Issues surrounding fracturing of geothermal systems - predicting thermal conductivity of reservoir rocks and evaluating performance of fracture proppants

Traditional geothermal systems have been limited to geologic systems in which elevated temperatures, abundant water, and high porosity and permeability are found. Engineered geothermal systems (EGS) have been proposed for thermal reservoirs in which insufficient water and/or permeability are present. The EGS model calls for the creation of large fracture networks which penetrate the hot rock resource. These fracture networks are formed by reopening sealed fractures or by creating new fractures using hydraulic fracturing methods common to the oil and gas industry. Application of hydraulic fracturing technologies in geothermal systems and operation of engineered geothermal systems present new issues including the formation of thermal fractures due to temperature differentials and rock shrinkage; and the performance of hydraulic fracturing materials such as proppants under geothermal conditions. The formation of thermal fractures in a geothermal reservoir will be governed by the thermophysical properties of the reservoir rock, including heat capacity, thermal conductivity, coefficient of thermal expansion, etc. Thermal conductivity may be estimated using data obtained from geophysical well logs. Multivariate data analysis methods such as principal components analysis and regression analysis have been used to interpret log data. Significant discrepancies between experimentally-determined thermal conductivity and model-derived thermal conductivity were noted. Possible sources of the discrepancies include rock anisotropy and insufficient data. However, principal components analysis proved to be a valuable resource for data interpretation. The resilience of proppants under geothermal conditions was evaluated. Three proppant types were tested in the presence of water and crushed granite at elevated temperatures for periods up to 11 weeks. Sintered bauxite proppant was found to be susceptible to dissolution in hot geothermal water. Quartz sand proppant and resin-coated bauxite proppant appeared to experience less dissolution. Sintered bauxite and resin-coated bauxite proppants were crush tested both before and after exposure to geothermal conditions and the resistance of the proppants to crushing remained unchanged. Based on the testing regime, resin-coated bauxite proppant appears to be well-suited for use in engineered geothermal systems.