A review of reservoir damage during hydraulic fracturing of deep and ultra-deep reservoirs

Deep and ultra-deep reservoirs have gradually become the primary focus of hydrocarbon exploration as a result of a series of significant discoveries in deep hydrocarbon exploration worldwide. These reservoirs present unique challenges due to their deep burial depth (4500-8882 m), low matrix permeability, complex crustal stress conditions, high temperature and pressure (HTHP, 150-200 degrees C, 105-155 MPa), coupled with high salinity of formation water. Consequently, the costs associated with their exploitation and development are exceptionally high. In deep and ultra-deep reservoirs, hydraulic fracturing is commonly used to achieve high and stable production. During hydraulic fracturing, a substantial volume of fluid is injected into the reservoir. However, statistical analysis reveals that the flowback rate is typically less than 30%, leaving the majority of the fluid trapped within the reservoir. Therefore, hydraulic fracturing in deep reservoirs not only enhances the reservoir permeability by creating artificial fractures but also damages reservoirs due to the fracturing fluids involved. The challenging "three-high" environment of a deep reservoir, characterized by high temperature, high pressure, and high salinity, exacerbates conventional forms of damage, including water sensitivity, retention of fracturing fluids, rock creep, and proppant breakage. In addition, specific damage mechanisms come into play, such as fracturing fluid decomposition at elevated temperatures and proppant diagenetic reactions at HTHP conditions. Presently, the foremost concern in deep oil and gas development lies in effectively assessing the damage inflicted on these reservoirs by hydraulic fracturing, comprehending the underlying mechanisms, and selecting appropriate solutions. It's noteworthy that the majority of existing studies on reservoir damage primarily focus on conventional reservoirs, with limited attention given to deep reservoirs and a lack of systematic summaries. In light of this, our approach entails initially summarizing the current knowledge pertaining to the types of fracturing fluids employed in deep and ultra-deep reservoirs. Subsequently, we delve into a systematic examination of the damage processes and mechanisms caused by fracturing fluids within the context of hydraulic fracturing in deep reservoirs, taking into account the unique reservoir characteristics of high temperature, high pressure, and high in-situ stress. In addition, we provide an overview of research progress related to high-temperature deep reservoir fracturing fluid and the damage of aqueous fracturing fluids to rock matrix, both artificial and natural fractures, and sand-packed fractures. We conclude by offering a summary of current research advancements and future directions, which hold significant potential for facilitating the efficient development of deep oil and gas reservoirs while effectively mitigating reservoir damage. (c) 2023 The Authors. Publishing services by Elsevier B.V. on behalf of KeAi Communications Co. Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/ 4.0/).