Brazilian Pre-Salt reservoir characterization and monitoring is a challenge for surface geophysical methods due to the inherently minute observable signals from the reservoirs, located below the massive salt, in depth ranges > 5000 m.

The status of a program to develop a wireline deployed 3-axis gravity sensor with a target sensitivity of ~ 5 μGal is firstly introduced. This is followed by a feasibility study for the potential deployment as a time-lapse gravity survey (4D gravity monitoring) within the pre-salt. A conceptual pre-salt reservoir model for the Libra field, Santos Basin, the offshore of Brazil, is developed, built on available pre-salt knowledge provided from seismic imaging and well log data, including reservoir and production characteristics. The model is based on water substituting oil over 6 months within and through the reservoir, with adoption of an oil production rate for the field published in 2017 by the ANP. These data are used to forward model gravity to understand the potential for a detectable signal and thereby establish a baseline for a time-lapse gravity survey (4D gravity) that could be used to monitor the Libra Field.

A clear gravity response >80 μGal is observed over a six month period in the reservoir, due to water replacing oil, at the defined oil production rate. In a three-axis measurement the vertical axis is directly related to the magnitude of the fluid substitution and the two horizontal axes are sensitive to the fluid movement directions. Together, these suggest that an annual survey with a limited well stock could be effective in monitoring this type of reservoir and that a wireline deployed 3-axis gravity tool is likely to provide significant additional surveillance to constrain a reservoir production strategy through better appreciation of the direction of water movement. The follow-on step would be to model further field data and run a baseline survey to develop a novel reservoir surveillance method within the Pre-Salt.

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