Mechanistic Explanation of the Impact of Pore Pressure on Hydraulic Fracture Propagation

Agrawal, Shivam (The University of Texas at Austin) | Ouchi, Hisanao (Japan Oil Engineering Co. Ltd.) | AlTammar, Murtadha J. (The University of Texas at Austin) | Sharma, Mukul M. (The University of Texas at Austin)

OnePetro 

ABSTRACT: The effect of non-uniform pore pressure field and saturation on fracture propagation is investigated using a peridynamics-based hydraulic fracturing simulator. The model solves for rock displacement, fluid saturation, and fluid pressure both inside and outside the fracture in a fully-coupled manner. When fractures initiate from multiple injection points, they can propagate towards each other by opening against the maximum stress. Laboratory experiments conducted on synthetic rock samples show that saturating a porous rock with fluid before fracturing it decreases the breakdown pressure. Under low far-field stresses in the laboratory, fractures are attracted towards the high pore pressure region. The strength of this attraction depends on both the magnitude of the pressure and the pressure gradients. The simulation results are completely consistent with experiments and show why this effect is observed in the lab. These results highlight the importance of poroelasticity and non-planar fracture growth behavior in hydraulic fracture modeling.

1. INTRODUCTION

Production of fluids from a reservoir reduces the pore pressure and creates pressure gradients in the rock. This modifies the stress state from its initial in-situ condition (Warpinski and Branagan, 1989; Wright et al., 1994). Numerous field studies have reported this phenomenon in the literature (Siebrits et al., 2000; Weng and Siebrits, 2007; Roussel and Sharma, 2012). In addition, laboratory experiments are often performed to understand the underlying mechanisms (Bruno and Nakagawa, 1991; Liu et al., 2008). Further insights into refracturing process are obtained by mathematical modeling and numerical simulations (Berchenko and Detournay, 1997; Wang et al., 2013; Agrawal and Sharma 2018).

Bruno and Nakagawa (1991) conducted fracturing experiments in the presence of a non-uniform pore pressure field and isotropic stresses. They showed that both mechanical and hydraulic fractures are attracted to the high pore pressure region. Their observations were justified by Berchenko and Detournay (1997) based on a deviation of the maximum stress trajectory towards the injector well. Recently, fracture propagation in the presence of different configurations of injection sources has been studied experimentally by (AlTammar et al., 2018). The purpose of this research is to simulate and explain their results using an effective stress law and to investigate the effect of pore pressure, fluid injection scheme, saturation conditions, and applied stress on fracture growth.