Highly accurate seafloor gravity data can detect small density changes in subsurface hydrocarbon reservoirs by precisely repositioning the gravimeters on the seafloor. In producing gas fields, these small density changes are primarily caused by production-related changes to the pressure and gas/fluid saturations in the reservoir pore space. Knowledge of the pressure and saturation changes is vital to optimize the gas recovery, especially in offshore environments in which wells are expensive and sparse. We assessed the feasibility of time-lapse seafloor gravity monitoring for the giant gas fields in Australia’s premier hydrocarbon province, the Northern Carnarvon Basin. We determined that gravity monitoring is more feasible for reservoirs with a large areal extent and/or shallow burial depths, with high porosities and high net-to-gross sand ratios. Forward modeling of the gravity responses using simple equivalent geometry shapes and full 3D complex heterogeneous models predicted that density changes in several of these producing gas reservoirs will result in readily detectable gravity signals (>5μGal) within just a year or so of gas production. In a pure water-drive production regime, this gravity response equated to a fieldwide change in the gas-water contact height of approximately 2–3 m, or in a pure depletion-drive regime, a pressure decline equated to approximately 3–4 MPa (435–580 psi). We assessed the feasibility of time-lapse seafloor gravity monitoring for producing gas reservoirs that is flexible and practical, and it may be useful for a wide range of subsurface fluid-flow monitoring applications.

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