Investigation on water-drive performance of a fault-karst carbonate reservoir under different well patterns and injection-production modes based on 2D visualized physical models

Abstract

Recently, a new type of carbonate reservoir, the fault-karst carbonate reservoir has attracted lots of attention, which usually develops large-scale karst caves, vugs, and high-angle fractures along the strike-slip fault zones. Despite containing plenty of hydrocarbon resources, the complex geological structure and the strong heterogeneity make it difficult to develop this kind of reservoir effectively and economically. To perform water-drive experiments and investigate the production performance under different well patterns and injection-production modes, two 2D visualized physical models were designed and fabricated based on the typical geological structures of the fault-karst carbonate reservoir in the vertical and horizontal directions, respectively. According to the experimental results, the average oil recovery can reach 62.96% for the vertical model and 58.18% for the horizontal model. In fact, gravity effect due to density differences can greatly improve the displacement efficiency of oil by water in the vertical direction; hence, the horizontal heterogeneity would have a relatively greater impact on waterflooding development. When water is injected into the reservoir from the middle section, a large amount of un-swept oil will remain at the bottom fractures. Thus, the Bottom Injection & Top Production (BI & TP) mode should be adopted in all well patterns to increase recovery. If non-inverted well patterns are employed, dominant water channeling would form quickly in the horizontal direction, and the swept area will be hugely limited. Therefore, the Center Injection & Edge Production mode (inverted well patterns) is recommended for water-drive applications. It is also found that the water breakthrough time negatively correlates with the effective flooding time for the vertical model and the total oil recovery is more impacted by water breakthrough for both models. Additionally, the oil-water exchange ratio is introduced to evaluate the utilization efficiency of injected water. Calculation results have shown that the BI & TP mode is optimal in terms of either total recovery or oil-water exchange ratio in the vertical direction, but the well pattern layout in the horizontal direction could impact the oil recovery and oil-water exchange ratio, which, in turn, affect the project economics. This study provides novel insights for optimizing well patterns and waterflooding measures in actual fault-karst carbonate reservoirs.