Rock Mechanics Analysis of Enhanced Geothermal Energy Recovery
AuthorKamichetty, Krishna Kumar
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Permeability enhancement during stimulation in a geothermal reservoir is mainly due to the development and extension of pre-existing fractures and joints in the rockmass. The success of this technique is primarily associated with a complex interaction between thermal, hydraulic, mechanical and chemical processes within the rockmass. Various solutions have been developed with the intention of modeling each single process and eventually couple them together in order to predict the resulting permeability and size of the man-created fracture network for given injection rates and rockmass properties. Different combinations of T-H-M-C processes have been studied and are largely based on theoretical frameworks [Taron, et al, 2009]. Most of the T-H-M-C applications occur in the fields of nuclear waste disposal, gas and oil recovery, and Hot Dry Rock Geothermal Systems. In Enhanced Geothermal Systems (EGS), fluid circulation in the fractured rocks is influenced in both the short term and long term by Thermal-Hydro-Mechanical deformation [Elsworth, et al, 2009]. There are many considerations in the design of EGS and most relate to the behavior of fractures. In this work, the Thermal-Hydraulic-Mechanical behavior of the fracture aperture in a Hot Dry Rock Enhanced Geothermal System is studied through numerical simulations of the Fenton Hill model site [Cramer et al, 1979] using a three dimensional distinct element code, 3DEC. A full study of the physical processes involved and their relation with the in-situ geological, structural, mechanical and hydrogeological conditions is a constant and essential component of the work. Different values of input parameters such as Young's modulus, Poisson's ratio, mechanical stresses, thermal load and fluid flow are applied to a planar fracture, and the resultant deformation of the fracture is obtained in the form of displacements. Coupled processes such as coupled T-M and H-M methods are employed and solved for the fracture aperture changes. From the study, it is found which of these individual parameters or coupled processes have significant effect on the fracture aperture change which in turn changes the modeling methods of an Enhanced Geothermal Reservoir.As a result the study offers a reference catalog of guidelines, directions and solutions for future considerations of EGS candidates and demonstrates the capability of the model as an assessment tool for the creation of fractures for an EGS energy recovery project.