Water Block Damage and Its Solutions for Tight Gas Reservoirs in Dabei-Keshen Area

Water block damage may arise in tight gas reservoirs in Dabei-Keshen area since the reservoir is associated with low porosity, low permeability, and strong capillary forces, resulting in worthless industrial exploitation. In order to investigate the mechanism of water block and put forward corresponding measures, water block index (WBI) is developed to appraise the damage degree of water block and thermostable surfactant systems are optimized to clean up water block through interfacial tension tests, wettability tests, and spontaneous imbibition. The result of WBI for core samples from the targeted zone is 70%, belonging to the type of strong water block. Sensitivity analysis shows that matrix permeability and displacement pressure are in positive correlation with WBI, while water saturation, content of clays, fluid viscosity, and interfacial tension are in negative correlation with WBI. Thermostable surfactant systems JY-2 (0.05FS-31 + 15% methanol) and JY-3 (0.5%HSC-25 + 15% methanol) are preferentially optimized. And JY-3 works best to reduce WBI from 66.2% to 30.4%. In the composite fluid system, surfactant is beneficial for reducing interfacial tension and methanol is benefit for reducing water saturation through accelerating evaporation in a short time. This synergy promotes the cleanup process of water blocks. Based on the study of mechanism of water blocks and experimental data, we are able to provide reference for economic and efficient development of gas fields.