Multi-Fracturing and Cyclic Hydraulic Stimulation Scenarios to Develop Enhanced Geothermal Systems - Feasibility and Mitigation Strategies to Reduce Seismic Risk

Abstract

Enhanced Geothermal Systems (EGS) are required to extract economic amounts of heat from low permeable geothermal reservoirs. We present various hydraulic stimulation scenarios to develop suitable down hole heat exchangers applying multi-fracturing designs and cyclic stimulation treatments. The aim is to control the fracture propagation and simultaneously reduce the risks of unwanted seismic events beyond a certain threshold (depending on the vulnerability and exposure of people, buildings and infrastructure). Stimulation treatments have to be designed individually for different environments. The layout depends on the geological environment as the recent stress field, preexisting fractures and fault zones and the properties of the different geological units involved. Based on the geological environment the technical and economical feasibility have to been proven. We present simulations and sensitivity analyses of hydraulic fracturing treatments from different geological environments. These include granites from the Upper Rhine Valley graben structure, precambrian basement rocks in Northern Alberta, limestone and a sandstone formations from the Western Canadian Sedimentary Basin, Rotliegend sandstones in the North-east German Basin and low permeability limestones from the western Malm Karst in the South German Molasse Basin. The simulations include gel-proppant treatments for tensile fractures as well as waterfrac treatments to address the different rock types and their self propping potential to achieve sustainable fracture conductivities. We show that cyclic stimulation schemes can reduce the hazard potential in terms of decreased induced seismicity (less number of events with lower moment magnitudes and higher b-value) if compared to constant rate injections. Multiple fracturing treatments in deviated wells offer the opportunity to stepwise develop a suitable heat exchanger in performing a number of small scale stimulations which reduce the total number and magnitude of unwanted seismic events.