Experimental Investigation of Hydraulically Induced Fracture Properties in Enhanced Geothermal Reservoir Stimulation

Abstract

Geothermal energy production by water circulation in man-made fracture systems is referred to as enhanced or engineered geothermal systems (EGS) production. The permeable zones of an EGS must be created by stimulation, a process which involves fracture initiation and/or activation of discontinuities such as joints by pore pressure and stress perturbations. In this work, we study the hydraulically induced fracture properties on a laboratory scale using acoustic emission (AE) cloud, spontaneous potential (SP), and tracer analysis. To achieve this goal, we have performed reservoir stimulation using 13x13x13 inch3 pre-heated cubical rock samples under representative in-situ stress regimes. During this stimulation stage and the subsequent production processes, sensors are used on the block surfaces and within cavities and wellbores to characterize and locate acoustic emissions (AE) caused by the stimulation, and to monitor local changes in fluid pressure, temperature and electrical self-potential (SP). Cold water is injected centrally and simultaneously collected from nearby miniature production wells. Water with tracer was injected after the circulation test, and concentrations in fluid collected from the production wells could be measured. The data collected could be then analyzed to develop a better understanding of the fractures and the induced fracture permeability and fluid/heat flow.