Experimental and Modeling Study on Proppant Flowback during the Entire Period of Deep Coalbed Methane Production.

Deep coal reservoirs in the Daji region of China have achieved high industrial gas production rates through large-scale volumetric fracturing. However, severe proppant flowback has significantly undermined coalbed methane recovery. Choke size management presents a practical and cost-effective approach to controlling proppant flowback. To quantify the relationship between proppant flowback and flow rate, this study conducted flowback experiments on actual coal fracture surfaces under both single-phase water production and gas-water two-phase coproduction conditions. The experiments examined the time-varying characteristics of flowback under constant flow rate, and a semitheoretical model for predicting cumulative proppant flowback was developed based on dimensional analysis. The results showed that flow velocity variations at the boundaries of flowback channels significantly influence proppant flowback rates. Under equivalent total flow conditions, the cumulative proppant flowback during the gas-liquid two-phase stage increased by 98.19% compared to the single-phase water production stage. When the fracture width increased to 6 mm, compression from the fracture walls significantly intensified proppant flowback, though the increase in flowback ratio tended to level off. When closure stress exceeded 15-20 MPa, the differences in cumulative proppant flowback became less pronounced. These findings provide theoretical guidance for choke size management, aiding in the optimization of production strategies while effectively controlling proppant flowback.