The Unique Aspects of Fracturing Western U.S. Coalbeds

Abstract

Summary Bottomhole treating pressure (BHTP) behavior during the fracture treatmentof western U.S. coalbeds is unique and often unpredictable because of thedistinctive material properties of coal. This paper assigns the most-probablecauses to specific BHTP behavior and shows ways to adapt to the downholeproblems associated with these BHTP anomalies. Introduction Large coal deposits exist in several western U.S. sedimentary basins. Motivated by a nonconventional fuel tax credit, many operators have completedwells in western U.S. coalbeds to evaluate the viability of methaneproduction. Fracture treatments are used routinely to complete western U.S. coalbedmethane wells. During these treatments, BHTP often acts anomalously, affectingthe treatment performance in various ways. 2 The objectives of this paper areto describe the BHTP behavior that is unique to coals, to assign amost-probable cause to specific BHTP behavior, and to recommend ways to adaptto the downhole problems associated with BHTP anomalies. To achieve theseobjectives, the characteristics of coalbed methane reservoirs and the factorsaffecting fracture treatments in coals are discussed. Coalbed treatments in the Green River southwestern Wyoming) and San Juan southwestern Colorado andnorthwestern New Mexico) basins are used as examples of anomalous BHTP behaviorand to describe innovations in coalbed treatments. Rock and Reservoir Properties. The physical properties of coal aredistinctive. Coal has a spongelike pore network. Coal gas is adsorbed on thesurfaces of micropores <20A [<2 nm] in diameter. Cross-settingshrinkage fractures, called cleats, are the large pore channels through whichfluid flows in the reservoir; these pore channels through which fluid flows inthe reservoir; these fractures formed when volatile materials (e.g., water, methane, and CO2) were expelled from the coal. Also, fractures caused bytectonic or differential-compaction stresses can exist in a mature coal. Natural fractures affect the fracturing-fluid leakoff rate and can serve asinitiation sites for multiple hydraulic fractures during stimulationtreatments. Most coal reservoirs must be dewatered to reduce the reservoir porepressure, so that gas can desorb and then diffuse into the natural fracturesystem. Coals have a high irreducible water saturation; consequently, theirrelative permeability to gas is low (i.e., 10% of absolute permeability). Thisinherent permeability restriction impairs gas flow to the wellbore.permeability restriction impairs gas flow to the wellbore. The compressivestrength of coal is low; near-wellbore coal degradation and coalfines movementduring drilling, completion, and production operations are common problems. Coal is a soft rock production operations are common problems. Coal is a softrock with a low elasticity modulus. This property facilitates the creation ofrelatively wide hydraulic fractures and the containment of fractures in thecoal interval when the interfacial shell strength across the coal/bounding-rockinterface is low. The softness of coal also facilitates proppant embedment. Other mechanical features of coal, such as high fracture toughness andporoelasticity in highly fractured, water-saturated coals, may poroelasticityin highly fractured, water-saturated coals, may elevate BHTP to a very highlevel during fracture treatment. Why Are Fracture Treatments Needed? Jeu et al. noted four main reasons whyfracture treatments are needed in cased-hole coalbed-methane completions.They bypass wellbore damage. Natural fractures can be invaded and pluggedby drilling solids and cement. Fracture plugging was observed in mine-throughoperations in a coal seam in the Black Warrior basin; cement was found incleats more than 100 ft [30 m] from the entry source, a vertical wellbore.They accelerate dewatering and pressure drawdown. Pressure drawdown must extenddeep into the seam to accelerate gas release from the micropores. Proppedfractures increase the effective wellbore radius when the fracture conductivitycontrast is high.They distribute the pressure drawdown to reduce finesproduction. This can be accomplished by reducing the pressure drop in thenear-wellbore area, where the coal is often degraded.They connect thewellbore to the entire reservoir effectively. Abrupt vertical discontinuitiesoften are present in coalbeds; perforations may not connect with all thefracture networks.