SC-CO2 Fracturing Research Articles

Experimental study on the supercritical CO2 fracturing of shale considering anisotropic effects

Hydraulic fracturing using freshwater as fracturing fluid is regularly employed in commercial shale gas or oil production. Many problems are brought by the fracturing fluid of water, such as water shortages, swelling of clay mineral, and the pollution of flow-back water. Replacement of water by supercritical CO2 (SCCO2) in the hydraulic fracturing treatment of shale reservoir has meaningful potential for the improvement of gas production. Hydraulic fracturing experiments, under different injection rate and stress state, were carried out for studying the SC-CO2 fracturing of shale considering anisotropy effects. Anisotropy of shale has a significant impact on the mechanical behavior and fracture propagation of shale in the experiment. There shows a downward tendency for breakdown pressure with the increase of bedding plane angle in general. Higher injection rate can lead to the higher breakdown pressure, while higher deviator stress can lead to the lower breakdown pressure instead. In addition, three patterns of fracture propagation can be observed in the experiment, relative to the bedding structures of shale specimen, including propagating along, propagating across and arresting. The maximum values of fracture width during experiment in shale with different bedding plane angle ranges from 0.29 mm to 1.05 mm, while the final fracture width after the fracturing experiment is kept within the range of 0.01 mm–0.04 mm under the injection rate of 0.3 ml/s.

Experimental investigation on fracture propagation modes in supercritical carbon dioxide fracturing using acoustic emission monitoring

Supercritical carbon dioxide (SC-CO2) fracturing has broad application prospects due to its potential to increase production, to reduce water requirements, and to minimize environmental impacts. To date, lowering fracture breakdown pressure and inducing multi-fractures using SC-CO2 as fracturing fluid have been recognized, but the process of fracture propagation has been poorly studied. Therefore, acoustic emission (AE) system with 12 channels is used to monitor the SC-CO2 fracturing. The results show that energy release rate of SC-CO2 fracturing is 1–3 orders of magnitude higher and the cumulative energy is also much larger compared to hydraulic fracturing. Then much available energy can be converted into driving force to generate dynamic multi-fractures. Meanwhile, the curves of cumulative energy are observed as shape of sidestep or zigzag. The SC-CO2 fracturing evolution of AE events is discontinuous, suggesting that intermittent fracture propagation occurs. The process and modes of SC-CO2 fracture propagation are proposed, respectively. The process of SC-CO2 fracture propagation can be divided into 4 stages: damage zone evolution, fracture initiation, fracture post-instability and fracture arrest. And fracture propagation modes are classified into 3 types, including single curved fracture (Type -I), fracture band (Type -II) and fracture network (Type -III). This study is helpful for designing appropriate treatment to optimize CO2 sequestration or to enhance stimulated reservoir volume.

Mechanism of supercritical carbon dioxide (SC-CO2) hydro-jet fracturing

Supercritical Carbon Dioxide (SC-CO2) hydro-jet fracturing can increase gas production and reduce carbon dioxide emission, and is considered as a promising alternative for shale gas exploration. SC-CO2 hydro-jet fracturing technology has resulted in multi-fracture network in the YanChang Well of Ordos Basin, China. However, the mechanism of SC-CO2 hydro-jet fracturing is still not clear. To address the problem, experiments and specimens were designed based on Orthogonal Experimental Design (OED) to investigate the influential factors, including jet pressure (Pin), jet distance (S), nozzle diameter (dn), perforation length (L) and perforation diameter (dperf). In addition, the evaluation indexes, ambient pressure and the fracture profile were deeply discussed. The investigation results indicated that crack initiation on the root of perforation was prevented by the higher SC-CO2 jet pressure. Particularly, when the nozzle diameter was more than the inlet diameter of casing (3 mm), the crack number on perforation dropped sharply. When the optimal perforation length and perforation diameter were respectively 50 mm and 6 mm, the peak values of the crack number and distribution range were observed. The results of Analysis of Variance (ANOVA) demonstrated that jet pressure and perforation length were the main influential parameters. With the increase in ambient pressure, the threshold pressure of fracture initiation increased. Three patterns of crack extension directions were found in the experimental specimens: surface fracture, longitudinal fracture, and transverse fracture.

