Simulation of Produced Water Reinjection Under Fracturing Conditions

Abstract

SummaryAn essential difference between conventional waterflood fracturing and unfiltered produced water reinjection (PWRI) under fracturing conditions is that in the latter case, the fracture conductivity cannot be assumed infinite anymore as a result of fracture fillup with injected solids. This relates to the important PWRI issue of where the injected solids go. Using numerical model calculations, we show that the pressure drop over a finite conductivity fracture can lead to a significant increase in fracture volume without necessarily leading to a significantly higher injection pressure. Thus, a picture emerges in which the fracture conductivity "adjusts" itself in order to accommodate injected solids. This picture allows the computation of well injectivity as a function of total injected water volume, solids loading, etc. This concept can also be used to qualitatively explain the PWRI field observation that injectivity appears to be partially or fully reversible as a function of water quality.A field example from the Middle East is presented. The effect of parameters such as water quality, formation stiffness, and filter cake permeability on well injectivity and fracture size is discussed. It is shown that for most practical applications, an approximate analytical formulation for the computation of well injectivity and fracture size provides good results.