CO2 Immiscible Flooding Technology Policy Research in Normal Pressure Tight Oil Reservoir: A Case Study of Honghe Chang 8 reservoir in South Ordos Basin

CO₂ is recognized as an effective EOR method in low permeability reservoir, but whether it is a feasible formation energy supplying method or not for the normal pressure tight oil reservoir is still unknown. Complex fractures are developed in Honghe Chang 8 tight sandstone reservoir and fractured horizontal wells were put into production, resulting in complicated flow and serious gas channeling since CO₂ flooding pilot was conducted in 2015. It is difficult to describe the fracture connectivity among wells quantitatively, even if the previous study shown two-stage fractures blocking technique is an effective method to control the gas mobility. And also it is still necessary to carry out further research work of inter-well fracture connectivity description and CO₂ flooding technology policy to achieve the highest enhanced oil recovery.

In the paper, based on geological and geophysics results, HH156 well block geological model was built up, according to CO₂ flooding pilot, setting up multiphase and multicomponent dual medium numerical simulation model. History matching showed geological model could not meet the requirement of gas response characteristics because the fracture connectivity among wells could not be described reasonably. Reservoir engineering method was established to describe the fracture connectivity, complicated fractures among I-P wells have been revised. Further by author previous research on two-stage gas mobility controlling technique optimization, CO₂ flooding technology policy optimizing was conducted with a new generation numerical simulator, on which gas injection mode, gas rate and CO₂ miscible degree were analyzed.

Research shows that for the wells along the principal stress direction, gas breakthrough quickly, presenting large-scale fracture developing, connective porosity is decided to be less than 0.1% and permeability is higher than 2000mD. Perpendicular to principal stress direction, gas breakthrough slowly, presenting small to medium-scale fracture development, connective porosity is more than 0.2% and permeability is lower than 500mD. Numerical simulation indicated that periodic asynchronous gas injection is the best way for CO₂ flooding. By effectively controlling gas mobility, with gas rate increasing, oil production improve gradually, the optimal gas rate is 24±t/d(0.011±HCPV/a). CO₂ flooding is immiscible process, miscible pore volume amounts to 0.86%HCPV. After 10 years, the average reservoir pressure keeps 101% level of original pressure. The sweep efficiency was improved from 1.4% (previous CO₂-foam flooding) to 24.1%, reciprocal of gas utilization ratio is 0.23t/t, oil recovery factor reaches to 6.08%.

CO₂ flooding is expected to be one of the important development modes for normal pressure tight oil reservoir. large-scale fractures should be avoided and small to medium-scale fractures are the favorable area for expanding CO₂ sweep efficiency. Results are not only advisable for the ordos normal pressure tight reservoir development, but also for the similar tight reservoir in the world.