Figure 16 shows a linear relationship between proppant volume and cumulative recovery of coalbed methane from the Cedar Cove Field of the Black Warrior Basin. Wells were treated with low viscosity fluids, so it is likely that proppant was not transported very far from the wellbore.
Coalbed methane is an abundant resource in the U.S.A., for example, in the San Juan Basin (50 TCF), and Black Warrior Basin (20 TCF). Amoco has drilled many wells in both basins in the last few years. Different completions and stimulations have been tried, and these are summarized as follows:
1. Openhole cavity - This has worked best in areas of the San Juan Basin where reservoir pressure and permeability are high. The wells can be prolific producers (up to 12 MMCFD). The physical mechanisms involved in the completion are discussed, and these are used to try to understand the difference in gas production between cavity completions and gel fracture stimulations.
2. Gel fracture treatments - These stimulations are conducted through perforations in coal seams, and sometimes two or more stages are used per well. High fracture conductivities are achieved by using 12/20 mesh sand to concentrations of 10 ppg, and the viscous gel should mean most of the coal seams are propped. Graduated proppant treatments (40/70 sand preceding the main 12/20 sand) are often used to alleviate screen-outs. Although some gel damage to the coal formation is evident, moderate productivity increases have been achieved.
3. Water fracture treatments Because of gel damage to the formation, fracturing treatments have been conducted using water as fracture fluid, plus 12/20 sand to concentrations of a few ppg. Although not all coal seams are expected to be propped (due to low viscosity of water), gas production is greater in general than offset wells with gel fracture treatments, and the water fractures are cheaper.
4. Sandless water fracture treatments - Recently water fracture treatments have been performed without sand, using ball sealers to open up more seams. Although their gas production may not be as good as wells fractured with water and sand, they can be substantially cheaper.
Fracture designs are summarized, and gas production is compared for the different completion/stimulation techniques.
Fracture treatments in general appear to fall into two different classes: First, those in which significant fracture height growth occurs, accompanied by falling pressures (vertical fractures). The second class are pressures (vertical fractures). The second class are those which are largely confined by the coal seams, and are accompanied by steady or rising pressures, proppant-induced pressure increases, and ISIP values greater proppant-induced pressure increases, and ISIP values greater than 1 psi/ft (T-fractures). Interpretations of fracture geometry, height growth or confinement, fracturing pressure, and proppant-induced behavior, are pressure, and proppant-induced behavior, are summarized.
Author(s): Amoco Production Co., I.D. Palmer
Paper Number: SPE 22395