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Review of Proppant Behavior and Fracture Conductivity Preservation in Deep Geothermal Systems
Yusuf Z. PAMUKCU, Coşkun ÇETIN, Alireza BIGDELI, Gökhan KARCIOĞLU, Cenk TEMIZEL
[California State University, Sacramento, USA]
The long-term performance of Enhanced Geothermal Systems (EGS) depends greatly upon the ability to maintain fracture conductivities long enough to allow for extraction of heat from the deep subsurface at economically viable rates. Hydraulic stimulation is an important tool used to create the conductive pathways required for this purpose. However, in the absence of proppants, hydraulic fractures will close as the in-situ stress increases. Proppant injection is important because proppants are used to create a permanent channel for fluids to flow through the created fractures so that they remain open, and the reservoir will continue to produce. Compared to other conventional oil and gas operations, there are some significant differences in using proppants for geothermal systems. First, the proppant has to maintain its mechanical strength and hydraulic performance at temperatures higher than 200°C. Second, the geochemical environment of geothermal reservoirs can be very aggressive and corrosive, which can cause the proppant to crush, undergo chemical changes and generate fines, all of which result in the progressive reduction of the fracture's ability to conduct fluids. To provide a comprehensive reference for the selection of proppants in geothermal systems, researchers have integrated experimental data from laboratory scale tests with field data and new technologies to provide a full scope of proppant selection criteria. The focus of this study is to evaluate how proppants behave mechanically and chemically when subjected to high temperature conditions that are representative of EGS and super-hot rock (SHR) projects. Combining experimental data with practical field experience will support the decision-making process for selecting proppants to sustain the conductivity of fractures over the life of geothermal energy production. This paper provides an overview of the current knowledge regarding the behavior of proppants under geothermal conditions, emphasizing the major factors contributing to the degradation of the fracture's ability to conduct fluids. Special emphasis has been placed on the development of new methods for testing proppants in SHR environments, where the temperature of the reservoir could be greater than 375°C. Additionally, the study explores the potential for developing technology or operating practices that would extend the time that the fracture's conductivity remains stable in deep geothermal reservoirs.
Topic: Reservoir Engineering