Oil and Gas at Your Door? (2005 Edition) - Earthworks

NON CONVENTIONAL OIL AND GAS
FIGURE I-24. TYPICAL COALBED METHANE WELL. Source: Ecos Consulting.
In the initial production stage of coalbed methane, the wells produce mostly water. Eventually,
as the coal beds near the pumping well are dewatered, the volume of pumped water decreases
and the production of gas increases. 161 Depending on the geological conditions, it may take
several years to achieve full-scale gas production. Generally, the deeper the coal bed the less
water present, and the sooner the well will begin to produce gas.
Water removed from coal beds is known as produced water. The amount of water produced
from most CBM wells is relatively high compared to conventional gas wells because coal beds
contain many fractures and pores that can contain and move large volumes of water. 162
CBM wells are drilled with techniques similar to those used for conventional wells. In some
regions where the coal beds are shallow, smaller, less expensive rigs, such as modified waterwell
drilling rigs, can be used to drill CBM wells, rather than the more expensive, specialized oil
and gas drilling rigs. 163
As with conventional gas wells, hydraulic fracturing is used as a primary means of stimulating
gas flow in CBM wells. 164 Another gas stimulation technique, unique to CBM wells, is known as
cavitation (also known as open-hole cavity completion).
Cavitation is a similar phenomenon to opening a shaken pop bottle, only on a much larger
scale. 165 Water, and air or foam are pumped into the well to increase the pressure in the reservoir.
Shortly thereafter, the pressure is suddenly released, and the well violently blows out, spewing
gas, water, coal and rock fragments out of the well. This action is sometimes referred to as
“surging,” and it is accompanied by a jet engine-like noise, which can last up to 15 minutes. 166 I-33