Relation Between Fracture Stability and Gas Leakage into Deep Aquifers in the North Perth Basin in Western Australia

Abstract

The Kockatea Shale is a proposed target for unconventional gas development in the North Perth Basin in Western Australia. This research is concerned with correlating the extent of thermogenic gas leakage into deep aquifers overlying the Kockatea Shale with an assessment of how close the formation is to mechanical failure. Data from two petroleum exploration wells located approximately 20 km apart were considered. Both have comparable stratigraphy; however, they differ by their local tectonic setting. The stress regime is strike slip at Arrowsmith 2 well and for an assumed hydrostatic pressure the Kockatea Shale is not close to frictional limits. Minor amounts of methane and trace amounts of short chain alkanes are leaking into deep aquifers pre‐development. In contrast, the stress regime is strike slip/normal at Woodada Deep well and the Kockatea Shale is close to frictional limits. Significant volumes of gas including methane and condensate are leaking into deep aquifers. The sealing capacity of the Kockatea Shale as evidenced by the variation in gas concentration in aquifers at the two sites indicates the formation is sensitive to stress. Additionally given the low permeability of the regional Kockatea Shale seal, it is assumed that at both locations gas leakage is via critically stressed faults. Deep aquifers proximal to the shale gas target are low salinity (<5000 ppm NaCl eq.) at Woodada Deep well and are saline at Arrowsmith 2 well. Based on this assessment, it is suggested that hydraulic fracture stimulation at the Woodada Deep well poses a significant environmental risk.

Fault Seal Analysis in an Onshore Unconventional Gas Target, North Perth Basin

“…Similar risk factors occur in the onshore Kockatea Shale. Gas leakage from the Kockatea Shale was correlated with critically stressed faults for the current strike slip/normal stress regime across the Woodada anticline (Mullen et al 2018). In addition, while linkages of Jurassic and Permian faults occur in the Kockatea Shale in the Woodada gasfield ( Figure 2) no modeling has been conducted to determine the integrity of such intersections for the current stress regime.…”

“…On the crest of the Woodada anticline at 1910 m depth in the Kockatea Shale, the pore pressure was assumed NGWA.org F. Mullen et al Groundwater to be hydrostatic (18.7 MPa). Vertical stress (S v ) was estimated to be 47.6 MPa from density logs, minimum horizontal stress (S hmin ) was 30.1 MPa from leak off tests and the maximum horizontal stress (S Hmax ) ranged between 45.8 and 50.8 MPa using rock strength and breakout data (Mullen et al 2018). For fault rock with no cohesion and a coefficient of friction of 0.6, the stress state was found to be transitional strike slip/normal.…”

“…In the Eneabba Formation, groundwater salinity is marginal in the southern half of Woodada gasfield based on water samples (Commander 1978). Groundwater in the Eneabba Formation and Lesueur Aquifer is brackish at Woodada 4 well based on resistivity log estimates (Mullen 2017). Groundwater is saline in both aquifers at the northern end of Woodada gasfield based on spontaneous log estimates .…”

“…While the Eneabba Formation and Lesueur Aquifer are hydraulically connected (Commander 1978), the Woodada Formation is hydraulically disconnected from the overlying aquifers (Mullen 2017). Returned water from the Woodada Formation during drilling of the Woodada 14 well indicated groundwater in the aquifer is saline (Consolidated Gas unpublished data).…”

“…In all cases, stress is varied between strike‐slip and normal. The maximum horizontal stress was constrained by assuming that cohesionless faults exist within the study area (Mullen et al 2018). This means that while the ΔP p estimate is absolute for the cohesionless case, the ΔP p for the healed case provides only a relative risk for the purpose of illustrating the sensitivity of the FAS to rock strength.…”