The Mishrif formation in Abu Dhabi comprises progradational shelf margin facies. The western platform sediments are characterized by stacked clinoforms of clean high energy carbonates, with generally good reservoir properties at the top deteriorating gradually toward the west flank. In contrast in the east the formation is thicker and characterized by more differentiated, coarsening, shoaling- upwards sequences. High quality reservoir facies occur only near the prograding shelf edges. The reservoir in this study had finely layered pillar grid models of the field using onlap and offlap layering to capture the vertical property heterogeneity and layering within the clinoforms implied by the depositional environment. However, this grid structure posed challenges for flow simulation as there were unphysical barriers and connections across the clinoform boundaries caused by the pinching out layers at boundaries between clinoform units. By construction, in a depogrid each clinoform may be independently gridded with coordinate lines that do not need to be continuous through the vertical extent of the reservoir. Layers within a sequence can truncate arbitrarily against bounding discontinuities since the cells are polyhedral and the grid globally unstructured. Therefore, an evaluation of the depogrid cut-cell grid was undertaken for the reservoir. A volume-based model was constructed using horizon surfaces and fault surfaces extracted from the pillar grid model, and the depospace transform calculated. The depogrid was created using the same areal resolution as the pillar grid. The layer parameters were set to give approximately the same number of layers in each zone as the pillar grid. The rock type base properties from the pillar grid were upscaled onto the depogrid, in the interest of time, to populate the static depogrid model. Reservoir fluid properties, relative permeability, and capillary pressure curves were taken from existing simulation models of the field. A high-resolution reservoir simulator was then run on the depogrid model. Run time comparisons of the pillar and depogrid simulations were made The evaluation concluded that the cut-cell stratigraphically layered depogrid provided a more geologically consistent representation of the complex stacked clinoform structural elements and required significantly fewer grid layers to achieve the required vertical resolution. The depogrid simulations could represent the expected connectivity for flow. Improved run times were observed

A small onshore brownfield in south Oman has low oil recovery because of its heavy oil and high water production, which together with reservoir uncertainties poses development challenges. Petrogas implemented an innovative field development planning approach to quantitatively compare multiple field development scenarios and optimize the operational choices within each. The workflow started with a single history matched model for each of the two geological structures in the field. A set of 14 field development scenarios were defined, on injection rates, well locations, and injection fluids. Identification and quantification of subsurface uncertainties were performed. These uncertainties were included in the geomodel for each scenario, which generated an ensemble of realizations and corresponding production forecasts. Two sets of economic results were produced—a simple, discounted cashflow model and the fiscal terms of the operator's service contract. Each ensemble was run against these models to generate probabilistic performance indicators for each scenario. Using cloud-computing capability, the field development study was drastically accelerated without losing on the quality. Almost 800 simulations were run over 5 days, covering 32 development scenarios in total (for two structures), automatically integrated with the economics workflow, providing in-depth analyses. The scenarios were compared in a series of dashboards that presented the economic metrics and their corresponding cumulative distribution functions. The analysis yielded several important insights: longer wells did not provide enough additional production to offset the increased costs. Moreover, peripheral drive with horizontal wells was more effective than irregular vertical wells. The waterflood scenarios improved production, but the polymer-injection option with short horizontal wells and peripheral infill well pattern was the highest-performing scenario. The study also helped identify areas where more detailed optimization studies should be performed, e.g., to optimize polymer-injection scheduling and polymer design. Traditionally, subsurface uncertainties analysis was restricted to a small number of discrete model realizations. Results were quantified in terms of production ranges only. Here, production forecasts were based on an ensemble of models, capturing the full range of uncertainties. In addition, evaluation criteria included economics.