Geomechanical Study on Hydraulic Stimulation in Enhanced Geothermal System: Field Observation Analysis and Analytical Estimation

Abstract

Enhanced Geothermal System (EGS) technology which intends to extract deep geothermal resource from the hot crystalline basement has been a frontier for the geothermal industry for the past 30 years. Massive hydraulic stimulation is one key to improve the reservoir permeability to allow fluid circulation at rates of commercial interest during EGS development. The EGS stimulation targets to inject fluid into the long open hole section with an interval of tens to hundreds of meters in the crystalline formation. Numerous field stimulation tests have been performed and increasing efforts have been made to study the mechanisms of stimulation and interpret the test observation. However there is much knowledge gap for understanding the essential details of the stimulation process, e.g., shearing initiation and propagation, and fracturing initiation and propagation, which is vital for interpreting the evolution of reservoir permeability and managing induced seismicity. Key characteristic test and performance parameters of field hydraulic stimulation tests on seven EGS or HDR (Hot Dry Rock) projects were reviewed in geomechanical perspectives, followed by comparative correlation analysis on reservoir conditions, test parameters and test observations. The analysis indicates the differential stress condition plays a controlling role in hydroshearing and induced seismicity. Generic geomechanical models were developed to estimate shearing initiation location, the required pressure and the overall shearing growth direction corresponding to hydroshearing mechanism. General studies on the effects of the stress condition on the shearing initiation and propagation captured some basic features related to the observed induced seismicity. Upward growth of shearing prevails for most stress conditions and a dense fluid favors downward shearing. The developed method is potentially applicable to provide primary assessment of shearing initiation and propagation during hydraulic stimulation in a fractured EGS reservoir. For hydrofracturing mechanism, the proposed generic model can estimate the fracture initiation in open hole section and the overall fracturing propagation during stimulation. General studies on the effects of in situ stress and open hole trajectory on hydraulic fracturing indicate that the fracture initiation at casing shoe section and the upward growth of vertical fracture prevails for common stress condition at deep EGS reservoir. An open hole with building up trajectory may shift fracture initiation location from cashing shoe to well toe by a lower breakdown pressure.