Modeling Shear Failure and Stimulation of the Barnett Shale After Hydraulic Fracturing

Abstract

Abstract In the Barnett Shale, microseismic bursts are caused by shear failure on planes of weakness well outside the central fracture plane. Some data show that greater gas flowrate is correlated with a larger "failed reservoir volume", and a higher net fracturing pressure. These aspects have been integrated into a theoretical model. Shear slip or failure along planes of weakness is instigated by pore pressure increases during injection of frac fluid, and pressure transient theory is used to describe that. A knowledge of in situ stress, and strength (or failure envelope) for the planes of weakness, is needed to predict how far away from the central fracture plane this failure zone extends. The failed reservoir volume can be matched to the volume of the microseismic cloud, using injection permeability is the matching parameter. The predictions appear to be reasonable. The injected permeability we obtain is greater than the virgin permeability, and this is interpreted as enhanced permeability due to shear or tensile failure away from the central fracture plane. The permeability enhancement increases as fracturing pressure increases. We have compared the permeability enhancement with history matching of gas production in Barnett Shale wells, and with theoretical matching of Cooper basin data in Australia. The model is a screening model, and may be viable to design a fracture treatment to optimize the enhanced permeability, and the gas production. An advantage of the screening model is its ability to quickly run a number of different scenarios, to choose an optimal stimulation. The model is completely general, and should be applicable to other formations, such as coals and tight sands. Introduction The Barnett Shale in the Fort Worth basin is the site of much recent activity as firstly horizontal wells, and secondly larger frac jobs, have boosted gas production. Large frac treatments (up to 8 million gals) result in a "fairway" zone of stimulation, as defined by detection of microseismic bursts. A fairway is generally interpreted as a region of complex fracturing where fracture branching, and multiple strands, have been created by shear and/or tensile failure. Microseismic bursts are caused by shear failure well outside the main fracture plane, and presumably occur along planes of weakness or natural fractures. Some data show that a larger "failed reservoir volume" identified by microseismic bursts implies a greater gas flowrate. Other data reveal that a higher net fracturing pressure is correlated with greater gas flowrate. These are pieces of a puzzle that we have integrated into a conceptual model, and developed into a theoreticl model. Shear failure of a formation is predicted outside a hydraulic fracture, induced by pore-pressure increase. Very little has been published on this subject1,2,3,4, but it may have significant impact on permeability around a hydraulic fracture, and therefore on production. A key question is how far away from a vertical fracture surface could a shear-failure zone extend? This has been addressed by Warpinski et al3. In an elegant study, they used geomechanics concepts to predict shear failure, based on structure (i.e., in situ stresses). The extent of the shear failure zone depends on several parameters, including fracturing pressure and whether the reservoir is water or gas-saturated. The failure zone is predicted to spread out further in a water-saturated reservoir, in general, and this seems to agree with most microseismic observations. This approach has also been followed, with some minor differences, in Palmer et al1. However, they applied the failure predictions to coals in a study applied to coalbed methane (CBM) wells. The size of the failure zone was shown to depend on fracture pressure, in situ stress, and coal rank, but no comparison with observation was available. It was pointed out that shear failure can be accompanied by permeability increase (due to dilatancy) or decrease (due to fines creation, movement, and plugging). Rahman et al4 also modeled failure (shear and tensile) induced by pore pressure increase during low-pressure injection (ie, below parting pressure) in a geothermal context. Nevertheless, they predicted very large permeability increases, up to a factor of 30 times the virgin permeability.