Page 246 - Fundamentals of Enhanced Oil and Gas Recovery

234 Alireza Keshavarz et al.

The original gas content of the cleats might be zero in some coals, while in other coals
there might be some initial gas available in the cleat system [2]. The uniqueness of these
reservoirs roots in the fact that opposite to conventional reservoirs in which the gas is
trapped in the reservoir’s void space, the gas is sorbed on the coal rock’s surface, and the
production requires desorption of the gas. These reservoirs are referred to as CBM,
because most of the reservoir gas content is accounted for by methane. The adsorption
sites consist of micropore and mesopore spaces inside the matrix, while cleats behave as
the fluid flow conduits. Desorption process is viable through depressurization of the res-
ervoir, and upon desorption from the rock surface, the gas diffuses within the matrix
porous media toward the cleats by gas concentration and partial pressure gradient.
The primary production is accomplished by declining the pressure of the reservoir
through depletion, thereby facilitating the gas desorption process. Since the gas release
from coal matrix into the cleat system in such reservoirs is controlled by the gas partial
pressure gradient rather than reservoir pressure gradient, and given that methane partial
pressure could not reach zero in cleats due to economic limitations, it is expected to
obtain a recovery factor less than 50% in the natural depletion condition [3].Therefore,
in order to get a desirable gas production rate from the reservoir, we need to keep two
pressure gradients at maximum: methane partial pressure gradient between matrix and
cleats and reservoir pressure gradient between cleats and the production wellbore. As
such conditions are not achievable in the natural depletion, the injection of a foreign
gas to the CBM reservoir, namely enhanced coalbed methane (ECBM) recoveryprocess,
was suggested in the early 1990s [4]. Another approach to improve gas production from
CBM reservoirs is to improve reservoir conductivity. Hydraulic stimulation techniques,
such as natural fracture stimulation and hydraulic fracturing, are the most common pro-
ductivity enhancement techniques in CBM reservoirs.
In this chapter, CBM and its associated recovery and enhanced recovery processes are
discussed in detail. First, the properties of CBM reservoirs including coal rank, macerals,
porosity and permeability, coal density, and coal rock mechanical properties are explored.
Subsequently, a typical production profile of CBM reservoirs is illustrated, and the flow
mechanism in such reservoirs is scrutinized. In this section, the most unique feature of
CBM reservoirs is introduced. Finally, the production enhancement approaches in CBM
reservoirs, split into stimulation techniques and ECBM, are investigated.

8.2 COALBED METHANE RESERVOIR PROPERTIES

Due to the unique behavior of CBM reservoirs in terms of gas storage as well as
production mechanism compared to conventional reservoirs, the key properties of