In-Situ Stress Estimation from Thermally Induced Tensile Failure of Breakouts in EGS Wells: A Case Study in Newberry, Oregon, USA
Reservoir behaviour in Enhanced Geothermal System (EGS) development is highly sensitive to the in- situ stress state. Fracture enhancement and propagation through induced fluid flow improves hydraulic connectivity; yet requires appropriate assessment of stress and fracture interactions to achieve commerciality. As minimum and especially maximum horizontal stress magnitudes are hard to obtain, it has been proven that breakouts and tensile wall fractures can be used to constrain the in-situ stress magnitude. An investigation has been conducted on unconventional, mode X type breakouts that were typically thought to be induced by wellbore cooling to originate through tensile micro-cracking process. A simple compressive and tensile failure model based on Mohr-Coulomb failure criterion for inclined wellbores has been developed incorporating temperature changes involved in EGS environment. Different type of breakouts have been successfully analysed using the model developed in our study. When the mode X breakouts occur, hoop stress can be used to estimate the maximum horizontal stress magnitude, not exactly at the direction of tensile failure but at the edge of the observed thermal elongations. This method has been applied to inversely determine the maximum horizontal stress magnitude where the B-mode breakouts have been observed and the horizontal stresses have been determined from hydraulic fracturing tests. Published study of two sub-vertical EGS wells (GPK1 and GPK2) at Soultz geothermal field in France has the similar observation with needed data. Mechanical and thermal properties of the granitic rock have also been studied and published. While well GPK1 and GPK2 are separated only by 500m, in well GPK1, conventional breakouts have been observed at E-W direction yet breakouts at NNW-SSE direction have been observed at well GPK2 indicating B-mode type. Applying the methods, Hoop stress is able to estimate the maximum horizontal stress. Then, this method is applied to Newberry EGS well, NWG 55-29. Similar to the Soultz case, tensile wall fractures have been observed at a nearby well in the direction of E-W while at the same direction, excessive breakouts occurred in well NWG 55-29. This observation also fairly consistent with fast shear direction from regional teleseismic study, which indicates maximum horizontal stress direction. Stress estimation with this method shows narrow window between the horizontal stresses with vertical stresses as the largest principal stress. This result is consistent with microseismic events showing scattered profile at horizontal plane. With better breakouts observation through failure analysis, thehoop stress canbeusedtoestimate maximum horizontal stress magnitude. Especially in EGS wells with high temperature, low matrix permeability, and highly fractured characteristics, long drilling time is needed. Hence thermally induced tensile failure is quite common.
AAPG Datapages/Search and Discovery Article #90346 ©2019 AAPG European Region, 3rd Hydrocarbon Geothermal Cross Over Technology Workshop, Geneva, Switzerland, April 9-10, 2019