts are given for comprehensive investigations of SC-CO 2 fracturing mechanism. • State-of-the-art and limitation of water-based and waterless fracturing fluids are reviewed. • Current development of SC-CO 2 fracturing apparatuses with respect to function units is briefly introduced. • A self-designed experimental apparatus is customized to maintain the super-critical state of CO 2 during CO 2 injection. • Obstacles remaining for field-scale SC-CO 2 fracturing are discussed.

Experimental study of micromechanical properties alterations of shale matrix treated by ScCO2-Water saturation using nanoindentation tests

Supercritical CO2 (ScCO2) fracturing is a preferred alternative to water-based fracturing for stimulation of the shale gas reservoirs, and also promotes CO2 geological sequestration. The injected ScCO2 will interact with shale minerals and then induce changes in rock matrix micromechanics. In this study, a series of nanoindentation tests was conducted on the shale sample to investigate the effects of ScCO2-water treatment on shale matrix micromechanics. X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS) were used to analyze the changing mechanisms of micromechanical properties. The dispersion of grid nanoindentation data of different minerals indicates the strong heterogeneity of shale’ micromechanical properties. After ScCO2-water treatment, different shale minerals experienced different reductions of Er and H because of the dissolution induced the alterations in crystal structures, and their contents also altered in varying degrees. The values of hf/hm and Wp/Wt can be combined with the hysteresis loops enclosed by the P-h curves to determine the dominant work and irreversible deformation that produced in the process of indentation. According to the observation of indentation morphology, unlike other minerals, considerable micro-fractures were generated in clay minerals, which is correlated to the significant plastic deformation and the layered crystal structure.

Experimental study on effect of effect time of supercritical CO2 fracturing fluid on mechanical properties of sandstone

Supercritical CO2 fracturing is a novel hydro-free fracturing technology, which has the characteristics of minor damage to reservoirs and higher efficiency. During the process of fracturing, the CO2 fracturing fluid will infiltrate into the pore of rock, resulting in the change of rock mechanics properties. The rock samples were drilled from the same layer, then the supercritical CO2 fracturing fluid was used to soak the rock samples to measure the changes of rock mechanical properties under different soaking time. When the pore pressure is 0 MPa, the fracture initiation stress of rock sample is about 40% of its peak strength, and with the increase of pore pressure, the elastic stage decreases and the plastic stage increases, while the pore pressure reaches 31 MPa, the core fracture initiation stress is about 20% of its peak strength. The results show that with the longer CO2 soaking time, the shorter elastic stage with the longer plastic stage and the weaker compressive strength, the smaller elastic modulus, and the higher Poisson's ratio. This indicates that the strength of rock decreases gradually with the increase of injection time in supercritical CO2 fracturing process, which is conducive to the formation of longer fractures.

Study on the cracking mechanism of hydraulic and supercritical CO2 fracturing in hot dry rock under thermal stress

To realize the effective extraction of high-temperature thermal energy in enhanced geothermal system (EGS), it is necessary to explore how to efficiently construct fracture network in hot dry rock (HDR). Considering the cryogenic induced thermal stress, we investigate the cracking mechanism when using supercritical CO2 (SCCO2) and hydraulic fracturing to stimulate HDR. Firstly, the established three-dimensional THMD coupling model is verified by conducting high temperature granite fracturing experiments. Then, characteristics of the heat and mass transfer and fracture propagation when using SCCO2 and H2O in HDR fracturing are explored. Finally, the cracking mechanism of hydraulic and SCCO2 fracturing in HDR under thermal stress is revealed. The results indicate that the cryogenic induced thermal stress can reduce the cracking pressure and tend to form branch fractures under the cooperation of injection pressure. Adopting SCCO2 in HDR fracturing more micro fractures can be generated in near-well zone. Hydraulic fracturing has better cooling efficiency and preferably capability of extending fractures. For HDR fracturing, the scope of fracture network tends to increase first and then decrease with the raise of injection mass flux. The lower viscosity and higher specific heat capacity of fracturing fluid can promote the formation of fracture network.

