History-Matching of Early Gas Breakout in a Saturated Oil Reservoir to Unlock Value and Optimise Further Field Development

A 3D dynamic simulation model remains a key tool in locating remaining oil and gas opportunities and testing enhanced oil recovery scopes in matured fields. Reservoir G3000X has produced for 52 years via seven completions with cumulative production of ca. 33 MMstb from an estimated STOIIP of 60 MMstb, with two completions still producing. Despite its maturity, whether the reservoir was originally saturated or undersaturated was still unclear. Conflicting Gas-Down-To (GDT) and Oil-Up-To (OUT) contacts have been logged from two early wells, while the PVT sample from a 3rd well indicated the reservoir to be undersaturated (Pb < Pr by ~120 psi). It led to additional uncertainty in the fluid-in-place estimate. An integrated 3D static and dynamic model was built using a commercial simulator. The model was initialised using both saturated and undersaturated fluid models, allowing for Gas-Oil-Contact (GOC) sensitivities to be carried out to history-match early gas breakout. The early spikes in Gas-Oil-Ratio (GOR) observed in one of the early wells during production were investigated as part of this study and found to be related to the presence of a free gas cap ab initio. The history-match sensitivities carried out allowed the determination of a representative GOC to re-initialise the model. A good match was found, leading to a re-calculation of the STOIIP and FGIIIP and further development of the reservoir. About 5 MMstb of oil was estimated to be recoverable from two re-completion opportunities under a waterflooding scenario. Accurate analyses and history-matching of a gas breakout of early wells signified by a pronounced spike in GOR could lead to proper reservoir volumetrics evaluation, improved history-match of GOR, water cut, and energy balance in the system (i.e., aquifer influx, fluid expansion, PV compressibility, etc.), as well as the generation of representative and realistic production forecasts.