Page 266 - Fundamentals of Enhanced Oil and Gas Recovery

254 Alireza Keshavarz et al.

network in the coal seam if the injection pressure is below the fracturing pressure
(natural fracture stimulation). The injected fluid can initiate and propagate new frac-
tures if the injection pressure is above the fracturing pressure (hydraulic fracturing). In
order to keep the conductivity of the fractures, after reproduction of the injected
fluid, small propped agents (proppants) are added to the injection fluid. The proppants
are placed inside the stimulated or induced fractures to mitigate fracture closure due
to injected fluid withdrawal and hydrocarbon production.

8.5.1.1 Hydraulic Fracturing
Hydraulic fracturing is a common technique for productivity enhancement in con-
ventional oil and gas reservoirs by inducing high-conductivity fractures from reservoir
to wellbore. Hydraulic fracturing was applied successfully for the first time in the
Hugoton gas field in 1947 [68]. Since that time, it has been recognized as one of the
main stimulation techniques for productivity enhancement and has been conducted in
many conventional oil and gas fields. Oil and gas industry has become more interested
in hydraulic fracturing treatment recently because of its unique role in productivity
enhancement from unconventional reservoirs (e.g., shale, CBM, tight sands, etc.).
However, the design and performance of hydraulic fracturing treatment in unconven-
tional reservoirs differs from that in conventional ones.
Hydraulic fracturing is the most common stimulation technique in CBM reser-
voirs and improves the gas productivity by connecting cleat network to wellbore [69].
However, fracture propagation is the main challenging issue as the injected fluid can
divert into the natural cleats in the intersection of hydraulic fracture and the existing
cleat system. In this case, short and noncontinuous cracks are created, instead of a sin-
gle massive fracture, within a complex system of natural cleats and joints [70 75].
The induced complex fracture geometry can significantly decrease the efficiency of
hydraulic fracturing treatment by accelerating fluid leak-off and causing ineffective
proppant transport [76,77]. In addition, uncontrolled fracture propagation may cause
some environmental issues such as contamination of nearby aquifers used for
agriculture.

8.5.1.2 Natural Fracture Stimulation
The aim of natural fracture stimulation is to improve conductivity of the preexisting
fracture network by injecting high-pressure fluids [78 80]. In this technique, injec-
tion pressure must not exceed the fracturing pressure. In natural fracture stimulation,
the following mechanisms lead to conductivity enhancement of the fracture network.
1. Improving conductivity of existing fracture network due to slip-dilation (shear dilation
stimulation).
In this stimulation technique, injection of high-pressure fluid causes shear stress
perturbation in the fracture network. Fracture surfaces slip to each other due to