Abstract:

Hydraulic fracturing is a complex physical process which involves the coupling of hydraulic and mechanical behaviours, as well as temperature effects in some cases. The interdependence of these various factors is complex, and cannot be fully captured by the relatively simple state-of-practice tools employed in investigations of hydraulic fracturing. In this paper, a new approach is presented for fully coupled hydro-mechanical simulation of hydraulic fracture propagation by using three-dimensional Voronoi geometries within the context of Distinct Element formulation. The block boundaries formed by the Voronoi tessellation, provide a random flow pathway for the fluid and the contact breakage due to the increase of the fluid pressure acting on them, replicate the hydraulic fracture propagation. While, the Voronoi approach for hydraulic fracturing simulation has been implemented previously in 2D models, this work puts forward a technique for extension of its application to 3D models. A series of verification tests are performed to investigate the suitability of the proposed approach. Finally, example applications are presented for simulation of single-stage and multi-stage hydraulic fracturing of intact rock by using the 3D Voronoi models at the reservoir scale.

Introduction

The hydraulic fracturing process in Massive Multi-stage Hydraulic Fracturing (MMHF) operations is repeated multiple times in stages along horizontal wells. The hydraulic fracturing process can be simplified into three stages:

  • Fracture initiation

  • Fracture growth/mobilization

  • Proppant injection

 

A primary goal of most mechanistically-focused models for hydraulic fracturing is to capture the fracture initiation and growth/mobilization of fractures (and the associated changes in matrix stresses and overall reservoir permeability) prior to the injection of proppant. This is both due to the importance of this initial stage as well as the additional complication associated with modelling proppant transport and deposition.

Once a fracture has developed and the fluid volume pumped into this fracture continues to increase, there are several competing factors which determine where the fluid travels and how the energy which is input into the system is re-distributed. These factors include; matrix permeability, rock mass stiffness, fracture toughness and the most important, existing natural fractures.

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