Current research into the use of supercritical CO2 technology in shale gas exploitation

The use of supercritical CO2 for shale gas extraction is a promising new technology. This paper explores current research into this process, looking at analysis of the mechanism of CH4 displacement in nanoporous shale, the positive and negative effects accompanying its use for sequestration as well as organic extraction, the migration of elements and the swelling process, and the macro and micro control mechanisms involved in permeability enhancement in reservoirs. Fruitful directions for future research are also considered through comparison with hydraulic fracturing. The research findings indicate that ScCO2 fluid replacement can be used to increase gas production and seal up greenhouse gases as an effective, clean and safe method of shale gas exploitation. It is particularly effective for promoting the desorption of CH4 in shale reservoirs that have developed fine neck-wide body pores, and the subtle structural changes effected by ScCO2 fluid in sensitive minerals in reservoirs with a high brittle mineral content also have a positive effect on permeability and storage capacity. The adsorption process has been characterized as consisting of three stages: short-term shrinkage, slow swelling, and stability; an expansion equation has been proposed for CO2/CH4 that incorporates competitive adsorption, collision desorption, and impingement re-adsorption. ScCO2 fracturing has been found to be more effective than hydraulic fracturing for dense reservoirs and more effective at linking up pore-micro-fissure-fracture systems.

Transport feasibility of proppant by supercritical carbon dioxide fracturing in reservoir fractures

The pure supercritical carbon dioxide (SC-CO2) fracturing prevents the clay from swelling and avoids the water lock as compared with the slick-water fracturing. The CO2 molecule could replace the CH4 adsorbed in organic matter and tiny particles on the clay mineral surface in the formation. This leads to an increased cumulative gas production rate. The SC-CO2 fracturing is an alternative waterless fracturing technique for an effective future development of shale gas reservoirs. Due to its low density and viscosity as compared with the slick-water, it attracts attentions for the proppant transport. In this paper, the two phase flow of the SC-CO2 and the proppant in fractures during the SC-CO2 fracturing is analyzed with the computational fluid dynamics method. The characteristics of the proppant transport by the SC-CO2 fracturing and the slick-water fracturing are compared. Moreover, a sensitivity analysis is also performed to see the influence of various parameters on the proppant transport ability of the SC-CO2 fracturing. It is shown that the proppants in the SC-CO2 and the slick-water have similar distribution characteristics. Reducing the proppant density, the proppant diameter, and the solid volume fraction as well as increasing the injection rate can all have similar filling effects on the fractures. The feasibility of the proppant transport by the SC-CO2 fracturing in fractures is revealed and a guidance is provided for the SC-CO2 fracturing design.

Experimental study of supercritical CO2 fracturing on initiation pressure and fracture propagation in shale under different triaxial stress conditions

Supercritical carbon dioxide (SC-CO2) fracturing technique used in the development of shale gas has attracted increasing attention in the past decades. Using a self-developed physical simulation system equipped with acoustic emission (AE) and computed tomography (CT) system, the fracture initiation pressure (FIP) and fracture propagation mechanism of shale in the process of SC-CO2 fracturing was invested. The results show that the FIP for the shale without pre-existing fractures obtained from experiment is consistent with the theoretical values under different triaxial stress conditions, indicating that the theoretical calculation for FIP is feasible. The FIP increased gradually with increasing in situ stress for the shale without pre-existing fractures. The pre-existing fractures will affect the FIP and the fracture propagation direction in the shale. AE signals are obviously observed in the fracture initiation and the fracture propagation stages. In addition, the hit, energy, and amplitude are increased stepwise with increasing fracturing time. In the triaxial stress state, the initiation position and the propagation direction of cracks are random in the process of the SC-CO2 fracturing of shale and are along the direction of weak structural planes or pre-existing fractures. The SC-CO2 fracturing technique integrated with AE monitoring and CT scanning can be used to analyse the crack initiation position, the direction and sequence of crack propagation, and the number of the fracturing-generated cracks in shale. This research lays a foundation for SC-CO2 fracturing of shale in the Longmaxi formation in the Sichuan Basin.

