Pore Pressure and Geomechanical Assessment of Nappamerri Trough Wells with Emphasis on the Exploration and Development of Unconventional Petroleum Resources
Samantha J. Conlay - 2014
Bachelor of Science (Petroleum Geology & Geophysics)
Australian School of Petroleum
The University of Adelaide
Evaluation of geomechanical and pore pressure petroleum data is essential to optimise the exploration and development of unconventional petroleum resources. The Nappamerri Trough, located within the Cooper Basin, central Australia, requires the evaluation of in situ stress, pore pressure and elastic rock properties for study wells; Holdfast-1, Encounter-1, Halifax-1 and Moomba-191. This will assist the development of Permian tight gas reservoirs observed to reside within the: Toolachee Formation, Roseneath Shale, Epsilon Formation, Murteree Shale, Patchawarra Formation and Daralingie Formation. It is fundamental to predict the behaviour of rock in response to hydraulic stimulation treatments, which are applied to these tight, impermeable formations to optimise the extraction of petroleum resources. Previous regional studies of the Cooper Basin highlight a complex stress field containing an east-west maximum horizontal stress orientation. However, scarce exploration has been conducted locally within the Nappamerri Trough and this study seeks to explore the variation of stress and pore pressure specific to this area.
The findings presented within this study are formulated using resistivity and acoustic image logs and other geomechanical petroleum field data. The reliability of results is strengthened through use of local Nappamerri Trough data and the application of multiple methodologies. However, the accuracy of each result varies depending on the type and quality of each utilised data set.
In comparison to the surrounding Cooper Basin region, the Nappamerri Trough is characterised by an overpressured zone at depths exceeding 2,700 m, which is observed to correlate with:
pore pressure-stress coupling including increased minimum horizontal stress (14 to 24 MPa/km) and maximum horizontal stress magnitudes (20.4 to 44.8 MPa/km); pore pressure approaching vertical stress magnitudes (21.15 to 22.37 MPa/km); the transition from a strike-slip to dominant reverse faulting stress regime; east-west and northeast-southwest horizontally and vertically orientated natural fractures; vertically oriented induced fractures associated with minimal differential stress; and, low ΔP required for fracture reactivation and initiation (non-cohesive: up to 116 MPa, but mostly less than 0 MPa indicating existing fracture planes; and, cohesive: up to 136 MPa).
This study identifies the overpressured zone as the most prospective location to perform hydraulic fracturing treatments, due to its increased fracture susceptibility and host of east-west trending natural fractures that are optimally orientated for shear reactivation. However, hydraulic stimulation complexity may be enhanced within highly pressured areas by increased stress magnitudes, decreased hydraulic flow rates and the reorientation of fractures.