Three single-cluster hydraulic fracture stages were pumped at the Frontier Observatory for Research in Geothermal Energy (FORGE) site in Milford, Utah. Our goal was to develop a robust model to accurately represent the formation of fracture networks in this naturally fractured geothermal reservoir. In this paper, a discrete fracture network (DFN) was built based on log and core data and a three-dimensional hydraulic fracturing simulator was used to propagate hydraulic fractures with the pumping schedule used in the field.

The natural fracture network (DFN) was built stochastically with the fracture density, length, and orientation distribution of natural fractures estimated from cores and logs. We then took one-dimensional synthetic cores to ensure that the number and density of fractures per unit length of the core matched with the actual measurements (for each fracture set) until the best realization of natural fractures was found. The length distribution of natural fractures was simulated using a power law distribution.

Hydraulic fracture treatments were simulated in the DFN using a simulator that fully couples stresses and strains using the displacement discontinuity method (DDM) with fluid flow inside fracture networks using a finite volume method. The interaction of multiple hydraulic fractures with a network of natural fractures is fully accounted for. The results show that the net pressure measured in the field matched well with the simulation results when using the pumping schedule in the field. Microseismic events recorded during the fracture treatment statistically agreed with those predicted by the simulation when the proper NF length distribution was chosen. The results also show that long natural fractures tend to increase the probability of fracture deflection along natural fractures.

Our simulator is capable of conducting a comprehensive sensitivity analysis to investigate the effect of different parameters and to conduct a Monte-Carlo uncertainty analysis based on a large number of DFN realizations. Our simulations, coupled with field data, allow us to better describe the complex fracture patterns that result from hydraulic fracturing naturally fractured granites.