Page 587 - Petrophysics

CHAPTER 9

EFFECT OF STRESS

N RESERVOIR

s OCK PROPERTIES

Fairhurst defines rock mechanics as "the fields of study devoted
to understanding the basic processes of rock deformation and their
technological significance" [l]. The significance of these processes to
petroleum engineers is considerable. For instance, being able to predict
the mechanical behavior of underground formations is key to avoiding
borehole instabilities during drilling. If rock deformation results in a
noticeable contraction of the wellbore due to the state of induced stress
in the rock formation immediately adjacent to the wellbore, the motion
of the drill bit may be restricted or the emplacement of the casing, after
drilling ceases, may be hampered. If the deformation results in a large
expansion of the wellbore, the rock formation may fracture and result
in lost circulation [2]. Predicting the mechanical behavior of reservoir
rock is essential for well completion or stimulation programs. Reservoir
compaction, which may lead to surface subsidence, is a critical factor
with respect to design of the casing platforms and to the overall reservoir
performance [3]. Figure 9.1 illustrates casing failure resulting from
compaction of reservoir rock. The production of oil, natural gas, and/or
water from underground rock formations results in a local change in the
stress and strain field in the formation due to the decline in pore pressure
[4]. In order to predict the compaction or compressibility behavior of
petroleum reservoirs due to this decline in pore pressure, it is necessary
to know the compressibility characteristics of the reservoir rock.
Rock mechanical properties, such as Poisson's ratio, shear modulus,
Young's modulus, bulk modulus, and compressibility can be obtained

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