A characterization of pressure records in inelastic rock demonstrated by hydraulic fracturing measurements in salt

Hydraulic fracturing tests in anhydrite interbeds in the WIPP, Marker Beds 139 and 140

“…There are, nevertheless, several uncertainties, some of which are to be addressed by the experiments in this report: (I) Large variability of formation (pore) pressure, permeability, and brine mass collected in drillholes in MI3139 leave open the possibility that MI3139 was not in the virgin state. This suggestion is consistent with the observation that stress field perturbations in rock salt, and therefore, also along salt-interbed boundaries, appear to propagate much farther than in hard elastic rock (Wawersik and Stone, 1989 ME3139 which could (i) remain in the marker bed or (ii) propagate vertically out of the interbed. The latter scenario would be likely if the horizontal in situ principal stress in anhydrite, a nearly elastic material at low and intermediate applied deviatoric stress, were significantly smaller than the overburden stress.…”

“…Based on Argiiello's analyses, mining of Room C1 produces significantly smaller reductions in the horizontal than in the vertical in situ stresses. It is suggested, therefore, that the horizontal stress magnitudes inferred above, 0-p 13 MPa (1890 psi), are indeed representative of the horizontal in situ stresses distant fiom the WIPP and that the vertical stress everywhere is equal to the integrated overburden density as was verified in rock salt in the G-drift (Wawersik and Stone, 1989).…”

“…The comparison of instantaneous shut-in pressures in MB139 (9.6 and 12.37 MPa; 1390 and 1790 psi MB139; Table 2) with the virgin in situ stress at the WlPP horizon (14.9 MPq Wawersik and Stone, 1989) indicates that the virgin stress field in MB139 was altered due to the presence of existing WIPP excavations. This result was not totally surprising in the light of numerical predictions of stresses in the floor salt of Room C1 (Argiiello, 1991).…”

The disturbed rock zone at the Waste Isolation Pilot Plant.

“…The phenomenon of high-pressure opening of fractures can he envisioned as hydrofractures that tend to open preferentially normal to the direction of the minimum principal stress. To examine hydrofracture in more detail, field studies (Wawersik and Stone, 1989) were re-evaluated. The results of hydraulic fracturing tests performed in WIPP salt, 3 to 100 meters from excavated rooms indicate that the pressures at which hydraulic fracturing is initiated, fall in the same range as hydrofracture tests performed in anhydrite Marker Beds 139 and 140 (Wawersik et al, 1997).…”

“…To examine hydrofracture in more detail, field studies (Wawersik and Stone, 1989) were re-evaluated. The results of hydraulic fracturing tests performed in WIPP salt, 3 to 100 meters from excavated rooms indicate that the pressures at which hydraulic fracturing is initiated, fall in the same range as hydrofracture tests performed in anhydrite Marker Beds 139 and 140 (Wawersik et al, 1997). Fracture initiation pressures for the anhydrite tests ranged from 7.36 to 12.46 MPa with an average initiation pressure of 10.5 MPa.…”