The F6 field is one of many high porosity carbonate buildups developed in offshore Sarawak, Malaysia. With maximum porosities in excess of 30%, the reservoir experiences pore collapse during depletion. As a result, the predicted reservoir compaction and surface subsidence can be in the order of 25 ft or more. This poses a significant risk in terms of the platform subsiding into the wave-base, requiring shut in of production and prompting costly "jacking up" of the platform. To mitigate the risks it is recommended to build a field-wide 3D geomechanical model, constrained by available compaction and subsidence measurements, which are used to forecast future platform movement to end of field life.

The F6 field finite element geomechanical model was constructed from the 3D static structural model and populated with the calculated in-situ stresses and pore pressure distribution in the field from the reservoir simulations. The rock properties assigned in the model were derived from both log data and numerous core tests, which also defined their deformation and failure behaviour.

The F6 field has an on-going monitoring program, including compaction logging data in the reservoir, GPS data on the platform (both lateral and vertical movements), and sonar data for the platform height above sea level. Intermittent data gathered includes sonar seabed surveys of the subsidence bowl. This paper describes the data collected and how it is used to constrain the 3D finite-element geomechanical model constructed of the F6 field. The key findings of this study include; firstly, the importance of hard data in terms of reservoir compaction (compaction logging) and how this is translated through the overburden to surface subsidence (GPS measurements). Secondly, the importance of an accurate pre-production baseline for the monitoring program; it is relatively easy to match the rate of subsidence predicted from the model with the monitoring data collected after production start-up, however, the lack of pre-production data can lead to significant differences in the predicted absolute subsidence at end of field life.

The results of this study show the importance and value of geomechanical modeling constrained by historical observations from a field monitoring program and the necessity to have it included as part of the field development plan prior to start of production. Use of this data with geomechanical forecasting is now a significant component of production assurance for the asset and has lead to plans to increase coverage and frequency of data gathering over these highly compactable fields.

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