Research on Mechanism and Parameter Optimization of CO2 Pre-pad Fracturing-- Taking the Mahu Conglomerate Reservoir as an Example

ABSTRACT: The Mahu conglomerate reservoir has low effective permeability, making it challenging to improve oil recovery. CO2 pre-pad energized fracturing technology can enhance oil recovery by improving the reservoir's physical properties and oil seepage capacity. However, the fracture propagation mechanism in the conglomerate reservoir and the dynamics of CO2 injection and backflow are not well understood. This study combines experimental testing, numerical simulation, and theoretical analysis to investigate the mechanisms of CO2 pre-pad energized fracturing in the Mahu reservoir and optimize construction parameters for effective development. The results show that CO2 aqueous solution dissolves acid-soluble minerals and weak cement more effectively than it precipitates clay particles, thereby improving the reservoir's porosity and permeability. With a longer reaction time between CO2 solution and reservoir core, carbonic acid generation saturates, and the dissolution of soluble matter completes, leading to a decrease in porosity and permeability increase rates. Using the optimized fracturing mode-fracturing fluid + CO2 + fracturing fluid the optimal fracture half-length is 150 m, and the optimal CO2 injection volume is 100 tons per fracture. Field tests indicated that, compared to conventional wells, the average daily oil production of the test wells increased by 18.1%, and cumulative oil production increased by 22.1%. These results offer guidance for optimizing CO2 pre-pad energized fracturing in similar conglomerate reservoirs.