Flow Simulation and Sensitivity Analysis of Paleokarst Carbonate Reservoirs

A paleokarst reservoir is a product of deactivation and degradation of karst by collapse and infill during burial. Paleokarst reservoirs contribute to about 30% of the world’s carbonate oil and gas reserves. However, a multiscale of heterogeneities that exist in paleokarst reservoir influence fluid flow behaviour resulting into production and forecasting challenges. This thesis presents a workflow to address these challenges by studying fluid flow sensitivities using synthetic paleokarst models with varying geometries, petrophysical properties, and well patterns using synthetic paleokarst models. Upscaling is also done to understand the extent of preservation of flow properties, and production forecast. Finally, the possibility of using 4D seismic (Time-lapse) paleokarst models to map changes in dynamic properties (saturation, and pressure) of the reservoir through its production lifetime is also tested. The test simulations were performed using two generic fine scale base models measuring 614x100x30 m3 and with cells of 2x2x2 m3. The base models represent an infilled cave system but are different from each other by the number of interconnected branches, loops of varying sizes, and complexity. Passage-diameters for both cases range from 2-18 m. On flow simulation, non-cave facies of permeability less or equal to 0.1 mD produced a flow pattern mainly through the cave geometry resulting into a relatively early water breakthrough. When the non-cave facies permeability is increased to 20 mD or more, the entire reservoir pay zone is uniformly swept with no preferential flow along the paleokarst structure. But non-cave facies of permeability as low as 5 mD had a better recovery comparable to the one from the cave region. In instances where the cave sections consisted of a combination of sand infill at the base and collapse breccia at the top inside the collapsed cave system, the recovery factor was observed to be high in less permeable sand infill than in the collapse breccia. This is only in section of 10-18 m diameter, but in cave sections with a diameter less than 10 m, the sweep is uniform, and the flow pattern is non-selective. Static and dynamic properties from flow simulation, and paleokarst rock physics models were used to produce synthetic seismic models at different production time-steps (4D seismic). Using the 4D seismic models, mapping of changes in saturation, and pressure changes in cave structures as a function of change in amplitude. Cave structures and the fluid flow path were captured when the fine grid paleokarst models were upscaled to 6 m, and 10 m grid scale, but further upscaling to 15 m, and 30 m grid smoothened most of the cave structures and the original fluid flow path. Future work should involve extending the results from this thesis and integrating them with real life paleokarst reservoir data for validation and to test if they can also be applied to other fields like geotechnical engineering, karst hydrogeology, geothermal energy, and to forecast geohazards in areas with karst carbonates.