ABSTRACT:

Modeling the interaction between hydraulic fractures and pre-existing natural fractures is important to geothermal and petroleum reservoir stimulation. This paper presents a boundary element-based method for modeling this interaction during hydraulic fracturing process. The model couples fluid flow to fracture deformation, and accounts for fracture propagation including the transition of natural fractures to a hydraulic fracture. The numerical model is used to analyze a number of stimulation scenarios with results presented in terms of the hydraulic fracture trajectory, fracture aperture, and pressures as a function of injection time. The injection pressure profile shows the complexity of the propagation process and its impact on stimulation design and proppant placement. In addition, sequential and simultaneous injection and propagation of multiple fractures is modeled. Results show that for sequential injection, the pressure needed to initiate the later fractures increases but the geometry of the fractures is less complicated than that obtained from simultaneous injection. It is also observed that when mechanical interaction is present, the fractures in sequential fracturing have a higher width reduction as the later fractures are formed.

1. INTRODUCTION

The propagation of hydraulic fracture in a naturally fractured reservoir is a subject of interest in both geothermal and unconventional petroleum reservoir development. Often, the fracturing in a reservoir having multiple natural fracture leads to a complex fracture network. In geothermal and shale reservoirs, it is believed that fracture complexity increases the heat extraction surface area, and drainage volume, respectively. However, poor placement of fracture stages can cause detrimental effects. In order to optimize the fracturing it is important to understand the HF-NF interactions and their effect on fracture system stability. A connected fracture systems plays a major role in heat extraction from geothermal reservoirs. Hydraulic fracturing is applied to enhance the system connectivity to increase the stimulated reservoir volume.

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