Experimental Fracture Creation in Cores: Permeability and Porosity Measurements of the Fractured Cores and the Use of Such Measurements in Analysis of Pressure Falloff Tests Following Well Stimulation

ABSTRACT This paper presents an assessment of the stimulation of an existing, research, geothermal well in a potential enhanced geothermal system (EGS) in Utah, adjacent to the Roosevelt hydrothermal field. The geothermal well has undergone stimulation with the objective to extract heat efficiently from the field's low-permeability granitoid hot rock. This research focuses on deciphering the information content of the stimulated well's pressure transient test using laboratory-measured core data as guides. Specifically, fractures were created in the laboratory in several cores from a non-related granite outcrop and from the Utah FORGE granitoid geothermal reservoir. Matrix and fracture permeabilities and porosities, and pore compressibility were measured and, where appropriate, were used both as guides and input data in the interpretation of the analytical and numerical solutions of flow equations in history matching of the geothermal well measurements. The information obtained from the core experiments was of great value in conventional interpretation of pressure falloff tests. From a pragmatic point of view, our laboratory data suggest that the contribution of the matrix flow to the fracture flow is extremely small, and probably some of the pre-existing natural fractures reopen during well stimulation to provide the main path for fluid flow and, possibly, heat exchange in the stimulated geothermal wells. INTRODUCTION The process of drilling and completion of a pair of parallel injection-production wells in hot, dry rock (HDR) system at 7000-10000 ft depths in the Utah FORGE area is an extensive effort (Moore et al., 2020). Because the permeability and porosity of this HDR system are very small, there is no mobile hot brine in the formation to circulate to sufficiently extract the heat from the formation. The aforementioned injection-production pair shown in Fig. 1 (injector 16A(78)-32 and producer 16B(78)-32) are vertically placed parallel to each other and are intended to be connected by a set of hydraulic fractures (Kumar, D., & Ghassemi, 2019). At this point field attempts have been made to connect the two wells which are 300 ft apart (Allis & Moore, 2019) with three large hydraulic fracture (HF) stages. Each fracture has a relatively large surface area to provide a means of extracting heat from the formation via the injected water flowing through the fracture. However, there is a need for additional surface areas that are perceived as the surface areas of the micro- and macro-fractures in the stimulated volumes between three sets of parallel hydraulic fractures placed in the injection well. One does not know how permeable these smaller fractures are; nonetheless, it is anticipated that some of the innate fractures and the newly created ones provide additional permeability connection to the HDR matrix (Bruce Hill, 2021).