Semi-analytical modeling of water injection under fracturing conditions by capturing effects of formation damage mechanisms to predict fracture propagation

Abstract

It is well known in the oil & gas industry that mature oil fields with a water drive produce increasing volumes of oily produced water which require treatment and efficient disposal. It is a major concern of all waterflood operations now in the industry, to adopt cost effective and environmentally sound water disposal systems. Re-injection into the subsurface has been chosen to be a potentially attractive option from environmental regulations view point, but to become the preferred alternative for produced water disposal, re-injection (PWRI) must also be economical and should not incur excessive risk in form of injectivity decline and damage to the formation. In this study, a semi-analytical model of contaminated water injection under fracturing conditions, including the effects of damage mechanisms over time is developed, that couples the reservoir engineering and fracture mechanics aspects of the problem. The main features of this model are finite fracture conductivity, external filter-cake build-up on the fracture face, internal plugging at fracture tip by injected solids, backstress resulting from pore pressure inflation (poro-elastic stress), backstress resulting from formation cooling (thermo-elastic stress), and fracture propagation. The most important characteristic is that the fracture is of finite conductivity due to fracture fill up by total suspended solids and oils in injected water. This is what differentiates this model from conventional waterflood induced fracturing simulators. This model is an extension to Koning’s model for waterflood induced fracturing, where fracture is of infinite conductivity. This model is largely based on work of Hoek et al. for finite conductivity fracture. This model incorporates a rectangular fracture that fully penetrates a permeable layer bounded by impermeable layers on top and bottom. The fracture is surrounded by three elliptically shaped zones, formation damages caused are included which are critical for fracture propagation, and the primary objective is to predict long term fracture growth and well injectivity. The governing Equations for fracture propagation are based on four fundamental physical phenomena which are fluid leak-off from the fracture into the formation, fluid flow inside the fracture, poro-elastic and thermo-elastic backstress on the fracture face and fracture propagation into the formation. The novelty of the new simulator is the ability to model waterflood induced fracture including all kinds of damage mechanisms, and accurately predict fracture propagation quickly.