Impact of Coupled Behavior of Natural Fractures on Geothermal Systems

Abstract

ParaGeo is a finite-element software specifically designed for solving coupled thermo-hydro-mechanical (THM) problems in the subsurface. ParaGeo is the result of almost 10 years of collaboration and co-development by Chevron and Three Cliffs Geomechanical Analysis Ltd (TCGA, Swansea UK), and is currently being used for a wide-range of geomechanical applications within Chevron, like mechanical earth models (MEMs), basin-scale geological restoration and forward modeling, fault stability analyses, water or CO2 injection, etc. Given the increasing interest of Chevron in geothermal opportunities around the world, a development effort is currently underway to make ParaGeo suitable for modeling problems such as enhanced geothermal systems (EGS) or advanced close-loop systems. With relatively minor developments, ParaGeo is now able to make predictions of temperature distributions, stress and pore pressure changes, temperature breakthrough, ground deformation, fault movement and re-opening of natural fractures due to changes of temperature and effective stress in the subsurface. ParaGeo is able to model fractures as discrete contacts and/or with an effective medium approach. Discrete contacts can be used for modeling large geological faults or man-made hydraulic fractures, whereas effective medium can be used to model natural fractures with relatively small spacing. In either case, ParaGeo can resolve the fracture aperture and hydraulic conductivity as non-linear functions of stress, fluid pressure and/or temperature, using either predefined functions (such as the Barton-Bandis model) or user-defined functions, which gives great flexibility to the model. Modeling results demonstrate the importance of leveraging fully-coupled THM tools such as ParaGeo to more accurately inform geothermal business decisions. Specifically, we show that for fully-coupled cases in which the fracture aperture is allowed to change with stress and temperature, parameters like the initial fracture aperture and stiffness have a significant effect on the model outputs including the predicted energy output, thermal breakthrough time and expected project lifetime. This brings to the forefront the need for proper characterization of the natural fractures, as well as non-linear constitutive models to describe fracture behavior that are calibrated with lab measurements.