Effect of Flow Velocity on Clogging Induced by Coal Fines in Saturated Proppant Packs: A Transition from Surface Deposition to Bridging.

Hydraulic fracturing is a widely used technique to enhance the production of coalbed methane reservoirs. However, a common issue is the invasion of coal fines into proppant packs, leading to pore clogging and reduced conductivity. This study investigated the impact of flow velocity on clogging by coal fines in saturated proppant packs to optimize the flow velocity and alleviate clogging during dewatering. Clogging experiments induced by coal fines were conducted on saturated proppant packs with varying superficial velocities. Throughout each experiment, the permeability and effluent concentration were monitored, and the process of clogging was visually observed using an optical microscope. The experimental results showed that both permeability and effluent concentration initially increased and then decreased with an increase in flow velocity, indicating the existence of a critical flow velocity for minimizing clogging in proppant packs. Microscale observations revealed that the dominant regimes of clogging induced by coal fines at low and high flow velocities were surface deposition and hydrodynamic bridging, respectively; a critical flow velocity was required to induce the occurrence of bridging. Removal efficiencies of coal fines in relation to surface deposition and straining against flow velocity were theoretically analyzed, aiming to provide insights into the mechanisms underlying the impact of flow velocity on clogging. The results showed that the overall removal efficiency by surface deposition and straining decreased with an increase in flow velocity. Theoretical data matched well with the experimental results at low flow velocities but failed to explain the outcomes at high flow velocities, primarily due to the onset of bridging at high flow velocities. This study highlights the necessity of developing a removal efficiency model for bridging to accurately describe clogging by coal fines in proppant packs and provides recommendations for clogging control in proppant packs.