A laboratory investigation of fracture propagation induced by supercritical carbon dioxide fracturing in continental shale with interbeds

Supercritical carbon dioxide (SC-CO2) used as a fracturing fluid in the shale fracturing stimulation is a promising technology as it can save much water and have some particular properties, such as low surface tension, large diffusion coefficient, strong permeable ability and adsorption capacity in shale. In previous studies, SC-CO2 fracturing presents great advantages in marine shale, granite and tuff. In this work, continental shale with interbeds from the Ordos Basin, China was used to conduct SC-CO2 fracturing experiments under the different stress conditions. Based on the observation of visible fractures before and after experiments, the fractures on the surface mainly propagate along natural fractures, bedding planes and interbeds, where crossing beddings and interbeds can also be observed. Besides, according to visualization and quantification of the main fractures and the main fracture surfaces by electronic microscope and three-dimensional scanner, with the increase in the stress difference, the main fracture can easily cross shale beddings and interbeds, and partial vertical fractures are formed, so tortuosity of the main fracture and roughness of the main fracture surface tend to increase. Furthermore, four main types of interactions between the main fracture and the fracture branch are obtained and the number of the fracture branches induced by the SC-CO2 fracturing is generally larger than that induced by the water fracturing. Meanwhile, compared with the water fracturing, it is implied that the breakdown pressure of SC-CO2 is generally lower, and its tortuosity of the main fracture and roughness of the main fracture surface tend to be higher under the same experimental conditions. Therefore, more curved and wavelike fractures with more branches can be generated in the continental shale SC-CO2 fracturing.

Calculation of proppant-carrying flow in supercritical carbon dioxide fracturing fluid

As a new clear and efficient fracturing fluid, supercritical carbon dioxide (SC-CO2) has many advantages in the development of unconventional oil and gas resources, such as no harm to the reservoir, improving the reservoir permeability and fluid flow resistance, quickly completely flowing back after fracturing. Based on the principle of suspension energy balance, an equilibrium flow rate model of proppant-carrying flow of SC-CO2 is derived, and calculation errors are 3.32% and 4.54%, respectively, compared with equilibrium flow rates obtained from experiments under different density and particle sizes. By considering the change of fluid internal energy and flow work, filtration characteristics, physical parameters, and SC-CO2–rock adsorption in the SC-CO2 fracturing process, a proppant-carrying flow calculation model of the SC-CO2 fracturing fluid is established based on temperature–pressure field coupling, fracture and filtration zone coupling, and SC-CO2 fluid and proppant coupling. In the experiment, the heights of nine groups of proppant embankments were measured, and the average model prediction error value is 3.43%. SC-CO2 has a similar proppant-carrying capacity to that of clear water under the high Reynolds number condition. Based on the proppant-embankment prediction model of SC-CO2 fracturing, through analyzing proppant-carrying characteristics of the SC-CO2 fracturing fluid, the proppant concentration distribution and proppant embankment section are obtained at different fracturing times. According to the calculation, as the fracturing fluid pumping rate increases, proppant density and proppant particle size will decrease, the viscosity of the SC-CO2 fracturing fluid system will increase, and proppant accumulation height can be reduced over a small range, while proppant accumulation length will experience a greater increase along the fracture length direction. During field operation of SC-CO2 fracturing, the proppant pumping process should be optimized according to the proppant embankment section and proppant concentration should be choose increased in small steps to meet fracture propping requirements.

Laboratory experiment on a toluene-polydimethyl silicone thickened supercritical carbon dioxide fracturing fluid

Supercritical carbon dioxide (SCCO2) fracturing fluid is an excellent application in CO2 re-use. It has many advantages than water-based fracturing fluids. However, the low viscosity hinders its application in unconventional oil and gas reservoirs. Therefore, we tried to improve the property of SCCO2 fracturing fluid by increasing the viscosity. A thickened SCCO2 fracturing fluid was constructed and the fracturing property of the fracturing fluid was investigated. The fracturing fluid consisted of toluene (cosolvent), polydimethyl silicone (thickener) and SCCO2. In SCCO2, toluene and polydimethyl silicone obtain low miscible pressure at 42 °C. In addition, the constructed fracturing fluid system obtains higher viscosity value (as high as 1.5 mPa·s), which can be 40 times high than the viscosity of pure SCCO2 at 42 °C and 20 MPa. These features match the viscosity requirement as a fracturing fluid. Molecular dynamics (MD) simulations were carried out to study the solubility parameter of the fracturing fluid system in molecular level. These MD simulation results show that, as a cosolvent, toluene helps polydimethyl silicon dissolve in SCCO2, which is consistent with laboratory experiment results. Moreover, the studied fracturing fluid obtains good properties as a fracturing fluid in terms of high proppant carrying capability, low fluid leakoff coefficient, and low formation damage rate.

