Abstract: In view of the frequent occurrence of problems such as bit-bouncing, torque distortion, and tripping resistance caused by peeling and chipping during drilling operations across the Yuquan and Pinghu formations of the oil and gas fields of the East China Sea, the present study investigates the characteristics of mud shale and drilling fluid techniques in the oil and gas fields of the East China Sea. X-ray diffraction, physical and chemical property tests, scanning electron microscopy, and high-pressure mercury injection were adopted to analyze the mechanisms of borehole destabilization in the mud shales of the East China Sea area. Based on these analyses, nano/micro pore sealing and balanced activity drilling fluid techniques were proposed to resolve the aforementioned issues. On this basis, a set of offshore reverse-osmosis-based drilling fluid techniques suitable for stabilizing deep mud shale borehole walls in oil and gas fields were developed for the East China Sea. Laboratory evaluations and field applications of these techniques were performed, and it was observed that compared with a neighboring well, the reaming time of the experimental well of size 12(1/4)" decreased from 137.75 h to 39.50 h and the circulation time decreased from 56.75 h to 35.75 h, resulting in time savings of close to 5 d for reaming and circulation for a single well.

Key words: oil and gas fields of East China Sea, deep-buried natural gas, borehole wall stability, drilling fluid, reverse-osmosisbased drilling fluid