Flow Pattern Controls and Sensitivity Analysis in Paleokarst Reservoirs

Paleokarst is a product of deactivation and degradation of karst by collapse and infill during burial. Geometries and petrophysical properties derived from these processes are preserved and frequently form key elements of petroleum reservoirs in carbonates. With nearly 30% of the world’s carbonate oil and gas reserves being associated with fracture cavities (Jiang et al. 2008), understanding the effect of paleokarst features on dynamic behavior is crucial. In this study, test simulations were done on a generic base model measuring 614x100x30m and cells of 2x2x2m. The model represents a collapsed and infilled cave system, including many interconnected branches and loops of varying sizes and complexity. Passage- diameters range from 2-18m. Two horizontal wells (an injector and a producer) were positioned in the reservoir to mimic uniform inflow from the left hand side and outflow through to the right hand side. At background/host rock permeability less or equal to 0.1mD, the flow closely follows the collapsed cave resulting into a relatively early water cut. However, when the background permeability is increased to 40mD or more, the entire reservoir pay zone is uniformly swept with no preferential flow along the paleokarst structure. 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, oil sweep efficiency was observed to be better in sand infill than in the collapse breccia (only in section of at least 10m diameter) while in sections of 6m, the sweep was uniform and the flow pattern was non-selective. Upscaling from 2m grid resolution to 6m and 10m yielded comparable results of total field oil production and retained preferential flow patterns along the collapsed cave. Further upscaling to 14m, 15m, and 30m grid cell resolution resulted in overestimation of the original total field oil production obtained from the high-resolution model and loss of the original flow pattern. The overestimation is mainly as a result of the complex geometry of the cave network.