Differentiated staged fracturing and testing for coal-tight sandstone composite reservoirs without interlayer barriers: a case study of Well WFD1

Coal-tight sandstone composite reservoirs without interlayer barriers are prone to interlayer fracture interference, ineffective stimulation, and difficulties in production evaluation due to strong contrasts in lithology, petrophysical properties, and in-situ stress. To address these challenges in Well WFD1, a differentiated staged fracturing and testing strategy was proposed and implemented based on reservoir characteristics. The coal rock interval was treated using moderate pumping rates (6.0–10.0 m³/min) with medium-scale fracture network stimulation, combined with variable-rate control of fracture height and multi-size proppant placement to enhance conductivity while mitigating water sensitivity damage. The tight sandstone interval was fractured using a moderate-scale, low proppant concentration design with composite proppants(150 μm ceramic fines + 212 to 425 μm quartz sand + 300 to 850 μm quartz sand), coupled with liquid nitrogen-assisted injection to improve flowback efficiency. During the production stage, a dual-packer pumping string was deployed to achieve effective zonal isolation, enabling independent flowback and production testing of the coal and sandstone intervals. Field results indicate that fracture height growth was effectively controlled, no interlayer communication occurred, and both reservoir intervals were successfully stimulated. Reliable production capacity and fluid properties of the coal-tight sandstone were obtained for each layer. These results indicate that the proposed differentiated staged fracturing and testing technology is effective for coal-tight sandstone composite reservoirs without interlayer barriers and provides a practical reference for safe stimulation and accurate zonal evaluation of similar coal-measure gas reservoirs.