MODELING DYNAMIC VARIATIONS IN PERMEABILITY WITH HYDROTHERM

The U.S. Geological Survey's HYDROTHERM model simulates three-dimensional, multiphase flow of pure water over the temperature range of 0-1,200^oC (Hayba and Ingebritsen, 1994, USGS WRIR 94-4025; http://water.usgs.gov/software/hydrotherm.html). It allows intrinsic permeability (k_x, k_y, k_z) to vary arbitrarily in space and also as a function of temperature (T). We recently added and tested a new capability that permits varying k according to user-designated functions of fluid pressure (P) and time (t), so that permeability can now be treated as k(T, P, t). For example, when P reaches a designated threshold, k_x, k_y, and k_z increase by designated amounts. The underlying assumption is that in any volume of fractured rock there is some appropriately oriented fracture that will slip if P increases sufficiently. The decay of permeability with time represents processes such as hydrothermal alteration and diagenesis and can be described by a variety of user-designated functions. Simulations of regional groundwater flow and heat transport with k depending on P and t show continuous variation in k due to the competition between fracturing and permeability decay. Simulated phenomena include permeability “waves” with periodic variations in permeability at any particular location. The amplitude and frequency of these periodic variations are highly sensitive to the fluid-pressure threshold, the magnitude of the permeability increase, and the subsequent rate of permeability loss (healing).