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Shales in Shear Zones and Their Effect on Cross-Fault Fluid Flow

Abstract

The geomechanical and hydraulic properties of shales are interrelated and are important for the characterization of unconventional reservoirs and evaluating lateral trapping or sealing capacity of fault-related plays in conventional hydrocarbon reservoirs. In this presentation, we examine the architecture and the mechanism of deformed shales in shear zones and their impact on fluid flow. The first category is of dilatant brittle fractures (both joints and shear fractures/faults) which result in increasing conductivity across and along shear zones. This mechanism and the resulting structures are similar to those in other brittle lithologies. The other category is about compactive plastic deformation generally associated with high degree of conductivity reduction. Two prototypes of these shear zones are: (1) Deformation bands responsible for a localized zone of deformed rock within otherwise undeformed or slightly deformed shales. It is inferred that the reduction in conductivity for cross-band flow due to this type of structure may be about three orders of magnitude relative to the undeformed shale. (2) Shale entrainment or smearing within fault zones surrounded by brittle lithologies such as sandstone or limestone of significantly higher conductivity. Two case studies are considered: One from a small-scale example of shale smear along a normal fault with about 8 meters throw in a mine shaft and another from a regional fault zone with about 180 meters apparent throw and 250 meters apparent lateral separation in a well-studied site with environmental concerns. They both have evidence for trapping fluids. The former, based on the laboratory measurements, suggests a capillary displacement pressure increase of about 30% across the fault zone. The latter provides a bulk hydraulic conductivity value inverted from an average hydraulic head difference between the two sides of a large fault in the range of 1×10-7 cm/sec to 1×10-8 cm/sec, which is 3 to 5 orders of magnitude lower than that of the surrounding rocks. These changes in the petrophysical properties of the deformed shales are attributed to the alignment of platy clay minerals, diagenesis, reduced porosity and shear induced compaction.