Page 169 - Applied Petroleum Geomechanics

CHAPTER 5

In situ stress regimes with

lithology-dependent and

depletion effects

Contents

5.1 In situ stresses in various faulting regimes 164
5.2 In situ stress bounds and stress polygons 165
5.3 Lithology-dependent in situ stresses and improved stress polygon 168
5.3.1 Lithology-dependent coefficient of friction of the fault 168
5.3.2 Poisson’s ratioedependent stress polygon 172
5.3.3 Relationship of the coefficient of friction of the fault and Poisson’s ratio 173
5.3.4 Lithology-dependent minimum and maximum horizontal stresses 175
5.4 Fault strength and in situ stresses 176
5.5 Depletion and injection impacts 177
5.5.1 Depletion-reducing horizontal stresses 177
5.5.2 Depletion and Mohr’s circle representation 181
5.5.3 Injection and shear failures 182
References 183
Abstract

Three in situ stress regimes are introduced, which can be used to describe the rela-
tionship of the vertical stress and two horizontal stresses. From Anderson’s faulting
theory, the lower and upper bound horizontal stresses are obtained either from the
coefficient of friction of the fault or from Poisson’s ratio of the formation. Both the
coefficient of friction of the fault and the horizontal stresses are lithology-dependent. If
formations in a fault are composed of shales, the coefficient of friction of the fault is
small. This implies that the fault zone that is composed of shales is weaker and more
likely to have shear failures than the fault zone composed of sandstones. An improved
lithology-dependent (Poisson’s ratio-dependent) stress polygon is presented, which
can reduce the uncertainty of in situ stress estimate by narrowing the area of the
conventional stress polygon. Pore pressure depletion by oil and gas extraction
decreases the minimum and maximum horizontal stresses. Field-measured depletion-
induced stress paths are also examined. Continuous depletion makes the formation
approach the critical stress state (shear failures), and in this state fluid injection or
repressurization may cause the formation shear failures.

Keywords: Depletion and injection; Horizontal stresses; In situ stresses; Lithology-
dependent; Poisson’s ratio dependent stress polygon; Stress polygon; Stress regimes.
Applied Petroleum Geomechanics
ISBN 978-0-12-814814-3 Copyright © 2019 Elsevier Inc.
https://doi.org/10.1016/B978-0-12-814814-3.00005-8 All rights reserved. 163