Laboratory Scale Investigation of Enhanced Geothermal Reservoir Stimulation

Abstract

Geothermal energy production by water circulation in natural and/or man-made fracture systems is referred to as enhanced or engineered geothermal systems (EGS). The permeable zones of an EGS must be created by stimulation, a process which involves fracture initiation and/or activation and propagation of discontinuities such as joints by pore pressure and stress perturbations. Economic design and operation of EGS can be achieved by reliable characterization of the stimulation results which relies mostly on analysis of MEQ and tracers. In this work we study the reservoir stimulation process on a laboratory scale to improve the current understanding and to help quantify the fracture geometry using AE cloud, spontaneous potential, and tracer analysis. To do so, we have developed a new poly-axial hydraulic fracturing test system which allows us to perform reservoir stimulation experiments on rock blocks with size up to 18’ x18” x 18” under pore pressure and representative in-situ stress regimes while simultaneously recording SP and AE during fracturing and circulation. The system can operate at relatively high temperatures (150oC). In this paper, we describe preliminary experiments conducted at room temperature. Results show excellent correlation between fracturing and the SP signal. Also, stimulation has been accompanied by AE events that are generally correlated with the stress state and fracture geometry.