Constraints on upward migration of hydraulic fracturing fluid and brine.
Bottom Line: Recent increases in the use of hydraulic fracturing (HF) to aid extraction of oil and gas from black shales have raised concerns regarding potential environmental effects associated with predictions of upward migration of HF fluid and brine.Consequently, the recently proposed rapid upward migration of brine and HF fluid, predicted to occur as a result of increased HF activity, does not appear to be physically plausible.Unrealistically high estimates of upward flow are the result of invalid assumptions about HF and the hydrogeology of sedimentary basins.
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Mentions: Permeability is partly dependent on effective stress, which controls the amount of compaction and fracture apertures in a given rock layer. Both the void space and connectivity decrease as effective stress increases, thereby restricting flow and lowering permeability. (Kwon et al. 2001) provided a pressure-permeability relationship for the Wilcox Shale based on laboratory experiments, k = k0[1 − (Pe/P1)m]3, where k0 is on the order of 10−17 m2, P1 is 19.3 (±1.6) MPa, m is 0.159 (±0.007), and Pe is the effective stress (Pe = Pc − χPp, where Pc is the overburden stress, Pp is fluid pore pressure, and χ is a constant that is approximately one for shales; Kwon et al. 2001). This relationship is plotted in Figure 3A. Note that the relationship of (Kwon et al. 2001) is for horizontal permeability (for flow parallel to bedding), which is typically higher than vertical permeability (for flow perpendicular to bedding). (Kwon et al. 2001) indicate that permeability decreases by 4 orders of magnitude as effective stress increases to 12 MPa (e.g., conditions that may be encountered at depths >1000 m).
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