SELF-PROPPING AND FLUID-FLOW IN SLIGHTLY OFFSET JOINTS AT HIGH EFFECTIVE PRESSURES

We have made laboratory measurements of joint closure and permeability of both well- and poorly mated, laboratory-produced joints in Westerly granite under joint normal stresses to 160 MPa, appropriate to midcrustal depths. We have also made detailed topographic measurements of the joint surfaces, both before and after pressure testing. The purpose of the study was to characterize and understand the hydraulic behavior of joints under conditions where fluid migration under natural settings is known to occur but where physical conditions, namely high lithostatic load, suggest that the presence of open, fluid-conducting joints might be improbable. Normal stress was applied by hydrostatic confinement of cylindrical samples, each containing a single axial tensile fracture. Experiments were performed at room temperature under conditions designed to minimize chemical effects of dissolution or precipitation. We found that while a mated joint can be completely closed by the application of high normal stresses, the same joint laterally offset by 0.53 mm remains open and many orders of magnitude more hydraulically conductive than the mated joint even at highest stresses. Detailed topographic observations are consistent with self-propping of the offset joint. Permanent damage to the surfaces of the offset joint is not widespread but occurs at isolated locales covering about 10% of the surface. Damage is confined to regions of negative (convex upward) curvature, such as summits and the edges of plateaus. Approximately 20% of the joint surface is in wall-to-wall contact at 160 MPa, and the flow tortuosity thus induced is probably the cause of the breakdown of the parallel plate approximation for fluid flow in the offset joint.