Rapid Evaluation of Geological and Reservoir Uncertainty in a Multi-Field Development

Abstract

An automated modelling process for rapidly evaluating differing multi-field development production strategies under geological uncertainty has been developed. We demonstrate the use of the technique on the Charlie, Alpha, and Bravo (CAB) fields lying in deepwater, offshore Angola. The names of the fields have been changed to preserve confidential data. CAB is a planned clustered development with all CAB reservoirs producing back into a single Floating Production Storage and Offloading (FPSO) vessel. Due to a challenging seismic environment causing considerable geological uncertainty, stochastic modelling was used to populate reservoir models bounded by seismically interpreted deterministic channel system surfaces. The CAB team developed a systematic modelling workflow combining static and dynamic data for simulation using a multi-field reservoir simulator. Evaluating each new geological model was a manual process taking up to 10 days. To accelerate the modelling process, the entire workflow was automated and BP's Top-Down Reservoir Modelling (TDRM™) philosophy and toolkit (Williams et al., 2004) was used to investigate the affect on hydrocarbon recovery of different possible geological models. Automation involved generating stochastic geological realisations for each CAB field and simulating simultaneous production from all three fields with reinjection of the produced gas. Up to 30 different multi-field simulations could be generated and simulated every 24 hours, with the workflow using 10 CPUs Approximately 700 multi-reservoir simulation runs (2100 individual reservoir simulations) were conducted within a two month period. By automating the workflow, more time was available for evaluating simulation results and less time was spent preparing data and running the simulations. The automated workflow has been used to decide FPSO capacities and to gain confidence that the Charlie field could take all produced gas without flaring or significant loss of production. The workflow continues to be used to quantify the effects of geological uncertainty on different gas management strategies. Introduction There have been many studies over the past decade which investigate the affect of uncertainty on predicted performance and net present value (NPV) of single reservoirs (Friedmann 2003, Hoffmann 2006, Campozana 2008). There are further studies which extend the approach to look at the effect of uncertainty on managing multiple reservoirs (Kabir 2005, Cullick 2007). Broadly, these papers deal with some combination of reservoir flow uncertainties, surface facility uncertainties and/or economic uncertainties. Where attempts have been made to integrate both facilities, wellbore and reservoir performance, it has often been required to include a simplifying step either by using a spreadsheet instead of specialised software (e.g. Rodriguez et al. 2008) or by generating a proxy model to avoid using a fully integrated simulator (e.g Friedmann 2003). In this paper, we look at the effect of geological uncertainty on multiple reservoir performance using the specialised geological properties software and reservoir flow simulation software which would normally be used for a single deterministic study. CAB is an 'ultra-deepwater', offshore development consisting of the Charlie, Alpha and Bravo fields. All gas produced from all three fields is expected to be temporarily injected into the Charlie field for approximately 3 years until full gas export to the Angola Liquefied Natural Gas plant becomes available. The seismic image of the CAB fields is not clear enough to see the internal architecture of each reservoir and so decisions about FPSO capacities and gas management strategies had to take into account the geological uncertainty.