Interaction of Multiple Non-Planar Hydraulic Fractures in Horizontal Wells
Authors: Xu Guanshui , Wong Sau-Wai 
Published: May 2016

The use of multi-fracced horizontal well technology is one of the key reasons for the recent success in the exploitation of unconventional resources such as shale gas and shale oil. This technology of placing multiple fractures in horizontal wells has provided economic production rates resulting in the prevalent development of unconventional oil and gas reservoirs. The fracture stimulation process typically involves placing multiple fractures stage by stage along the horizontal well using diverse well completion technologies. The effective design of such massive fracture stimulation requires an understanding of how multiple hydraulic fractures would grow and interact with each other in heterogeneous formations. This is especially challenging as the interaction of these fractures are subject to the dynamic process of subsurface geomechanical stress changes induced by the fracture treatment itself.

This paper presents a new three dimensional (3D) hydraulic fracture computational simulator which describes non-planar hydraulic fracture growth in heterogeneous formations. It addresses the geomechanical interaction of multiple fractures, and can be extended to considering interaction with natural fractures as well. In this model, the interaction of multiple non-planar fractures is meticulously captured by means of boundary integral formulation with dislocation segments solution techniques. The flow of proppant laden frac fluid within a fracture is represented by a power-law fluid model according to the Reynolds lubrication theory. The derived non-linear fracture growth and fluid flow equations are solved in a coupled manner via a proprietary, robust and efficient algorithm where mass conservation (i.e., frac fluid and proppant) is strictly observed. Examples are presented to demonstrate that the present numerical approach can be used to provide a much needed insight into the growth of multiple fractures under the influence of subsurface geomechanical stress 'shadows' and thus, serve as a valuable tool for optimization of multiple hydraulic fractures design.

Introduction

Hydraulic fracturing or fracture stimulation improves well productivity by establishing conductive fractures hydraulically in the tight formation/reservoir and connecting them to the well. Multiple fractures are now placed in sequential stages in horizontal wells. This technology of placing multiple fractures stage by stage along horizontal wells has provided economic production rates resulting in the pervasive development of unconventional oil and gas reservoirs, especially in shale gas and oil formations in the North Americas (US DOE, 2009). Typically, four or more fractures are pumped in each stage simultaneously, and it is not uncommon to place up to 20 to 40 stages in a single horizontal well. Attempts have also been made to simultaneously fracture two adjacent horizontal wells to generate more complex fracture networks thought to be beneficial for production (Mutalik and Gibson 2008; Waters et al 2009).

One of the key considerations in designing multiple hydraulic fractures is the changes to subsurface geomechanics due to fracture stimulation, especially the in-situ stress regime. That in turn influences how fractures would grow and develop. The fracturing induced formation stress change is also called 'stress shadowing'. This geomechanical effect needs to be understood and quantified for optimization and to avoid problems such as fracture 'screen-out'.