Drilling fluid technology based on well stability and reservoir protection

In Huizhou area of eastern South China Sea, the Paleogene coal bed in the upper part undergoes sloughing easily and there are risks of well collapse after the hydration swelling of hard brittle shale, while the Pre-Paleogene volcanic rock in the lower part is characterized by high temperature, tightness, low porosity and low permeability. These stratigraphic characteristics require drilling fluid system to have good capacities of inhibition, sealing, temperature resistance and reservoir protection, but conventional drilling fluid systems cannot meet all these requirements simultaneously. Based on neighboring well data, lithological characteristics and geological requirements, the strong-sealing polymer drilling fluid system is applied in the Paleogene, which has strong shale inhibition capacity and presents good sealing capacity when the dosage of combined sealing agent is only 4%. The high-temperature solid free drilling fluid system is used in the Pre-Paleogene, which can meet the temperature requirement of 150 ℃ at the bottom hole. In addition, the recovery rate of core permeability is up to 85%, indicating its reservoir protection capacity is good. The field application indicates that applying two different drilling fluid systems in the upper and lower parts respectively solves the problems of coal bed sloughing and hard brittle shale collapse in the Paleogene and satisfies the requirements of high temperature and reservoir protection in the Pre-Paleogene, so as to ensure the safe downhole operation while guaranteeing the smooth testing operation. The research results provide the reference for the selection of drilling fluid systems in the similar strata.