Numerical Analysis of Parameters Affecting Hydraulic Fracture Re-orientation in Tight Gas Reservoirs

Abstract

In tight gas formations where the low matrix permeability prevents successful and economic production rates, hydraulic fracturing is required to produce a well at economic rates. The initial fracture opens in the direction of minimum stress and propagates in the direction of maximum stress. As production from the well and its initial fractures declines, re-fracturing treatments are required to accelerate recovery. The orientation of the following hydraulic fracture depends on the actual stress-state of the formation in the vicinity of the wellbore. Previous investigations by Elbel and Mack [1] demonstrated that the stress alters during depletion and a stress reversal region appears. This behavior causes a different fracture orientation of the re-fracturing operation.

For the investigation of re-fracture orientation a two-dimensional reservoir model has been designed using the software package COMSOL Multiphysics. The model represents a fractured tight gas reservoir of infinite thickness. A coupled simulation of fluid flow and geomechanics is realized by the use of Biot’s theory of poroelasticity.

The simulation shows that the poroelastic behavior develops an elliptical shaped stress reversal region around the fracture, if the difference between minimum and maximum horizontal stresses is small. The time dependent analysis indicates that the dimension of the region initially extends quickly until it reaches its maximum. Subsequently, the stress reversal region shrinks slowly until it finally disappears. The reservoir characteristics influence the dimension and time development of the stress reversal region in this process. Different case studies shown in this work illustrate the sensitivity of various parameters to this behavior and the orientation of the resulting fractures.

The success of re-orientated fractures depends on the actual dimension of the stress reversal region. Therefore the work aims to help predict the optimum timing of re-fracturing in order to maximize production. Further, the paper will show numerical investigations for vertical, horizontal and multifractured wells. The findings will support effective field development in tight gas reservoirs.