Pore pressure in the Beetaloo Sub-basin

Abstract

The Beetaloo Sub-basin’s (Beetaloo) Proterozoic source rocks are among the world’s oldest and best-preserved hydrocarbon accumulations. The potential of the Beetaloo gas resource is material in the context of the Australian energy sector. As with all unconventional reservoirs, pore pressure determination is a key parameter for commercial development potential. It is also a critical input for geomechanical modelling and analysis, and for fluid flow potential – particularly in tight reservoirs (10s to 100s of nanodarcies). Direct measurements of pore pressure in very low permeability unconventional reservoirs are challenging, where most conventional methods are not applicable. Post closure analysis (PCA) of fracture injection tests, as an indirect method, can constrain the in situ stress magnitudes and estimate pore pressure in low permeability rocks. In this study, we summarise pore pressure estimates from diagnostic fracture injection test (DFIT) data from previous research and publicly available well completion reports. In addition, we estimate pore pressure from sonic log data from six wells. Reported DFIT analysis reveals variable pore pressure gradients across the Beetaloo including normally pressured in shallow parts and overpressured regimes (up to 0.65 psi/ft) in deeper parts of the basin. To have a continuous profile of pore pressure in the studied wells, we employ modified Eaton’s equation in deep wells to initiate the pore pressure mapping across the Beetaloo for future geomechanical analysis.