Preparation and Performance of Supercritical Carbon Dioxide Thickener.

The low sand-carrying problem caused by the low viscosity of supercritical carbon dioxide (SC–CO2) limits the development of supercritical CO2 fracturing technology. In this study, a molecular simulation method was used to design a fluorine-free solvent-free SC–CO2 thickener 1,3,5,7-tetramethylcyclotetrasiloxane (HBD). Simulations and experiments mutually confirm that HBD-1 and HBD-2 have excellent solubility in SC–CO2. The apparent viscosity of SC–CO2 after thickening was evaluated with a self-designed and assembled capillary viscometer. The results show that when the concentration of HBD-2 is 5 wt.% (305.15 K, 10 MPa), the viscosity of SC–CO2 increases to 4.48 mPa·s. Combined with the capillary viscometer and core displacement device, the low damage of SC–CO2 fracturing fluid to the formation was studied. This work solves the pollution problems of fluoropolymers and co-solvents to organisms and the environment and provides new ideas for the molecular design and research of SC–CO2 thickeners.

Development of Novel Silicon-Based Thickeners for a Supercritical CO2 Fracturing Fluid and Study on Its Rheological and Frictional Drag Behavior

To address low viscosity of CO2 in supercritical CO2 fracturing, the authors studied silicon-based thickeners for a supercritical CO2 fracturing fluid. Two silicon-based thickeners, namely, PMSQ-VAc and PDMS-VAc, were prepared by solution polymerization and emulsion polymerization, respectively; for each of the thickeners, molecular structural characterization, observation of dissolution in supercritical CO2 with a pco.dimax cs1 high-frequency microscope camera, and exploration of viscosity-enhancing mechanisms were performed; a differential pressure method for fluid flow in a high-pressure delivery pipe was used to calculate viscosity and frictional drag coefficient of the supercritical CO2 fracturing fluid, and rheological behavior and frictional drag behavior of the fracturing fluid under different experimental conditions were evaluated. The test results show that, by solution polymerization, the PMSQ-VAc thickener could be prepared from vinyl acetate and polymethylsilsesquioxane at a mass ratio of 13.95:7.51, with a yield of 84%, a molecular weight of 49,170 g/mol, and a viscosity of 12.86 mPa·s; by emulsion polymerization, the PDMS-VAc thickener could be prepared from dimethylcyclosiloxane, vinyltriethoxylsilane, and vinyl acetate at a mass ratio of 2:1:3, with a yield of 93%, a molecular weight of 76,190 g/mol, and a viscosity of 21.47 mPa·s; in the absence of any cosolvent, both silicon-based thickeners had a good solubility and viscosity-enhancing effect in supercritical CO2, among which PDMS-VAc had a better solubility and viscosity-enhancing effect, enabling the viscosity of supercritical CO2 to be up to 10.12 mPa·s; after being incorporated with the silicon-based thickeners, the supercritical CO2 fracturing fluid exhibited rheological behavior in agreement with shear behavior of pseudoplastic fluid and thus belongs to non-Newtonian fluid; the viscosity of the supercritical CO2 fracturing fluid varied in a nonlinear manner with increasing temperature and pressure, i.e., increasing and then decreasing, and with increasing thickener concentration, the viscosity increased and then tended to be stable; the frictional drag coefficient of the fracturing fluid in pipe flow decreased gradually with increasing flow rate and pipe diameter and increased gradually with increasing viscosity of the fracturing fluid; two fracturing fluid systems consisting of silicon-based thickeners and supercritical CO2 barely or weakly damaged the reservoir core. The silicon-based thickeners are practicable to some extent during supercritical CO2 fracturing and likely to become high-performance, low-cost, pollution-free supercritical CO2 thickeners.