A Method for Evaluating the Effects of Confining Stresses and Rock Strength on Fluid Flow along the Surfaces of Mechanical Discontinuities in Low-permeability Rocks
Published:January 01, 2012
Milton B. Enderlin, Helge Alsleben, 2012. "A Method for Evaluating the Effects of Confining Stresses and Rock Strength on Fluid Flow along the Surfaces of Mechanical Discontinuities in Low-permeability Rocks", Shale Reservoirs—Giant Resources for the 21st Century, J. A. Breyer
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Changing confining stress can modify not only rock properties, such as porosity and permeability, but can also affect the ability of fluid to flow along planar mechanical discontinuities, such as faults, shear fractures, tensile cracks, or bedding planes. The degree to which the flow of fluids can be altered with varying confining stresses depends on the spatial orientation of the mechanical discontinuity and the strength of the rock. Similarly, if hydraulic fracture stimulation occurs in the vicinity of a mechanical discontinuity and the pressurized fracture fluids enter the discontinuity, then the high-pressure fluids can alter the effective stress on the mechanical discontinuity. These changes can cause the mechanical discontinuity to reactivate in shear, possibly resulting in an increase in the ability of the mechanical discontinuity surface to experience fluid flow, potentially diverting the stimulation fluids in a direction other than anticipated.
A key component in the characterization of fluid flow along mechanical discontinuities is an understanding of the surrounding subsurface stress field. To constrain the present-day horizontal stress magnitude, a stress-strength equilibrium approach can be taken using overburden rock density estimation and information on the present-day tectonic setting. Horizontal stress orientation and magnitudes can also be inferred from structural geology principles via the interpretation of mapped active features and wellbore information, such as drilling history and image logs. Once information about stress magnitudes and orientation is available, one can calculate the shear and normal stress magnitudes acting on planar mechanical discontinuities of all possible orientations. Furthermore, one can evaluate what magnitude of fluid pressure within each mechanical discontinuity would be required to encourage shear failure reactivation. An example from the Barnett Shale play is presented here as an application of the method, offering various solutions to the likely orientations of fractures that could interact with hydraulic fracture treatment.
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Shale Reservoirs—Giant Resources for the 21st Century
In the early 1970s, most exploration geologists in the United States considered subeconomic or marginally economic petroleum resources such as coalbed methane, shale gas, and tight-gas sands as unconventional resources (Law and Curtis, 2002). Tax incentives and federally funded research beginning in the late 1970s helped make these resources economically viable in the last two decades of the 20th century. Economics aside, two important geologic attributes characterize most unconventional petroleum resources (Law and Curtis, 2002). Conventional petroleum systems are buoyancy-driven accumulations found in structural or stratigraphic traps, whereas most unconventional systems exist independent of a water column and are generally not found in structural or stratigraphic traps.
Shale reservoirs are not new. The first commercial hydrocarbon production in the United States was from a well drilled in 1821 in a shale gas reservoir. By 2000, more than 28,000 wells had been drilled in shale gas reservoirs. Rising gas prices and technological advancements in horizontal drilling and hydraulic fracturing associated with the development of the Barnett Shale led to a boom in shale gas development in the early years of the 21st century. Now the exploitation of shale reservoirs is turning to natural gas liquids, condensate, and oil. Far from being isotropic and homogeneous, as once naively envisioned, shale reservoirs are complexly layered accumulations of fine-grained sediment. Geologic variation on scales ranging from that of stratal architecture to that of lamination within individual beds must be understood in order to locate and exploid areas of higher production within shale reservoirs. Shale reservoirs remain largely geologic plays - notmerely lease plays or strictly engineering plays made possible by improvements in drilling and completion technology.