Page 270 - Fundamentals of Enhanced Oil and Gas Recovery

258 Alireza Keshavarz et al.

The natural depletion (primary production) of CBM resources may result in less
than 50% methane recovery [3]. Besides, the methane depletion rate is usually
marginal, due to one or a combination of different factors, such as (1) low initial pres-
sure, (2) low cleats permeability and connectivity, (3) low gas diffusivity, and (4) high
water production. In the natural depletion, at the beginning, mainly water is pro-
duced (in undersaturated conditions). Once the cleat pressure falls below the equilib-
rium pressure corresponding to the adsorbed gas content, the liberation of gas from
the matrix starts. The equilibrium pressure depends on the coal matrix adsorption
capacity, gas type, and total adsorbed gas. The liberated gas diffuses from the matrix to
the cleats and then flows along with the remaining water, from the cleats to the well-
bore. The gas rate usually increases, by the reduction of water-in-place, and the two-
phase flow continues until the water saturation reaches the residual saturation or until
the system energy (pressure) becomes insufficient for pushing water to the wellbore.
Subsequent to the two-phase flow, there will be dominantly a single-phase flow of
gas. The gas production continues until a point at which the desorption rate becomes
insufficient for having an economically viable production, as a result of matrix gas
content reduction and/or cleats closure.
To enhance the recovery and improve the production rate at the same time, (1) the
reservoir pressure should be maintained at a reasonably high value (creating the pressure
gradient for effective convection flow in cleats and also avoid cleats closure), and
(2) methane partial pressure gradient, between cleats and matrix, should be maximized
(accelerating desorption and diffusion process). A continuous injection of a foreign gas
into the coalbed allows us to minimize methane partial pressure in cleats and simulta-
neously increase total cleat pressure. The injected gas sweeps methane from the cleats,
resulting in methane partial pressure drop, while the injected mass into the system main-
tains the reservoir pressure, keeping the cleats open and boosting flow in cleats. The
performance of ECBM is controlled by several factors, including the competitive
sorption characteristics and the geomechanical and petrophysical properties of the coal.
One of the early studies on ECBM was conducted by Fulton et al.; they carried
out laboratory analyses on five core samples from Pricetown mine in West Virginia
[97]. By introducing the CO 2 to the coal samples, through a cyclic or single-stage
injection, the recovery factor increased, compared to a natural depletion mechanism.
¸
Sinayuc and Gu ¨mrah performed a simulation study based on the data of Zonguldak
coal basin [98]. In the study, an increment of 23% CH 4 production was observed by
injecting CO 2 . The first field application of CO 2 -ECBM injection was piloted in
Allison unit of San Juan basin, operated by Burlington Resources. CO 2 was injected
for 6 years (1995 2001). The results revealed an increment of methane recovery, one
additional volume of methane per three volume CO 2 injected [99].
An additional benefit of the CO 2 -ECBM is the carbon sequestration, since the
injected CO 2 is adsorbed on coal matrix [100]. Normally, coal affinity toward CO 2 is