Inferring geothermal reservoir processes at the Raft River Geothermal Field, Idaho, USA through modeling InSAR-measured surface deformation

摘要

Ground surface deformations detected with Interferometric Synthetic Aperture Radar (InSAR) provide valuable information for inferring subsurface reservoir processes that are difficult to observe directly. This study aims at building a reservoir model that honors the available geological, hydrological and geo-mechanical data and also produces ground surface deformation consistent with InSAR measurements. In our coupled thermo-hydro-mechanical (THM) model, the reservoir deforms as a result of the rock's poroelastic response to changes in hydrologic pressure and thermal expansion/contraction. The computations are performed using a massively parallel multi-physics code (GEOS) at the scale of the geothermal field. At Raft River, the results indicate that the observed deformation cannot be caused solely by pressure changes in the deep production reservoir, and that pressure increase in a shallower reservoir that accommodates the injected fluid (likely in the Salt Lake Formation) must be involved. The rising pressure in the shallow reservoir generates strong uplift at ground surface. The combination of this uplift with surface subsidence around the production wells creates a complex pattern of surface deformation in which the center of subtle subsidence significantly deviates from the location of the production wells. The net pressure in the shallow reservoir may gradually diffuse into the moderately permeable layer underneath, resulting in additional slow deformation. Therefore, the surface deformation captured by InSAR represents a combination of multiple mechanisms acting over different time scales. A parametric study suggests that the Bridge Fault Zone is likely a barrier, impeding laterally flow cross the fault, although the fault could serve as a fast flow path along the strike direction. The surface deformation data appear to be insensitive to the presence of the Narrows Structure (i.e., a poorly defined northeast-southwest trending structure within the deep geothermal reservoir), since removing the Narrows Structure from the model does not substantially change the modeled deformation pattern at the surface. A flow barrier likely exists to the east of the site, where the surface uplift forms a band striking from north to south. This case study demonstrates the utility of a forward model that honors available known information and THM coupled processes in understanding geothermal reservoir characteristics.