Experimental study on filtration behavior and control methods of pre-pad CO₂ fracturing fluids in shale oil reservoirs

To investigate the filtration behavior and control mechanisms of novel carbon dioxide (CO₂) fracturingfluid systems suitable for shale reservoirs, systematic comparative experiments were conducted under simulated formation conditions using natural shale cores from a block of Shengli Oilfield with a high-pressure dynamic filtration experimental system, aiming to provide theoretical guidance for efficient fracturing technology in shale reservoirs and CO₂ injection strategies in carbon capture, utilization, and storage (CCUS). The influencing mechanisms of temperature, pressure, thickener concentration, and formation crude oil on the filtration behavior during single-phase CO₂ flow were analyzed, with particular focus on investigating the dynamic filtration characteristics of CO₂ under different temperatures and pressures in the supercritical state. The filtration coefficient was calculated to reveal the effects of temperatures and pressures on the filtration behavior of liquid CO₂ in shale. The results showed that when CO₂ was in a single-phase flow state, viscosity was the key factor controlling the filtration rate. As temperature increased or pressure decreased, the viscosity of CO₂ decreased, leading to an increase in the filtration coefficient. The addition of thickeners to CO₂ could effectively reduce the filtration rate, with the filtration coefficient decreased by approximately one order of magnitude compared with pure CO₂. The presence of formation crude oil further inhibited filtration by forming gas-liquid two-phase flow and additional flow resistance. When CO₂ transitioned from liquid phase to supercritical state in the formation, two-phase flow effects and volume expansion became the dominant mechanisms affecting filtration rate, effectively controlling the filtration rate. In the supercritical state, the filtration coefficient decreased or tended to stabilize as formation temperature increased or pressure decreased. This study clarifies the dynamic filtration patterns of CO₂ fracturing fluid systems in shale, and it provides important data support for the optimized design of efficient CO₂ fracturing technology in shale reservoirs.