Comprehensive Geomechanics Study Helps Mitigate Severe Stuck Pipe Problems in Development Drilling in Bohai Bay, China
Authors: Zhou Mingxin , Tang Xinguo , Wu Bailin , Tan Chee , Yang Shiduo , Wang Shunchang 
Published: March 2015

Abstract
Guan Jia Pu Oil Field is located in the western part of Bohai Bay, China. The field was developed with high angle and horizontal wells drilled from an artificial island. The development drilling had experienced severe stuck pipe problems in a short period, which resulted in hole collapse, equipment being left in hole and sidetracking. This paper presents a comprehensive geomechanics study with the aim of identifying the root cause of the stuck pipe problem and developing mitigating measures for future drilling.
A rigorous review of the drilling experience in the field revealed that the stuck pipe incidents mainly occurred 1) in high angle and horizontal wells in Sa He Jie shale overlying the main reservoir; 2) during tripping out hole whilst no problem was reported during drilling; and 3) with the stuck pipe wells appeared to be not correlated with a particular wellbore azimuth.
Mechanical Earth Models based on an isotropic mechanical property assumption were then constructed using mainly well logs and data extracted from the drilling and completions reports. Wellbore stability analyses were conducted for the stuck pipe and non-stuck pipe wells which demonstrated that extensive borehole failure in Sa He Jie shale was the main cause of stuck pipe. The arc of calculated borehole failure was greater than 90° due to the use of insufficient mud weight. This could have generated large size shale cavings which would be difficult to circulate out of the hole. For non-stuck pipe wells, the calculated borehole failure extent was much less.
Mechanical deformation and strength anisotropy of Sa He Jie shale were evaluated in laboratory using ultrasonic measurements and scratch testing equipment on large shale caving blocks. It was observed that the strength along the bedding planes of the shale was much weaker than the intact shale material. A numerical model was then applied to assess the effect of mechanical anisotropy on wellbore stability. Bedding plane failure in Sa He Jie shale affects wellbore stability for high angle and horizontal wells (deviation greater than 60°) drilled close to the minor horizontal stress direction. The additional mud weight to counteract bedding plane failure is approximately 0.03SG. Based on the study, a mud weight program was proposed for a planned well, which had been drilled without significant geomechanics-related problems.

Introduction
Guan Jia Pu Oil Field is located in the western part of Bohai Bay, China with a shallow water depth (5 metres). The field was developed with high angle and horizontal wells drilled from an artificial island. The development drilling had been hindered by severe stuck pipe problems experienced in several high angle and horizontal wells in a short period, which resulted in hole collapse, equipment being left in hole and sidetracking. This paper presents a comprehensive geomechanics study on the stuck pipe problem. The primary objective of the study was to identify the root cause of the stuck pipe problem and to develop mitigating measures for future drilling. Specifically, the objectives are• Review the drilling events occurred in the field, in particular wellbore instability in Sa He Jie shale, and extract relevant information from drilling data for mechanical earth modeling.• Integrate and analyze the existing relevant logs, field and other data, to generate Mechanical Earth Models (MEMs) describing stress magnitudes, stress orientation, pore pressure and formation mechanical properties.• Calibrate and refine the MEMs, using existing caliper and drilling data, to constrain and reduce uncertainties associated with limitations and availability of the existing data.• Identify the mechanical mechanisms of the stuck pipe problems that were encountered while drilling the existing wells through back-analysis.• Predict and recommend operating mud weight for the next planned well.