Micro-mechanical Aspects of Hydraulic Fracture Propagation and Proppant Flow and Transport for Stimulation of Enhanced Geothermal Systems
– a Discrete Element Study –
The study presented in this thesis uses micro mechanical approach to better understand hydraulic fracture initiation, propagation, proppant flow and transport and proppant settling during hydraulic fracturing of enhanced geothermal systems. Discrete Element Method (DEM) is used in two dimensions to address some of the current problems of hot dry rock fracturing. In particular, Bonded Particle Model (BPM) is used to simulate granite behavior and hydraulic fracturing. BPM is further improved using a novel convective-conductive heat transport model for studying hydro-thermo-mechanical fracturing processes. The new contributions regarding micromechanical understanding of fracturing fluid and rock behavior coupling, effect of fracturing fluid properties on fracture shape, branching, secondary fracturing and the relationship between tensile and shear micro-cracks are presented. The effects of temperature difference between fracturing fluid and surrounding rock on fracture initiation and propagation, as well as, heating of the fluid and cooling of the rock during fracture propagation are studied. Along with the process of hydraulic fracturing proppant is placed in the fracture for keeping its stable long-term aperture. Proppant is transported in the fracture in dense slurries with high viscosity fluids. The Discrete Element Method is coupled with Computational Fluid Dynamics (DEM-CFD) for studying horizontal proppant flow and transport in narrow fracture zone and proppant settling in a narrow rough granite fracture. High proppant concentrations are usually used in practice, but such systems exhibit frequent particles collisions, especially where the ratio of fracture width and particle diameter is small. The new contact model is built within DEM-CFD code that accounts for effects of fluid lubrication force on particle collisions. A thin layer of fluid between two approaching particles yields the lubrication force that dissipates particle kinetic energy. As a result, in high viscosity fluid particles remain in close vicinity and start to form agglomerate, while fluid flows around it. A comprehensive study that investigates the effects of fluid viscosity, particle and fracture width, and pressure drop and proppant concentration is given. Better understanding of conditions that lead to particle agglomerations and proppant clogging is obtained for horizontal proppant flow and transport and proppant settling in rough granite fracture.