Near-Surface And Deep Hydrofracturing Stress Measurements In The Waterlog Quartzite

We conducted hydrofracturing stress measurements in two adjacent holes drilled from the surface into the Precambrian Waterloo quartzite, some 3 km NE of Waterloo, Wisconsin. In one hole we carried out three tests at 17 m, 163 m and 238 m depth; in the other we ran ten tests between 2 m and 74 m. Calculations based on the recorded breakdown and shut-in pressures and fracture impressions in both holes reveal two apparent stress fields: one within the top 20 m from the surface (the near-surface field), and the other within the rest of depth range tested (the deeper field). In the near-surface stress field the largest horizontal compressive stress (sHmax ) varies between 1.5 to 3.0 MPa and is directed at N30°W 25°, and the least horizontal compressive stress (sHmin) is in the range of 1.0 to 1.5 MPa at N60°E 25°. In the deeper stress field sHmax •= 9.4+ 0.0ld at N60°E 15°, and sHmin = 5.4 + 0.01d at N30°W (where magnitudes were obtained from linear regression analysis, stresses are given in MPa, and d is depth in meters). The vertical stress, based on rock density, is equal to 0.026d. Note that the horizontal principal stresses at all tested depths are larger than the vertical. The deeper stress field is very similar to that at Montello, Wisconsin and appears to represent the regional stress regime. The partial relief and 90° rotation of the horizontal stresses in the near-surface field could have been caused by the free boundary created by a gentle slope of the outcrop where the holes are located. This slope dips WSW toward a nearby creek 15 m lower in elevation. Our present results indicate that surface and shallow measurements may misrepresent stress conditions at depth. Consideration of topography and comparison with deeper tests are recommended as a prerequisite before regional and tectonic implications are made.