Permeability change during experimental fault smearing

Abstract

[1] Smear sourced from shale or clay is thought to provide an across-fault barrier to fluid flow in sedimentary rocks. Permeability changes of this phenomenon were investigated using a triaxial testing machine. Experimental specimens consisting of interlayered siltstone (low initial permeability, $10 À16 m 2 ) and sandstone (high initial permeability, $10 À13 m 2 ) were subjected to 20, 30, and 40 MPa of effective normal stresses on a precut surface, dividing each specimen at a 30°angle to its axis, at axial shortening velocities between 0.14 and 1.41 mm s À1. Permeability was measured by the oscillation method, and permeability changes categorized in three distinct regimes that corresponded to progressively increasing rock deformation: regime 1, rapid reduction due to compaction of the siltstone layer prior to fault movement; regime 2, constant and minimum permeability while the smear developed; and regime 3, permeability recovery caused by smear thinning and loss of smear continuity. The duration of regime 2 and of smear continuity recorded provide measures of the sealing potential produced by the smear. The shale smear factor SSF, defined as (fault throw)/(thickness of low-permeability layer), shows that there may be a relation between seal potential and effective normal stress. Both factors in SSF can reach higher values when effective normal stress is 40 MPa than when that is 30 MPa.

Using probabilistic shale smear modelling to relate SGR predictions of column height to fault-zone heterogeneity

“…Observations of smears at many scales show that as displacement increases (i.e. SSF increases) then ultimately the smear on the fault plane becomes disrupted or discontinuous (Lindsay et al 1993;Lehner & Pilaar 1997;Takahashi 2003;Faerseth 2006). Hence we can define a critical SSF value, SSF c , below which the smear is continuous and above which the smear is discontinuous (Fig.…”

Geologic carbon storage is unlikely to trigger large earthquakes and reactivate faults through which CO2could leak

“…We contend that close analysis of fault zone architecture reveals that CO 2 will not easily penetrate into the portions of the fault contained within shale rocks (31). Fault permeability, which is highly variable in reservoir-caprock sequences (32,33), decreases several orders of magnitude for increasing clay content, leading to a much lower permeability in the caprocks than in the reservoirs (34,35). Rocks with low clay content, like reservoirs, tend to fracture, increasing the width of the damaged zone and usually increasing permeability in response to shear (34).…”

Changes in Electrokinetic Coupling Coefficients of Granite under Triaxial Deformation

“…To measure the permeability continuously during the deformation experiments [35], we adopted the sinusoidal oscillation method [36][37][38][39]. The method is based on the measurement of an attenuation and a phase retardation of an oscillation of the pore-fluid pressure as it propagates through the specimen.…”

“…The upper specimen face is connected to the downstream reservoir of a volume of 9.52 cm 3 . To calculate the permeability from the measured attenuation factor and phase lag, we used a custom-made code following Takahashi [35].…”