Laboratory Measurements of Permeability and Porosity of the Matrix and Created Fractures: Application to the Pressure Falloff Analysis Following Well Stimulation to Determine Effective Fracture Spacing in Field

Abstract

Abstract We present an analysis of several multi-rate pressure falloff data following well stimulation in the Utah FORGE enhanced geothermal system (EGS) injection well. The purpose of the analysis was to determine the effective stimulated permeability and representative fracture spacing in the stimulated formation volume. The Utah FORGE geothermal system is a very low-permeability, dry granitoid; therefore, it is necessary to create permeable flow channels in the formation for the injection water to penetrate and extract heat for power generation. We have previously used the same pressure falloff analysis procedure in numerous unconventional reservoirs. We measured the permeability of the matrix rock and the permeability of an embedded fracture, as well as the porosity for several 1-1/2-inch by 2-inch cores. Then, we used the measured permeabilities and porosities in the analysis of multi-rate pressure falloff field data to determine the effective macro-fracture spacings adjacent to the hydraulic fractures. The cores used for the experimental study were outcrop granite, geothermal granitoid, and several unconventional (Niobrara, Codell, Wolfcamp) reservoirs. To determine a representative macro-fracture spacing associated with the hydraulic fractures, the pressure falloff field data analyses were fine-tuned via numerical modeling iteratively. Finally, the experimentally measured core porosities and permeabilities were used as input data for the interpretation of the analytical model used in the pressure falloff test analyses. Such analysis indicates that fracturing fluid entering the rock surrounding the hydraulic fracture is dominated by the micro-fractures. Nonetheless, it is important to note that FORGE rock is highly water wet; thus, water imbibition plays a significant role in water entering the rock pores despite their low permeabilities. Finally, one can appreciate the importance of the stimulated fractured zone both in providing surface area for heat extraction in geothermal wells and for mass transport in improving primary production and EOR in unconventional shale reservoirs.