Trajectory design of ultra-deep horizontal wells in the marine hard-to-drill fractured strata of western Sichuan

The average burial depth of the marine gas reservoirs in western Sichuan is over 6 000 m. The second member of Xujiahe Formation-Xiaotangzi Formation encountered in the angle building section is characterized by ferruginous cements and tight rocks with drillability grade over 8 and rate of penetration (ROP) only 1.19 m/h. The reservoir of Leikoupo Formation has developed bedding and is seriously fractured locally, so wellbore tends to be unstable easily. In view that large-scale development in the late stage is mainly based on high-angle directional wells and horizontal wells, the design idea of four-increase section with kick-off point optimization and separate-layer deflection rate design as the cores was proposed based on the engineering and geological characteristics of the marine gas reservoirs in western Sichuan Basin,The kick-off point shall be selected according to the principle of decreasing the deflection rate and drilling footage of the hard-to-drill strata, i.e., Xujiahe Formation-Xiaotangzi Formation. The deflection rate of the first angle building section is decreased to 10-15 (°)/100 m so as to increase the orientation efficiency. The second angle building section drills into the second Member of Maantang Formation, which has better drillability, so fast angle building and direction turning are dominant. The formation drilled by the third angle building section is the first Member of Maantang Formation, so it is necessary to increase the rate of penetration (ROP) to reduce the deflection rate. The fourth angle building section drills into the bottom of the first Member of Maantang Formation and the target layer (Leikoupo Formation), so deflection rate shall be designed as low as possible while ensuring the vector target-entering. This design scheme can improve the orientation efficiency and reduce the drilling risks and is of great significance to guide the fast and safe drilling of marine ultradeep horizontal wells in the western Sichuan Basin.