Page 254 - Fundamentals of Enhanced Oil and Gas Recovery

242 Alireza Keshavarz et al.

8.2.6 Coal Rock Mechanical Properties
The mechanical properties of rocks are concerned with the rock physical changes as a
result of subjection to force from its physical environment. The mechanics of reservoir
rocks is of vital importance in naturally fractured reservoirs, particularly CBM, in
which the fissures serve as the circuit to fluid flow and vary in response to changes in
effective stress due to reservoir depletion. These properties are also among the main
design criteria to assure a promising hydraulic fracturing in the reservoir [35]. Among
key mechanical properties, the elastic properties mainly include Young’s modulus and
Poisson’s ratio. These properties are obtainable either by triaxial laboratory examina-
tions of representative samples or through analyzing the field data.
Young’s modulus represents the stiffness level of the rock, or in other words, it
quantifies the rock resistance against the compressive stress. Young’s modulus, with
pressure unit, could be calculated through dividing the tensile stress by the extensional
strain which are given by

F n
σ x 5 (8.7)
A o
ΔL
ε x 5 (8.8)
L o
where σ x is the tensile stress in the x direction, F n is the normal force, A o is the area,
ε x is the strain in the x direction, dimensionless, ΔL is the change of length, L o is the
initial length.
Equating (8.7) and (8.8) for Young’s modulus, we will obtain
FL o
E 5 (8.9)
ΔLA o
Eq. (8.9) vividly describes that for high Young’s modulus values, being the property
of stiff formations, a large force on the rock area would result in a minor change in the
rock length. Therefore, for stiff rocks, the permeability reduction due to increase in
effective stress during reservoir depletion is lower compared to softer rocks. The effect
of this elastic property of coal rocks has been investigated in some works. Palmer and
Mansoori suggested that for soft rocks of low Young’s modulus, a considerable perme-
ability reduction within the reservoir depletion is expected, since stress effects play the
most important role compared to matrix shrinkage [36]. In fact, coals with smaller
Young’s modulus compress more in subjection to an increase in effective stress com-
pared to coals with larger values of this property [37]. Geertsma and De Klerk pro-
posed that the major effect of Young’s modulus is on the width of the fractures in the
coal rock. They claimed that the maximum width of a fracture near the wellbore is
inversely proportional to the fourth power of Young’s modulus; that is, the lower the
value of Young’s modulus, the higher the width of the fracture network [38].