Surface characteristics and permeability enhancement of shale fractures due to water and supercritical carbon dioxide fracturing

Carbon dioxide (CO2) is an alternative working fluid to water for hydraulic fracturing in shale reservoirs. It offers advantages as a substitute for the use of large quantities of potable water and for the concurrent sequestration of CO2, however sorption and swelling effects, and their impact on permeability may be detrimental and are undefined. Hence, it is of great importance to understand the mechanism of supercritical carbon dioxide fracturing in shale and its effect on shale permeability enhancement. We conduct hydraulic fracturing experiments on shale samples using both water (H2O) and supercritical carbon dioxide (Sc-CO2) as fracturing fluids to explore the surface characteristics and permeability evolution of fluid-driven fractures. We use profilometry to measure the roughness and complexity of the resulting fracture surfaces and measure the permeability of the fractures. Results indicate that: (1) Sc-CO2 fracturing creates fractures with larger tortuosity relative to H2O fracturing (macroscale); (2) the topography of Sc-CO2 fracture surfaces is more rough and complex compared to that of H2O fractured surfaces; (3) larger mineral grains are removed and relocated from induced fracture surfaces by Sc-CO2 fracturing – these acting as micro proppants that result in a larger fracture aperture; (4) correspondingly, the permeability of shale fractures increases by ∼5 orders of magnitude with Sc-CO2 fracturing and this enhancement is ∼3 orders of magnitude higher than that by traditional hydraulic fracturing. This observation potentially validates the feasibility of Sc-CO2 as a fracturing fluid for the stimulation of shale reservoirs.

Effects of post-instability induced by supercritical CO2 phase change on fracture dynamic propagation

The effects of non-aqueous fracturing on unconventional reservoir stimulation have been investigated by laboratory and field work. Supercritical carbon dioxide (SC-CO2) is a notable non-aqueous fracturing fluid due to its potential to increase production, to reduce water requirements, and to minimize environmental impacts. In this paper, laboratory experiments conducted on polymethyl methacrylate (PMMA) are used to study the post-instability induced by SC-CO2 phase change on fracture dynamic propagation. Experimental results show that the fracture propagating speed of SC-CO2 fracturing is two order greater than that of hydraulic fracturing, and extensive hackle fractures occur in SC-CO2 fracturing. SC-CO2 phase change are mainly responsible for fracture post-instability, which dominates fracture propagation in SC-CO2 fracturing. Meanwhile, an equilibrium energy model of fracture dynamic propagation is proposed to describe the energy variation and transformation in porous medium. The concept of fracture propagation coefficient, i.e., the ratio of energy supplied by injection to fracture energy, is defined. SC-CO2 fracture propagation coefficient is much larger compared to hydraulic fracturing, suggesting that enough kinetic energy can accelerate fracture growth. This research may contribute to field application that SC-CO2 fracturing is likely to be an effective stimulation treatment, even in reservoir of undeveloped natural fractures.

Macroscale Mechanical and Microscale Structural Changes in Chinese Wufeng Shale With Supercritical Carbon Dioxide Fracturing

Summary Waterless fracturing for shale-gas exploitation using supercritical carbon dioxide (scCO2) is both effective and environmentally friendly, and has become an extensive research topic. Previous researchers have focused on the chemical and physical properties and microstructure of sandstone, carbonate, and shale caprock, rather than on the properties of shale-gas formations. The macroscale mechanical properties and microscale fracture characteristics of Wufeng Shale exposed to scCO2 (at greater than 31.8°C and 7.29 MPa) are still not well-understood. To study the macroscale and microscale changes of shale subjected to scCO2, we obtained Chinese Wufeng Shale crops (Upper Ordovician Formation) from Yibin, Sichuan Basin, China. The shale samples were divided into two groups. The first group was exposed to scCO2, and the second group was exposed to nitrogen (N2). Scanning-electron-microscope (SEM) and X-ray-diffraction (XRD) images were taken to study the original microstructure and mineral content of the shale. To study the macroscale mechanical changes of Wufeng Shale immersed in scCO2 or N2 for 10 hours, triaxial tests with controlled coring angles were conducted. SEM and XRD images were taken after the triaxial tests. In the SEM images, tight bedding planes and undamaged minerals (with sharp edges and smooth surfaces) were found in N2-treated samples both before and after testing, indicating that exposure to N2 did not affect the microstructures. However, the SEM images for the microstructure scCO2-treated samples before and after testing were quite different. The bedding planes were damaged, which left some connected microfractures and corrosion holes, and some mineral types were broken into small particles and left with uneven mineral surfaces. This shows that scCO2 can change rock microstructures and make some minerals (e.g., calcite) fracture more easily. The complex microscale fractures and the decrease in strength for scCO2-treated shale aid the seepage and gathering of gas, enhancing shale-gas recovery. Knowledge of the multiscale physical and chemical changes of shale exposed to scCO2 is not only essential for scCO2 fracturing, but it is also important for scCO2 jets used to break rock and for the geological storage of CO2.

