Fracture Growth Kinematics in Tight Sandstone Reservoirs
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Opening-mode fractures—joints and veins—are widespread structures in sedimentary rocks even in slightly deformed and flat-lying sequences. Understanding the growth and connectivity of fractures in low permeability sandstone reservoirs is essential for optimal hydrocarbon exploitation. In a linear elastic fracture mechanics framework, it is generally assumed that fractures widen in aperture while they propagate in length or height. However, it is also conceivable that a phase of proportional aperture to length or height growth is followed by a phase of aperture growth with relatively slow or arrested tip propagation. Slow propagation relative to aperture opening can occur by non-elastic deformation processes or if the material elastic properties change over time. Fracture propagation in length or height can be halted by material strength heterogeneities. To test for concurrent length versus aperture growth of these fractures, I reconstructed the crack-seal opening history for multiple cement bridges sampled at different distances from the tip of three opening-mode fractures in Travis Peak Sandstone of the SFOT-1 well, East Texas. Crack-seal cement bridges have been interpreted to form by repeated incremental fracture opening and subsequent precipitation of quartz cement that bridges the fractures. Crack-seal cement textures were imaged using a scanning electron microscope with a cathodoluminescence detector, and the number and thickness of crack-seal cement increments determined. Trends in crack-seal increments number and thickness are consistent with fast initial fracture propagation relative to aperture growth, followed by a stage of slow propagation and pronounced aperture growth. Cumulative fracture opening displacement based on palinspastic reconstruction of two cement bridges was compared to analytical solutions for a stationary and a propagating fracture aperture as a function of position relative to the fracture tip in an elastic medium. Based on this comparison, I conclude that the crack-seal cement record reflects largely the phase of dominant aperture growth and subcritical fracture propagation under constant loading stress.