(149e) The Chemical and Thermal Stability of Proppants Under Geothermal Conditions

EGS reservoir fracture creation and management may require the use of proppants to maintain fracture conductivity. Hydraulic fracturing accompanied by the addition of proppant is a common procedure in the oil and gas industry. In contrast, the use of proppants by the geothermal industry has been limited. A variety of proppants are available, and the most commonly used consist of silica sand, ceramic, resin coated sands, and bauxite. These solids remain in newly formed fractures to keep them open. In geothermal systems, proppant will need to withstand high temperatures, acidified fluids, acid treatments, and cleanouts while maintaining the porosity and permeability of the fracture. However, proppant particles may act as nucleation sites and promote precipitation, or they may dissolve, consequently affecting fracture performance. Thus, the performance of proppants in geothermal reservoirs must be understood to avoid costly mistakes in the creation and maintenance of EGS reservoirs. A series of static experiments have been conducted at 200^oC using 30/60 Sinterball bauxite proppant provided by Sintex Minerals and with bauxite mixed with granite. The proppant consists of corundum with minor quartz, hematite and chlorite. The granite is representative of low permeability rocks typical of EGS systems. Deionized water or deionized water spiked with tetraethyl orthosilicate has been used for the fluid phase. The tetraethyl orthosilicate has been added to provide silica concentrations that would be in equilibrium with quartz at the test temperatures. The experiments are run for periods of 1 month to ten weeks. X-ray diffraction, scanning electron microscope and QEMScan analyses were performed on the proppant before and after testing to evaluate textural, mineralogical and compositional changes that occurred as a result of water rock interactions. Only minor changes were observed. These changes include the presence of surface pitting, resulting from dissolution of impurities in the proppant grains and deposition of minor amorphous silica and aluminum oxides.