Impact of Pore Pressure Depletion on Stress Reorientation and Its Implications on the Growth of Child Well Fractures

Abstract

The most important factors governing hydraulic fracture propagation are completions and treatment design, in-situ stresses, and reservoir heterogeneity at different length scales (including natural fractures and bedding planes). However, it has been recognized that in depleted reservoirs, stress changes arising due to reservoir drainage significantly affect the growth of fractures and attract them towards the depleted regions.

Using a poroelastic hydraulic fracturing simulator based on the theory of peridynamics, stress reorientation due to production and its sensitivity on Biot's constant, pressure drawdown and reservoir fluid type is studied. It is shown that a tensile region is created between the fractures of a producing parent well. Consistent with previous studies, it is verified that a fracture from a newly drilled child well grows asymmetrically towards the depleted regions. When the child well lateral is not landed centrally between the parent wells and there is unequal depletion of these wells, asymmetry in the geometry of the child well fracture may further be accentuated. Similar observations can be made when production is from a gas reservoir. This asymmetric fracture growth leads to parts of the undepleted reservoir remaining unstimulated. Re-pressurization of the parent well fractures is shown to revert the stress state closer to the in-situ conditions, thereby reducing the attraction of the child well fracture towards depleted regions resulting in a better stimulation treatment.

1. Introduction

Hydrocarbons are economically produced from unconventional reservoirs by hydraulically fracturing wells. However, production from these fractures declines rapidly due to the ultralow matrix permeability of shale reservoirs. It is common to drill and fracture new child (or infill) wells in the regions left undrained by the parent well fractures. These child well fractures see the modified stress state due to production from the parent well, rather than the in-situ stress state.

The influence of non-uniform pore pressure fields on fracture propagation has been investigated both experimentally and numerically (Bruno & Nakagawa 1991, Berchenko & Detournay 1997, Agrawal et al. 2018). Changes in magnitude and orientation of stresses around producing fractures have gained interest due to the popularity of infill well drilling (Singh et al. 2008, Roussel et al. 2013, Manchanda et al. 2018). In addition to the depleted pressure field, these stress changes play a crucial role in determining the geometry of the child well fractures (Gupta et al. 2012, Rezaei et al. 2017, Safari et al. 2017, Ajisafe et al. 2017). When the horizontal stress anisotropy is low, Gupta et al. (2012) revealed that stress reversal may occur close to the infill well leading to the formation of longitudinal fractures. Rezaei et al. (2017) argued that asymmetric fracture propagation towards depleted regions leads to fracture interference and ineffective reservoir stimulation. Such interference has been shown to be manifested in the form of pressure hits and microseismic data (Yadav and Motealleh 2017, Kumar et al. 2018, Courtier et al. 2016).