The Effect of Trajectory Optimization on Wellbore Stability of Bedding Shale of Longmaxi Formation in Changning, Sichuan, China
Authors: Chao F. , Qian W. , Hongwei J. , Zhaowei C. , Ying W. , Xiao F. , Wenbao Z. , Shaoyun C. 
Published: September 2022

ABSTRACT:
The development of weak plane of Longmaxi formations brings great challenges to the safe drilling of horizontal wells. Since 2018, hundreds of sticking problems caused by wellbore instability occur every year. It is seriously restrict the efficient development of shale gas. In order to clarify the mechanism of wellbore instability and take methods to solve the problems, the wellbore stability model was studied in well NingA. Based on lithology division, rock mechanical parameter profiles of different lithology are established, and formation pore pressure and original in-situ stress are restored. Consider the role of intermediate principal stress, the wellbore stability analysis models of weak plane failure criteria and Modified Lade criterion are established. According to the model, the drilling fluid column pressure (collapse pressure) to maintain the wellbore stability is calculated. Properly increasing the density of drilling fluid above the collapse pressure is an effective solution to wellbore stability, but there may be a certain risk of leakage. By properly optimizing the well trajectory in the collapse section, it can maintain the wellbore stability and avoid leakage due to excessive drilling fluid density. Therefore, it is recommended to adjust the mud weight as required based on the trajectory of the planned well which can greatly improve the wellbore safety.

1. INTRODUCTION
Longmaxi Formation in Sichuan-Chongqing is a highly-fractured hard brittle shale formation with strong heterogeneity, and the complex problems such as sticking and lost circulation occur frequently in the process of shale gas drilling, which directly restricts the shortening of drilling cycle. Cui et al. (2020) and Liu et al. (2021) indicate that it is necessary to strengthen the research on the mechanism and countermeasures of reducing complexity, so as to provide scientific basis for formulating safe drilling technical measures and fundamentally solve the problems of shale gas drilling. In the drilling process, more and more researchers have noticed that the trajectory of directional or horizontal wells has a great correlation with wellbore stability. Based on the linear elastic model, the stress distribution around directional wells is studied by Fairhurst and Cook (1968). Chen et al. (1996) research shows that mechanical factors have more influence on wellbore instability in highly deviated wells than chemical factors. Zhang et al. (2006) point out that in order to ensure the wellbore stability in horizontal section, the drilling azimuth should be fully considered. According to different ground stress types, the optimal borehole stability orientation is analyzed to guide the optimal design of horizontal drilling trajectory by Zare-Reisabadi et al. (2012). Bahrami et al., (2013) consider both wellbore stability and fracturing effect, shale gas drilling has special requirements for wellbore trajectory design.