The Shushufindi field (Fig. 1) was discovered in 1972 and is sparsely developed, with only 181 wells covering an area of approximately 400 km2. Recovered whole core from the well SSF-151D presents very thin streaks of quartz material, along with carbon and amber, which form vertical permeability barriers in the cored section, approximately one inch thick. Production profiles for many wells in the field present a very gradual increase in the water cut from an active edgewater drive aquifer and demonstrate a layered system. This implies that the vertical permeability barriers are areally extensive, and that they have a substantial impact on production predictions and development scenarios. The challenge for the subsurface team is to represent these thin barriers in a predictive simulation model.
A standard Kv:Kh ratio is insufficient to mimic the observed production response. A very fine-scale geomodel could be considered but is computationally expensive. Vertical transmissibility multipliers are useful in a simulation model, but experiments demonstrate that the lateral extent of the barriers is discontinuous. Diagnostic plots of the well water/oil ratio (WWOR) versus the cumulative liquid production (WLPT) can identify groups of wells with layered production profiles. This information provides a basis to infer the potential for a layered production response from existing well logs. To introduce such discontinuities, a series of randomly generated vertical baffles have been created which provide an improved simulation response in a relatively coarse simulation model.
Early water breakthrough, and excessive water production in some wells caused premature re-completions into other zones. This type of sub-surface modeling provides support for the introduction of intelligent completions to isolate specific layers as part of the development plan.
This paper presents the techniques implemented by the team, the results to date, and a prognosis for the future of the field.