A coupled geomechanics and flow modeling study for multistage hydraulic fracturing of horizontal wells in enhanced geothermal systems applications.

摘要

The US has significant amount of underutilized geothermal resources that have recently gained more attention due to the technological advancements and practical knowledge brought from the horizontal drilling and multistage hydraulic fracturing operations in tight oil and shale gas reservoirs. The learnings from these unconventional efforts are the subject of this research study in order to conduct the feasible transformation of applying the technical and operational expertise into Enhanced Geothermal System (EGS).;A commercial hydraulic fracturing simulation model coupled with geomechanics and fluid flow concepts was used in a geothermal case study, allowing the simulating of hydraulic fracture creation while considering natural fracture network. First, concepts of the coupled unconventional model were studied in a shale reservoir with input parameters obtained from drilling, completion, and stimulation treatments utilizing well logs and production data to validate the integrated model. Then, a coupled Unconventional Fracture Model (UFM) was used for creating a suitable fracture network to be implemented for design in an EGS application. The role of in-situ stress state, pre-existing fracture network characteristics, injection fluid and proppant properties was investigated to optimize the design parameters and the economics for the EGS feasibility study.;The effects of various distribution patterns in natural fractures, complex fracture geometries, stress anisotropy, injection rates, surface and bottom holes pressures, fluid viscosity and fracturing proppant concentration were studied through simulations of hydraulic fracturing treatment. Modeling results confirmed that when designing for EGS, a widely distributed pre-existing natural fracture network can lead to interactions between hydraulic fractures and natural fractures and therefore raise the level of complexity of the total fracture network, which is not the desired fracture geometry for a successful EGS application. Though the overall complexity of the fracture network is also depended on many other factors such as lithology, temperature, formation fluid pressure and in-situ stress state, results from optimized simulation indicated that it can be minimized by utilizing treatment parameters such as proper fluids viscosity, proppants concentration, pumping rate, fracture stage spacing and well spacing during actual stimulation operations.