An Experimental Study of the Effect of Long-Term Time-Dependent Proppant Behavior Under HP-HT Reservoir Conditions

Abstract

Abstract The long-term sustainability of fracture conductivity in a geothermal system using proppant will be affected by crushing pressure and geothermal fluids. Previous experiments have shown that for short-term periods, field testing results have indicated some performance improvements and a few experiments for long-term periods have shown that different types of proppants and the crushing test results suggest probable geomechanical degradation of the proppants under the test conditions. So, some proppants, such as ceramics and kryptospheres, showed a significant degree of mechanical strength degradation after exposure to high temperatures and geothermal water formation. In addition, the crushing test does not replicate downhole conditions because the crush test compares the conductivity test and downhole conditions. So, all types of proppants do not crush in the same manner. Sand-based proppants tend to shatter, ceramics tend to cleave, and resin-coated proppant deform as the internal substrate breaks. Moreover, according to the literature review, the EGS projects have used proppants such as quartz sand, ceramic LT, and ceramic HSP. However, in recent years, new types of proppants have been designed to face these challenging conditions. For that reason, supplementary testing is required to comprehensively understand the long-term behavior of the proppants in geothermal reservoirs with low permeability, which we want to use proppants. This investigation analyzes the long-term proppant behavior under high-temperature reservoir conditions and shows that some types of proppants can improve fracture conductivity and support long-term performance in harsh aqueous environments with many thermal cycles on fields such as Utah FORGE or other locations with similar characteristics. So, proppants should work for long-term conductivity and thermal pressurization cycles, between 15000 to 500°F and 5000 to 15000 Psi, respectively. This study presents the results of experimental investigations of crushing tests for different proppants at 9000, 12000, and 15000 psi and the development of a numerical model for hydraulic fracturing design to compare how it affects the fracture permeability with a permanent fracture permeability and variable fracture permeability. Also, it is developed water analyzed to compare the chemical properties before and after combining the proppants with water formation at 260 oC after two weeks. The study focuses on evaluating the permeability drop of the artificially generated fracture throughout its lifetime. The artificial fracture permeability is affected by high reservoir temperature, lithostatic pressure, and chemical components of the water formation. According to previous studies, closure stress and geothermal fluids affect proppants reducing the fracture conductivity for long-term sustainability; instead of keeping the area generated and the high permeability in the artificial fractures. Moreover, the fracture permeability is diminished, and the fracture conductivity and width are reduced, affecting the flow rate and heat extraction. So, different kinds of proppants were evaluated under different closure pressure with a crushing test to determine the percentage of destruction after two weeks under high temperatures (260 °C); new proppants have a high crush resistance and withstand stress cycling to ensure that fracture conductivity and connectivity are sustained long-term to optimize production. Hence, a numerical simulation model was developed to compare economically different scenarios of fracture permeability (permanent and variable), fracture length, fracture width, flow rate, and reservoir temperature to determine the feasibility of developing an EGS project stimulated by HF.