Page 46 - Introduction to Petroleum Engineering

30 THE FUTURE OF ENERGY
Basin of Wyoming (Mavor et al., 1999) to 600 SCF per ton in the Appalachian
Basin (Gaddy, 1999).
Coal gas was historically known as coalbed methane. The term coal gas is used to
better convey that gas from coalbeds is usually a mixture. Other terms for gas from
coal include coal seam methane, coal mine methane, and abandoned mine methane.
The practice of degasifying, or removing gas, from a coal seam was originally used
to improve coal miner safety. Today, people recognize that coal gas has commercial
value as a fuel.
Coal gas bound in the micropore structure of the coalbed can diffuse into the
natural fracture network when a pressure gradient exists between the matrix and the
fracture network. Fractures in coalbeds are called “cleats.” Flow in the fractures is
typically Darcy flow which implies that flow rate between two points A and B is pro-
portional to the change in pressure between the points.
The ability to flow between two points in a porous medium is characterized by a
property called permeability, and a unit of permeability is the darcy. It is named after
Henry Darcy, a nineteenth‐century French engineer. Permeability typically ranges
2
−12
2
from 1 millidarcy = 1 md (or 1.0 × 10 m ) to 1 darcy = 1 D = 1000 md (or 1.0 × 10 m )
−15
for conventional oil and gas fields. Permeability in the cleat system typically ranges
from 0.1 to 50 md.
Recovery of coal gas depends on three processes (Kuuskraa and Brandenburg,
1989). Gas recovery begins with desorption of gas from the internal surface to the
coal matrix and micropores. The gas then diffuses through the coal matrix and micro-
pores into the cleats. Finally, gas flows through the cleats to the production well. Gas
flow rate through the cleats depends on such factors as the pressure gradient in the
cleats, the density of cleats, and the distribution of cleats. The flow rate in cleats
obeys Darcy’s Law in many systems but may also depend on stress‐dependent per-
meability or gas slippage (the Klinkenberg effect).
The production performance of a well producing gas from a coalbed will
typically exhibit three stages. The well produces water from the cleat system in the
first production stage. The withdrawal of water reduces pressure in the cleat system
relative to the coal matrix and establishes a pressure gradient that allows coal gas
to flow into the cleat system. The gas production rate increases during the first
stage of cleat system dewatering and pressure depletion. The amount of water
produced during the second stage of production is relatively small compared to
gas production because there is more gas present in the cleat system relative to
mobile water. Consequently, the gas production rate peaks during the second stage
of production and gradually declines during the third stage of production as coalbed
pressure declines.
The injection of carbon dioxide into a coal seam can increase coal gas recovery
because carbon dioxide preferentially displaces methane in the coal matrix. The
displaced methane flows into the cleat system where it can be extracted by a pro-
duction well. The adsorption of carbon dioxide in the coal matrix can be used to
sequester, or store, carbon dioxide in the coal seam. Sequestration of carbon
dioxide in a coal seam is a way to reduce the amount of carbon dioxide emitted into
the atmosphere.