Large reserves of coal exist in many of the western U. S. sedimentary basins. Motivated by a non-conventional fuel tax Credit1, many operators have been completing wells in western U. S. coals to evaluate the economic potential of methane production.
Fracture stimulation treatments are routinely used in the completion of western U. S. coalbed methane wells. However, the bottomhole treating pressure (BHTP) has exhibited anomalous behavior during many of these treatments2; this has affected treatment performance in various ways. The main objectives of this paper are to identify treating behavior that is unique to coals, evaluate reasons for occurrence of the unique behavior, and offer recommendations for mitigation of and/or adaptation to the down-hole problems associated with BHTP anomalies.
This paper will review rock and reservoir properties of coals, as well as the need for and factors affecting fracture treatments in coals. Case histories of coalbed treatments in the Green River (Southwest Wyoming) and San Juan (Southwest Colorado and Northwest New Mexico) basins will be used to demonstrate anomalous BHTP behavior and describe equipment and design improvisations to facilitate treatments.
Coalbeds possess characteristic pore network and rock mechanical properties that influence or affect treatment design and outcome. Coal exhibits a spongy, multiple-porosity network. The bulk of methane gas in coals is adsorbed on the surfaces of microspores less than 20A (2.0E-09 m) in diameter3. Macrospores are in the form of cleats, or cross-setting shrinkage fractures, which develop during the process of coalification, when volatile constituents (e.g., water, methane, CO2) are expelled from the coal. Additional fracturing can occur in a mature coal because of tectonic or differential compaction phenomena4. Tectonic fractures may be oriented differently than the cleat fractures if the in-situ horizontal stress field has reoriented over geologic time. Multiple natural fracture sets could greatly affect fluid leak-off rate and hydraulic fracture propagation paths during stimulation treatments.
Most coals are initially water saturated and require a de-watering period early in the life of the reservoir to induce a pore-pressure drawdown. The drawdown initiates gas desorption and enables gas diffusion into the natural fracture system. Coals have a high irreducible water saturation; the relative permeability to gas in coals is typically low (e.g., ±10% of absolute permeability)3. This inherent permeability restriction impairs the capability of gas to flow to the wellbore.
The compressive strength of coal is low. Accordingly, near-wellbore coal degradation and coal fines movement during drilling, completion and production operations are common problems. Coal is a soft rock, possessing a very low modulus of elasticity. This latter property facilitates the creation of relatively wide hydraulic fractures and the containment of fractures to the coal interval when the interfacial shear strength across the coal/bounding rock interface is low. The softness of coal promotes proppant embedment, which can be severe in this type of rock. Other mechanical features of the coal, such as high, posssl.bly scale dependent fracture toughness5, and poro-elastic response in highly fractured, water-saturated coals6, can cause the BHTP to elevate to a very high level during a fracture treatment.