Evolution of permeability in a single granite fracture under various temperature, confining pressure, and pH conditions

Abstract:

A series of long-term permeability experiments utilizing granite samples, with a single artificial fracture, was conducted to examine the evolution of rock permeability under relatively high temperature (up to 90 degree-C) and confining pressure (up to 10 MPa) conditions. Permeant with various pH ranging 6-13 was also used to examine the effects on the permeability evolution. Effluent element concentrations were measured throughout the experiments. Before and after flow-through experiments, rock samples are prepared for X-ray diffraction, X-ray fluorescence, and scanning electron microscopy coupled with energy dispersive X-ray spectroscopy to examine the mineralogical changes between pre- and post-experimental samples. Although there are exceptions, the observed, qualitative evolution of permeability is found to be generally consistent in the samples – the permeability decreases with time. This reduction may result from the removal of the mineral mass from the bridging asperities within the fracture. Post-experimental observations by scanning electron microscopy, coupled with energy dispersive X-ray spectroscopy (SEM-EDX), revealed the formation of several kinds of secondary minerals such as silica and calcite. However, the precipitated minerals seemed to have had little influence on the flow characteristics within the fracture, because the precipitation was limited to quite local and small areas. The evolving rates and ultimate magnitudes of the fracture aperture are likely to be controlled by the stress exerted over the contacting asperities and temperatures, and by the prescribed flow conditions. Thus, this complex behavior should be attributed to the coupled chemically- and mechanically-induced effect.