Prevention of Formation Damage Induced by Mud Lost in Deep Fractured Tight Gas Reservoir in Western Sichuan Basin

Western Sichuan deep tight gas reservoirs are characterized by ultralow permeability, natural fractures, partial ultralow water saturation, and a hard brittle shale interlayer. The matrix permeability varies from 0.001 to 0.1 md. The natural-fracture width varies from several micrometers to 3.0 mm, but it could be up to 5.0 mm during well operations. Lost circulation-inducing severe reservoir damage and increasing nonproductive time-has frequently occurred during well drilling and cementation during the past 10 years. The traditional lost-circulation-control techniques such as physical, chemical, or physicochemical methods, which used to permanently choke the lost-circulation passage of the nonpay zone, are not suitable for the pay zone. Several technologies, including air underbalanced-drilling fluids, noninvasive drilling fluids, and traditional temporary-shielding-fluids (TSF) technology, were tried to prevent formation damage owing to lost circulation but none of them worked well. Air underbalanced drilling has to be given up because of formation-water influx and wellbore instability. Noninvasive drilling fluids are ineffective because of the low percentage of return permeability and low bearing strength of the mudcake in the fractured formation. Traditional TSF technology is applicable for the damage prevention only in reservoirs with fractures less than 100 mu m in width. Temporary-sealing-loss (TSL) fluids with millimeter-sized agents take advantage of acid-soluble bridging particles to rapidly form a tight plugging zone near the wellbore that efficiently seals the pore throats and fractures. TSL fluids were developed to prevent formation damage in leaky fractured reservoirs. With the application of the new TSL fluids, Well W2, in the second member of the Xujiahe formation, obtained a gas-production rate of 52.16x10(4) m(3)/d. Furthermore, lost circulation never occurred during drilling of Well W101.