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Exogenous microorganisms are introduced into the shale gas reservoir during hydraulic fracturing, but the microbial-mediated changes in the process are yet unclear. Here, a brine–shale–microorganism interaction was simulated in the laboratory for 150 days. Biogenic methane was generated rapidly in 0–16 days, then slowly in 16–40 days and stopped after 40 days, accompanying by brine alkalization. Formation trend of biogenic methane may be independent of the substrate type under the same conditions. Biofilms accelerated mineral dissolution and facilitated secondary mineral formation. Microbial action enhanced the complexity and heterogeneity of macropore structure. Archaeal community structure changed little, while bacterial community structure altered significantly, with bacterial diversity decreasing. Predominant Proteobacteria in the early stage almost disappeared, while Firmicutes, Bacteroides and Desulfobacterota gradually increased. Firmicutes showed a transfer tendency from brine to biofilms. Proteiniphilum and Petrimonas were mainly distributed in brine, whereas Sedimentibacter and Clostridium_sensu_stricto_13 dominated in biofilms. Desulfovibrio was highly distributed in both brine and biofilms. Loss of microbial diversity and the self-assembly of symbiotic microorganisms with cooperation and competition occurred in response to the changes in the shale environment, which also affected methane production, brine pH, and both mineral composition and pore structure. [ABSTRACT FROM AUTHOR]

Conventional water-based fracturing treatments may not work well for many shale gas reservoirs. This is due to the fact that shale gas formations are much more sensitive to water because of the significant capillary effects and the potentially high contents of swelling clay, each of which may result in the impairment of productivity. As an alternative to water-based fluids, gaseous stimulants not only avoid this potential impairment in productivity, but also conserve water as a resource and may sequester greenhouse gases underground. However, experimental observations have shown that different fracturing fluids yield variations in the induced fracture. During the hydraulic fracturing process, fracturing fluids will penetrate into the borehole wall, and the evolution of the fracture(s) then results from the coupled phenomena of fluid flow, solid deformation and damage. To represent this, coupled models of rock damage mechanics and fluid flow for both slightly compressible fluids and CO are presented. We investigate the fracturing processes driven by pressurization of three kinds of fluids: water, viscous oil and supercritical CO. Simulation results indicate that SC-CO-based fracturing indeed has a lower breakdown pressure, as observed in experiments, and may develop fractures with greater complexity than those developed with water-based and oil-based fracturing. We explore the relation between the breakdown pressure to both the dynamic viscosity and the interfacial tension of the fracturing fluids. Modeling demonstrates an increase in the breakdown pressure with an increase both in the dynamic viscosity and in the interfacial tension, consistent with experimental observations. [ABSTRACT FROM AUTHOR]

Supercritical CO 2 (SC–CO 2) 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-CO 2 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 CO 2 temperature variation at a high sampling rate during the entire SC-CO 2 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-CO 2 as fracturing fluids and seepage tests with nitrogen (N 2) 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-CO 2 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. • A self-designed apparatus for fracturing cylindrical rock specimens with SC-CO 2 was introduced. • The experimental apparatus was flexible because its functional modules could be used interchangeably. • The performances of the designed packer and down-hole temperature measurement structure were emphasized. • The experimental apparatus was used for fracturing and seepage tests on shale specimens. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]