Abstract
Fault-karst reservoirs are featured by complex geological characteristics, and accurate and fast simulation of such kind of reservoirs using traditional simulator and simulation methods is pretty hard. Herein, we tried to discrete the complex fault-karst structures into one-dimensional connected units connecting the well, fracture and cave based on reservoir static physical parameters and injection-production dynamics. Two characteristic parameters, conductivity and connected volume, are proposed to characterize the inter-well connectivity and material basis. Meanwhile, the high-speed non-Darcy seepage term is introduced into the material balance equations for well-fracture-cave connected units to describe the actual seepage characteristics within the fault-karst reservoirs, and to better simulate the oil/water production dynamics. Based on this method, a fracture reservoir model of 1 injection-3 production was established. The change of oil–water action law in different injection and extraction systems under two production regimes of fixed production rate and fixed pressure is analyzed. A case study was conducted on S fault zone, where the flow of oil and gas did not follow the Darcy seepage rule and with a β value of 103–104, the single well flow pressure and oil production were perfectly matched with the real data. In addition, connected units with more prominent high-speed non-Darcy features were found to have better connectivity, which might shed light on the more accurate prediction of inter-well connectivity. Moreover, an improved injection-production well pattern and was proposed based on connectivity prediction model and reservoir engineering method to solve the problems of insufficient natural energy supply and overhigh oil production rate in Block S. Furthermore, the injection/production rate as well as the timing and cycle of water injection was predicted and optimized so as to better guide to site operations.
Highlights
In conventional oil and gas reservoirs, the seepage velocities of oil and gas phase are small, and the seepage processes satisfy the Darcy flow rule
Journal of Petroleum Exploration and Production Technology interwell connectivity model was established on the basis of material balance equations and Berkeley displacement equations for non-Darcy flow
Most of the current numerical simulation methods for fault-karst reservoirs have neglected the fact that the seepage flow in caves was no longer Darcy flow
Summary
In conventional oil and gas reservoirs, the seepage velocities of oil and gas phase are small, and the seepage processes satisfy the Darcy flow rule. For a typical Darcy flow, only the effect of viscous force is considered and the pressure gradient is proportional to the seepage velocity (Wen-Juan et al 2014). Most of the current numerical simulation methods for fault-karst reservoirs have neglected the fact that the seepage flow in caves was no longer Darcy flow. In Block S, only a few fault-karst units are in their stable elastic production stage, and most wells have already entered into the declining stage. Considering the high gas-oil ratio, high saturation pressure and high formation collapse risk in Block S, it is of urgent necessary to optimize the production rate, the injection-production well pattern and other operation parameters to solve or mitigate the problems occur in or follow the late elastic production stage. Hui (Hui et al 2017, 2014) was improved as:
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