Appalachian Stress Study: 1. A detailed description of in situ stress variations in Devonian shales of the Appalachian Plateau

Abstract

We describe an experiment to measure variations in the state of stress within a horizontally bedded Devonian shale/sandstone/limestone sequence in western New York. A total of 75 stress measurements were made in three wells a kilometer or so apart using a wireline‐supported hydraulic‐fracturing system. The stress profiles indicate that a major drop in horizontal stress level occurs in the generally massive shales. This drop occurs principally across the lowermost member of a group of sand beds and corresponds to an offset in Sh and SH of 3.5 and 9 MPa, respectively. Above the sands, “thrust” regime conditions prevail, although the amount by which Sh exceeds Sv is undetermined since instantaneous shut‐in pressures (ISIPs) were clipped at the level of Sv due to fracture rotation. Below the sands, the regime is strike slip with both horizontal stresses showing lateral uniformity despite substantial variations in topography. The magnitude of Sh in the sand beds themselves and a lower limestone remains at least as great as Sv despite the decline in shale stress. Hence stress contrasts between these beds and neighboring shales become pronounced with depth. The contrast in Sh and SH between the lowermost sand and the immediately underlying shale is at least 6 and 14.5 MPa, respectively. SH levels in the lower strike‐slip regime are about 1.75 times greater than Sh and are less than the value required to initiate slippage on favourably oriented frictional interfaces. For the upper thrust regime and the sand and limestone beds, however, the inferred lower bound on SH is close to the slippage threshold for a Coulomb friction coefficient of 0.6. The orientation of SH is ENE with a standard deviation of 20°. Fracture traces were usually splayed, occasionally spanning 30° of well bore. No systematic correlation between mean orientation and lithology is evident. Significantly different orientations were obtained for adjacent tests in which almost identical ISlPs were observed, suggesting that the fractures quickly reorient themselves to propagate normal to the least principal stress direction. Similarly, vertical traces were observed in those tests where ISIPs apparently reflect Sv, suggesting that rotation to horizontal was rapid.

The importance of characterizing uncertainty in controversial geoscience applications: induced seismicity associated with hydraulic fracturing for shale gas in northwest England

“…This particular dataset might be regarded as problematic given that the vertical stress values are much larger than expected from the loading effect of the overburden; however, in this formerly glaciated region, the possibility exists that the vertical stress has not re-equilibrated following removal of the former ice load (cf. Evans et al, 1982Evans et al, , 1989. Furthermore, the E-W orientation of the maximum principal stress determined in modern measurements is consistent with the design of this mine (Bauer et al, 2005), indicating that the mining activity has not perturbed the orientation of the stress field.…”

“…The layered stratigraphy ( Fig. 2(a) presents a further complicating factor, it having long been recognized both observationally and via numerical modelling (e.g., Evans et al, 1989;Warpinski, 1989;Brenner and Gudmundsson, 2004) (2014) reported is ~7500 psi which corresponds to ~52 MPa; if this point were not mentioned, readers might be puzzled as to the mismatch between the earlier statements that the BHP is subject to considerable uncertainty and the portrayal of precise values for this parameter by Clarke et al (2014).…”

“…In this locality the maximum principal stress was reported as oriented N83±15°E-S83±15°W, the differential stress at the stated depth being ~18.6 MPa. At South Canisteo, New York, at 1000 m depth in the Wilkins Well, Evans et al (1989) reported maximum and minimum horizontal stresses of ~38 and ~20 MPa and a vertical stress of ~26 MPa. In this locality the maximum principal stress was reported as oriented N68.5±24°E-S68.5±24°W, the differential stress at the stated depth being ~18 MPa.…”

Joint initiation in bedded clastic rocks

“…These values were selected to simulate remote stresses 15% higher in the shale interlayer than in the adjacent siltstones. This is a reasonable stress ratio based on estimates of in-situ stress in layered rock (Evans et al, 1989). An applied strain of 1 £ 10 23 results in a theoretical crack-normal stress (s xx r ) of 16.5 MPa in the siltstones and 19 MPa in the shale layer.…”

Continuation of a deep borehole stress measurement profile near the San Andreas fault. 1. Hydraulic fracturing stress measurements at Hi Vista, Mojave Desert, California

“…If bypass did occur, however, the slow pressure decays following shut-in and the similarity of these shut-in decay curves between cycles ( Figure A1) suggest that the resulting leak rates are small. In this regard, Evans et al [1988b] describe a number of hydraulic fracturing tests in shale for which'packer bypass was indicated and note that this did not appear to have a significant effect upon the determined ISIPs.…”