AAPG Latin America and Caribbean Region Geoscience Technology Workshop

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4D in Libra Pre-salt reservoir: Insights via full wave seismic modeling


High quality 4D has appeared as a key component of Libra integrated field management, especially as an enabler to proactive gas management. Yet Libra 4D setting is challenging, firstly because our 4D signal is very weak, secondly as high quality pre-salt reservoir imaging remains difficult, in 3D and as a 4D difference, with complex salt geometry such as found on Libra. One needs to address those challenges with technologies that push the limits of 4D detection further than required in any context to date, and to prepare for a significant 4D effort. Detection of weak 4D signal is only possible when 4D noise level is pushed down enough that even weak signal can still be robustly seen and interpreted. As the dominant cause of 4D noise is non-repeatability, firstly of receivers, then of sources between surveys, suitable technology is required. On Libra, a major step forward has effectively been to prepare to deploy an optical PRM, a real-time permanent reception system with high reliability and perfect repeatability. Given this, we also seek further 4D improvement on source and processing sides. However, with significant costs expected and even more significant stakes on reservoir side, we need to derisk the expected outcomes of technologies never used in pre-salt to date. We must also efficiently prepare for frequent acquisitions, planned to start in 2023. To achieve these goals, Libra JPT has been going beyond the classical 4D feasibility exercise. A significant seismic full wave modeling effort has been put in place, with the objective to address the geometric contribution of repeatability 4D noise, important in this context, by providing realistic synthetic shots as well as migrated volumes of base and monitor reservoir base case scenario, and enable not only to help study specific design aspects of survey design and specifications, but also to anticipate potential difficulties and enable the construction of an optimal 4D workflow, from acquisition to processing, differential imaging and estimation of the most suitable 4D change attributes, as will be illustrated in the presentation. By modeling realistically both 4D signal and 4D noise, we intend to anticipate future detection limit and better prepare the integration of 4D information into reservoir management. Using massive computational effort therefore helps prepare a data-driven reservoir management that we expect to be a critical component of the future Libra Digital integrated field management.