Sample dilation and fracture in response to high pore fluid pressure and strain rate in quartz‐rich sandstone and siltstone

Abstract

Natural hydraulic fractures (NHFs) are inferred to form where pore pressure exceeds the least compressive stress by an amount equal to the tensile strength of the rock. We improved upon an experimental protocol that meets the NHF criterion within cylindrical samples with the most tensile effective stress parallel to the sample axis. The effective tensile stresses achieved during these experiments ranged from 17–47 MPa. The pore fluids used had higher viscosities than water and the axial strain rate was rapid (∼10−3s−1) to delay dissipation of fluid pressure by flow. Four experiments on St. Peter Sandstone samples and two on an Abo Formation siltstone sample were performed under these conditions and under drained conditions. None of the drained experiments resulted in failure, but all of the sandstone and one of the siltstone samples fractured in response to elevated pore pressures. Consistent with field and theoretical studies, mechanical heterogeneity was a first order control on fracture location. In the absence of mesoscopic heterogeneity, fracture location coincided with the maximum pore pressure. Samples responded to elevated pore pressures and differential stresses by dilating, the magnitude of which was sufficient to achieve atmospheric pore pressure. Samples failed 2 to 250 s after experiencing the greatest pore pressures, when the effective stresses were no longer tensile. Thus, the high pore pressures and effective tensile stresses experienced early in the experiments were sufficient to fracture the rocks, even though they were not sustained until failure. These results provide insight into processes of fluid‐driven fracture formation.

Particle-size distributions of low-angle normal fault breccias: Implications for slip mechanisms on weak faults

“…High Pf is a popular hypothesis to explain slip on weak faults (e.g., Hubbert and Rubey, 1959;Rice, 1992;Axen, 1992;Axen and Selverstone, 1994), but the level of Pf relative to s1 (s1 z sv in Andersonian normal faulting stress fields)necessary to cause dilation and/or slip has not been well-quantified by laboratory experiments (French et al, 2012). New experimental data on natural hydraulic fracturing suggest that high Pf may not be sustained for long after dilation, and that weakening by microfracturing may allow slip, as opposed to having sustained fluid pressures (French et al, 2012). In the active low-angle Altotiberina normal fault, Italy, CO 2 -bearing fluid overpressures relative to the lithostatic load (lv: lv ¼ Pf (pore-fluid pressure)/sv (vertical stress)) are 0.85 based on borehole data from 4.75 to 3.7 km depth (Chiodini and Cioni, 1989).…”

Quantifying vein attributes in massive mudstones (Triassic, SW England): Implications for progressive evolution of opening-mode fracture networks

“…The difficulty in understanding opening-mode fractures is that they could arrest or even heal, when the fluid pressure in the fractures drops during fracture propagation (Renshaw and Harvey, 1994;Cosgrove, 2001;French et al, 2012). However, the characteristics of opening-mode fractures can be well-preserved if the fractures have been cemented by mineral aggregates precipitated from focused fluid flow in fracture channels.…”