A review of experimental apparatus for supercritical CO2 fracturing of shale

Supercritical carbon dioxide (SC–CO2) 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-CO2 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, CO2 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-CO2 fracturing of reservoir rocks. In this regard, the currently developed SC-CO2 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 CO2 phase and the temperature–pressure measurement of CO2 during fracturing. Third, inspired by the literature review, a self-designed experimental apparatus is developed to conduct SC-CO2 fracturing tests. In particular, an integrated structure for the temperature–pressure measurement associated with CO2 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, CO2–rock interactions, and CO2 phase control, have impeded the field-scale application of SC-CO2 fracturing for shale gas development. Hereby, future apparatus design concepts are given for comprehensive investigations of SC-CO2 fracturing mechanism.