Page 341 - Numerical Analysis and Modelling in Geomechanics

Chapter 11
Reservoir compaction, subsidence and well

damage
L.Brun Hilbert, Jr.

Introduction
This chapter is concerned with the numerical simulation of compaction of
hydrocarbon-bearing rocks, more commonly referred to as reservoir rock. With
respect to operations in the petroleum industry, rock compaction may be defined
as the reduction in overall volume of a subsurface stratum of rock as a result of
production of fluids (oil, water and gas) stored within the pore space of the
reservoir rocks. During production, both the volume of fluid and the fluid
pressure decrease. The weight of the Earth’s crust above the reservoir provides a
near-constant state of vertical compressive stress on the reservoir, which results
in vertical compressive strain within the reservoir rock as pore pressure is
reduced during production. While rock compaction can be an important factor to
be considered in reservoir engineering and analysis in gravity drained reservoirs,
large amounts of compaction can lead to significant subsidence at the surface
above the reservoir and can lead to damage and failure of wells in and around the
reservoir. Reservoir compaction in the petroleum industry has been responsible
for large financial losses to the operators of oil or gas fields due to well repairs
and failures, and even costly damage to offshore platforms. Therefore, the
numerical simulation of compaction has played an important role in the
development of many fields and in the continued development of fields with
compaction-sensitive rocks.
First, a brief synopsis of the nomenclature associated with compaction is
presented. The most common mechanisms for casing damage due to compaction
are discussed, with the focus placed on damage due to localized shear
deformation as a result of shearing of thin weak clay layers, faults or fractures.
Then historical compaction problems in the petroleum industry are reviewed,
with a focus on efforts to analyze compaction, subsidence and casing damage.
The theory of deformation of fluid-filled porous media is presented. The finite
element approximation of the governing equations of such media is presented
incorporating both coupled and uncoupled forms. Finally, the results of a finite
element analysis of the compaction and casing damage which occurred at the
South Belridge field near Bakersfield, California, USA are presented. The South