Abstract: In spite of abundant shale gas resources in Changning Block, Sichuan Province, the severe lost circulation of drilling fluid in horizontal well drilling is prominent restricts the exploration and development of shale gas resources in Changning Block. Based on single-well logging data and seismic interpretation data, a distribution model of shale gas formation fractures, crustal stress and formation pressure of shale gas reservoirs, Changning Block was established, and a study was conducted on the response relationship between them and lost circulation. The results showed that, the fracture density was high in the high position of the tectonic anticline in Changning Block, the formation was buried shallowly, the crustal stress was low, the critical pressure of opening and extension of closed fractures was low, the formation pore pressure was low, and the lost circulation of drilling fluid was severe; the fracture density in the low position of the tectonic syncline was lower than that in the high position, the formation was buried deeply, the crustal stress was high, the critical pressure of opening and extension of closed fractures was low, the formation pore pressure was high, the formation pore pressure was high, and the lost circulation of drilling fluid was subtle. The faults and fractures were developed in Changning Block, and the lost circulation of drilling fluid was mainly caused by pressure difference and closed fracture expansion. The results of the study disclose the lost circulation mechanism of drilling fluid in deep shale in Changning Block, and clarify the main influencing factors, which can be used as a scientific basis for lost circulation prevention and efficient development of shale reservoirs in Changning Block.

Key words: lost circulation of drilling fluid, fractures, crustal stress, formation pressure, shale, main controlling factors