20
Water for Energy and Fuel Production
coal bed methane will be very important for all parts of the world where significant
coal deposits are found.
In coal bed methane, the gas is trapped along with water in the porous coal bed.
The pressure of water keeps methane loosely attached to coal and therefore preserves methane in coal deposits. The water pressure also allows bacterial actions
between microorganisms in the water and coal to continue to generate new methane.
The water needs to be released to allow methane to escape the coal bed. When the
water is removed from the coal bed by pumping, the gas pressure is decreased and
this allows methane to detach from coal and flow up to the gas well. As shown in
Figure 2.1 [5,9], in the initial production stage of methane from the coal bed, the
well mostly produces water. Once the water is nearly removed, the production of
gas increases. Depending on the geological conditions, it may take several years
to achieve a full-scale gas production. Generally, the well with the deeper coal bed
produces gas with a short initial time lag due to the presence of less water. In general, the water produced from the coal bed is much higher than that obtained from
conventional wells [6,7,12].
Coal bed methane wells are drilled using techniques similar to those used for conventional wells (Figure 2.2) [42]. When coal beds are shallow, less expensive modified water well drilling rigs can be used. In general, however, hydraulic fracturing
(or fracking) is used as a primary means of stimulating gas flow in coal bed methane
wells. The gas can also be stimulated using a cavitation technique [5,6,11,12]. In
this technique, pressure in the reservoir is increased by the injection of water and
air (or foam) into the well. The pressure is then suddenly released and this causes a
violent blowout of gas, water, coal, and rock fragments from the well. This “surging action” can be repeated several times leaving larger holes and more fractures
within coal seams, which in turn causes a faster rate of gas release. The quality of
the produced water in both hydraulic fracturing and cavitation techniques mainly
depends on the geology of the coal formation. Typically, saltier water is produced
Dewatering
stage
Decline
stage
Water production rate
Time
M e th an e p ro d u ct io n ra te
Volume
Stable
production
stage
FiGUre 2.1 (See color insert.) Typical production curve for a coal bed methane well showing relative methane and water production. (Adapted from Rice, D., “Coal bed methane—
An untapped energy resource and environment concern,” US Geological Survey, Energy
Resource Surveys Program, USGS Fact Sheet FS-019-97, 1997.)
Water for Energy and Fuel Production
coal bed methane will be very important for all parts of the world where significant
coal deposits are found.
In coal bed methane, the gas is trapped along with water in the porous coal bed.
The pressure of water keeps methane loosely attached to coal and therefore preserves methane in coal deposits. The water pressure also allows bacterial actions
between microorganisms in the water and coal to continue to generate new methane.
The water needs to be released to allow methane to escape the coal bed. When the
water is removed from the coal bed by pumping, the gas pressure is decreased and
this allows methane to detach from coal and flow up to the gas well. As shown in
Figure 2.1 [5,9], in the initial production stage of methane from the coal bed, the
well mostly produces water. Once the water is nearly removed, the production of
gas increases. Depending on the geological conditions, it may take several years
to achieve a full-scale gas production. Generally, the well with the deeper coal bed
produces gas with a short initial time lag due to the presence of less water. In general, the water produced from the coal bed is much higher than that obtained from
conventional wells [6,7,12].
Coal bed methane wells are drilled using techniques similar to those used for conventional wells (Figure 2.2) [42]. When coal beds are shallow, less expensive modified water well drilling rigs can be used. In general, however, hydraulic fracturing
(or fracking) is used as a primary means of stimulating gas flow in coal bed methane
wells. The gas can also be stimulated using a cavitation technique [5,6,11,12]. In
this technique, pressure in the reservoir is increased by the injection of water and
air (or foam) into the well. The pressure is then suddenly released and this causes a
violent blowout of gas, water, coal, and rock fragments from the well. This “surging action” can be repeated several times leaving larger holes and more fractures
within coal seams, which in turn causes a faster rate of gas release. The quality of
the produced water in both hydraulic fracturing and cavitation techniques mainly
depends on the geology of the coal formation. Typically, saltier water is produced
Dewatering
stage
Decline
stage
Water production rate
Time
M e th an e p ro d u ct io n ra te
Volume
Stable
production
stage
FiGUre 2.1 (See color insert.) Typical production curve for a coal bed methane well showing relative methane and water production. (Adapted from Rice, D., “Coal bed methane—
An untapped energy resource and environment concern,” US Geological Survey, Energy
Resource Surveys Program, USGS Fact Sheet FS-019-97, 1997.)