Methane gas from wells completed exclusively in coal seams has become a major energy resource in the USA, and it is being evaluated in many other countries. In all but a very few cases, stimulation by hydraulic fracturing is required for adequate production rates. The application of fracturing to improve degasification of coal beds prior to mining began in 1974, but in recent years its application has expanded such that many completions are independent of any expected future mining operations. Wells are often completed in multiple coal seams with possibly hundreds of feet between the completion zones.
The hydraulic fracturing fluids, equipment, and designs used for coalbed methane wells have seen major evolutionary changes from the early treatments when completing in seams to be mined. When fracturing became common in coal seams where mining was not being considered, roof integrity was no longer a concern and the treatment designs began to undergo more accelerated changes.
This paper will trace the historical application of hydraulic fracturing in the two major commercial coalbed methane producing areas: The Black Warrior Basin of Northern Alabama and the San Juan Basin of Northwest New Mexico/Southwest Colorado. Recent applications in the Raton and Piceance Basins of Colorado and the Central Appalachian Basin will also be addressed.
Conventional fracturing technology cannot always be directly applied to fracturing coal seams. Coal is a reservoir rock that has many unusual characteristics, such as:
Wide variety of treating pressures, often abnormally high, with pressure gradients commonly above 1.0 psi/ft (even though most pressure gradients commonly above 1.0 psi/ft (even though most believe vertical fractures are still the predominant occurrence).
The very high leakoff of the fracturing fluid into the coal cleat system and coal's mechanical response are not well modeled by the mathematics used in conventional design simulators for sandstones and carbonate reservoirs.
The mechanism of methane production is quite different from traditional gas reservoirs.
Postcompletion problems of coal and proppant production are added difficulties.
Although not the only anomalies associated with completing coalbed methane (CBM) wells, the points listed above have been the dominant factors which have influenced the way fracturing has been applied as part of the completion procedure. When early fracturing treatments were performed in seams to be mined, safety concerns over cavings from a weakened mine tool would often have the effect of limiting pump rate, fluid volume, or fluid viscosity. These concerns could often lead to very limited effectiveness of the stimulation treatment.
Mining operations allowed the industry some firsthand observations of the resultant fractures. Beginning in 1974 and continuing through present operations, many investigators have had the opportunity to study some areas where the fracture(s) occurred within the coal seams. Diamond and Oyler presented an excellent report on 22 government-sponsored mineback investigations following fracturing treatments. However, we must use discretion in our application of these observed results. The response seen in seams only a few hundred feet deep may not always be an accurate indication of what will occur within more deeply buried coal seams. At shallow depths, there is a higher probability that the two horizontal stress components may be of similar magnitude. This will lessen the probability of achieving a single biwing planar vertical fracture. probability of achieving a single biwing planar vertical fracture. Current coal completions typically involve depths of 1000 to 4000 ft. More shallow coals are occasionally included, and many operators are evaluating the economic potential of deeper seams.
During the mid 1980's the industry began to seriously consider coal seams as a commercial gas reservoir. The classification of coalbed methane as an "unconventional gas" offered operators a significant tax credit, which greatly improved the economics of coal wells. Crouse gives a good discussion of the effects of this tax incentive, including economic comparison for Fruitland coal seam gas production with and without the tax credit. production with and without the tax credit. Drilling activity in 1990 reached a level few had imagined possible only a few years earlier. Possibly the greatest leap in proven possible only a few years earlier. Possibly the greatest leap in proven reserves of coalbed methane occurred during 1988.