Abstract

In this study, we used the Particle Flow Code 2D (PFC2D) to simulate interaction of hydraulic fractures and natural fractures in low permeable hard rock. Natural fractures are simulated using the smooth joint model of PFC2D. We modified our fluid flow algorithm to model larger fracture permeability, and we investigated interactions of hydraulic fractures and natural fractures by varying the angle of approach and viscosity of the fracturing fluid. We also investigated seismic events evolving in a complex fracture network. The results demonstrate that our modelling tool is able to capture all possible interactions of hydraulic and natural fractures: Arrest, Crossing, Slippage of hydraulic fracture, Dilation of natural fracture, Closing/Opening of natural fracture. With low angle of approach, the hydraulic fracture coalesces with the natural fractures and results in hydro-shearing and propagation of hydro-wing fractures at the tips that are mostly Mode I type. We tested the model containing multiple natural fractures with varied fluid viscosity. Hydraulic fracture generated by high viscosity fluid tends to be localized, linear and less influenced by the natural fractures. In the complex network of natural fractures, fluid columns built along the fracture network increase the local state of stress by stress shadowing. Hydro-shearing of the natural fractures that were under increased stress state can be explained as the main mechanism responsible for occurrence of larger magnitude microseismic events.

1. Introduction

Geothermal energy stored in the deep subsurface especially in low permeable and hard crystalline rock mass can be accessed by enlarging the surface area where the injected fluid makes contacts to rock mass containing geothermal heat and by increasing the flow rates of the injected fluid through natural pre-existing fracture. These two concepts contribute in the oil and gas industry and enhanced geothermal system to improve the productivity of the hydrocarbon and the geothermal heat energy, respectively.

One of the key factors in successful creation of an efficient subsurface heat exchanger is to predict the propagation of hydraulic fractures (HF) and their interaction with natural pre-existing fractures (NF). There are a number of numerical codes that can simulate interactions of HF and NF, e.g. UDEC [1]. However, not many of these codes are able to simulate the dynamic processes involved in the HF-NF interaction, i.e. evolution of seismicity in HF-NF interactions.

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