HYDRAULIC TESTING OF A FAULTED SANDSTONE RESERVOIR - THE BIG HOLE FAULT IN THE NAVAJO SANDSTONE, NORTHERN SAN RAFAEL SWELL, UTAH

Six in situ hydraulic tests were conducted on the Big Hole fault, a normal fault cutting the Navajo Sandstone in the northern San Rafael Swell of east-central Utah. Two locations along the fault were tested, one with about 8 meters of displacement and the other near the tip where there is about 3 meters of displacement. Each location had one injection well and one or two (at the tip site) observation well(s). The footwall, hanging wall and fault core were isolated in each well using pneumatic packers. One test was conducted in each interval at each location by injecting water at a variable rate in order to maintain a constant head, and monitoring pressure increases in all three intervals of the observation wells.

The response curves were analyzed using type curves developed by Hsieh and Neuman (1985) and Theis (1935). The response curves were a poor match for the Hsieh and Neuman type curves, and failed to give a positive definite hydraulic conductivity tensor. Theis analysis showed transmissivity varied over four orders of magnitude. There was no correlation between displacement and the hydraulic properties.

The hydraulic properties of the fault are highly variable, possibly due to changes in fault core thickness and slip surface density over small distances. The fault is both heterogeneous and anisotropic with regard to fluid flow, acting both as a barrier to and a conduit for flow. Finally, injection testing at this intermediate scale is not effective in determining the hydraulic properties of faults in sandstone reservoirs with deformation band style faulting.