The stress history results from a published viscous layer folding solution are used as the basis for a fracture mechanics analysis of the factors that control hinge‐parallel extension fracturing in tangential‐longitudinal strain folds. The analysis incorporates published results for the change in sedimentary rock mode I fracture toughness at increasing confining stress to examine the relationship between regional strain rate, depth of burial, pore fluid pressure, initial crack size, layer viscosity, and the amount of fold shortening required for the propagation of a bed‐perpendicular, hinge‐parallel extension fracture. Tangential‐longitudinal strain folding of layers can occur at all scales in a foreland thrust system and is the result of the buckling and bending of stratigraphic units during the development of décollement, fault bend, and fault propagation folds. Hinge‐parallel extension fractures oriented perpendicular to bedding are a common fracture set observed in tangential‐longitudinal strain folds. The fractures propagate as a result of local tensile stresses that develop by the stretching of layers in the outer arc of fold hinges during bending. We considered a range of geologically reasonable boundary conditions to show that at one extreme, fracturing can occur as a result of only minor shortening by folding to the other extreme where a tight fold can form with no associated extension fracturing. For folds formed at shallow depths, where the confining stress on the system is less than the bending stresses in the layer and where the confining stress has not greatly increased the fracture toughness of the rock, hinge‐parallel extension fractures can grow under hydrostatic fluid pressure conditions. As depth increases, however, much higher pore fluid pressures are required to cause fracturing under similar strain rates. The observed controls are used to hypothesize how hinge‐parallel extension fracturing in fault bend folds can vary spatially and temporally across a thrust belt as a function of strain (thrusting) rate, the amount of bending at thrust ramps, and the depth of folding.
Hydraulic fracturing of gas shale formations involves pumping a large volume of fracking fluid into a hydrocarbon reservoir to fracture the rock and thus increase its permeability.The majority of the fracking fluid introduced is never recovered and the fate of this lost fluid, often called "leak off," has become the source of much debate. Information on the capillary pressuresaturation relationship for each wetting phase is needed to simulate leak off using numerical reservoir models. The petroleum industry commonly employs airwater capillary pressuresaturation curves to predict these relationships for mixed wet reservoirs. Traditional methods of measuring this curve are unsuitable for gas shale's due to high capillary pressures associated with the small pores present. A possible alternative method is the water activity meter which is used widely in the soil sciences for such measurements. However, its application to lithified material has been limited. This study utilized a water activity meter to measure airwater capillary pressures (ranging from 1.3 -219.6 MPa) at several water saturation levels in both the wetting and drying directions. Water contents were measured gravimetrically. Seven types of gas producing shale with different porosities (2.5 -13.6%) and total organic carbon contents (0.4 -13.5%) were investigated. Nonlinear regression was used to fit the resulting capillary pressurewater saturation data pairs for each shale type to the Brooks and Corey equation. Data for six of the seven shale types investigated were successfully fitted (median R 2 = 0.93), indicating this may be a viable method for parameterizing capillary pressuresaturation relationships for inclusion in numerical reservoir models. As expected, the different shale types had statistically different Brooks and Corey parameters. However, there were no significant differences between the Brooks and Corey parameters for the wetting and drying measurements, suggesting that hysteresis may not need to be taken into account in leak off simulations.
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