Impairment mechanism of thickened supercritical carbon dioxide fracturing fluid in tight sandstone gas reservoirs

Non-aqueous supercritical CO2 (scCO2) fracturing stimulation techniques have acquired much attention in the development of tight gas reservoirs recently. However, few researches have devoted to the impact of scCO2 fracturing fluid on permeability and relevant mechanisms. In this study, phase behaviors of thickener polydimethylsiloxane (PDMS) and co-solvent kerosene in scCO2 were studied. The regained permeability investigations reveal that the low filtrated volumes of scCO2 fracturing fluid have instant and major deterioration of sandstone permeability. The regained permeability ratio reaches the equilibrium with the further increase of filtrated volumes. The compositions of scCO2 fracturing fluid also have impact on the regained permeability. Another regained permeability tests demonstrate that slick-water causes more permeability damage to lower-permeable samples, while scCO2 fracturing fluid causes more permeability damage to higher-permeable samples. The relevant mechanisms are proposed. XRD in clay swelling tests detects the existence of the clay minerals in the tight sands and eliminates the potential of clay swelling effect of scCO2 fracturing fluid. Our investigations indicate the operational feasibility of scCO2 fracturing fluid in tight gas reservoirs and the addition of high content of co-solvent should be avoided as to maintain the low permeability damage to gas reservoirs.

Experimental study on fracture initiation and propagation in shale using supercritical carbon dioxide fracturing

Supercritical carbon dioxide (SC-CO2) fracturing is a promising technology for developing shale gas because it can effectively solve problems related to shale swelling and lack of water resources. This work conducted simulation experiments on SC-CO2 fracturing in shale for the first time. Compared with hydraulic fracturing, using SC-CO2 as the fracturing fluid reduces the pressure needed to initiate fractures by more than 50%. This reduction is due to the increased percolation and pore pressure effects of using SC-CO2. Acoustic emission tests were used to monitor the progress of fracturing in shale and high-energy CT scanning documented fracture morphology. CT scanning shows that SC-CO2-induced fractures are irregular multiple cracks. These numerous crooked cracks are more likely to induce secondary fractures in shale and to connect with natural fracture and bedding to form complex fracture networks than those formed by hydraulic fracturing. The volume of rock fractured by SC-CO2 is several times that fractured by hydraulic fracturing and the surfaces of the fractures opened by SC-CO2 are more complex and rugged. SC-CO2 fracturing can achieve better fracture networks for reservoir stimulation in shale than in sandstone, and the degree of bedding development has a great influence on the complexity of the SC-CO2 induced fractures. Namely, using SC-CO2 as fracturing fluid can increase fracture conductivity and hence achieve increased shale gas production. This study also determined how fractures propagate under different horizontal stress regimes.

Supercritical carbon dioxide fracturing in shale and the coupled effects on the permeability of fractured shale: An experimental study

Supercritical carbon dioxide (ScCO2)-based reservoir fracturing associated with CO2-enhanced shale gas recovery is a promising technology to reduce water utilization in shale gas production and has the potential for CO2 sequestration. In the current research, experiments were conducted to explore the effectiveness of ScCO2 fracturing and the permeability of fractured shale under in situ stress and pore pressure. Computerized tomography scanning (CT scan) was used to characterize the fracture morphology. The results indicate that ScCO2 fracturing can induce complex fractures with various branches, which benefits the reservoir stimulation. There is a negative power relationship between the effective stress and permeability. However, the permeability reduction with effective stress depends on the stress path. The permeability substantially decreases with increasing effective stress, which is caused by the increase of the confining pressure. Nevertheless, the permeability decreases slowly when the increase of effective stress results from a decrease of the pore pressure. In addition, CO2 adsorption induces shale matrix swelling, influences the mechanical properties of shale, which significantly decreases the permeability of the shale, and the effect of adsorption on shale permeability is related to the stress state.