Openhole instability during drilling and production remains a serious problem in oil and gas industry. Previous research has almost exclusively focused on the stability of a circular borehole. It is well known that drilling operations and/or rock failure can often result in a noncircular borehole cross-section. Since it is inevitable that stress and pressure conditions in the borehole and/or reservoir can change over time, it is important to understand how the stability of non-circular boreholes would be affected by the changes in borehole and reservoir conditions. Presented in this paper is a semi-analytical solution for linear elastic stress and displacement fields for a noncircular borehole based on complex variable method. To gain a better understanding of the effects of noncircular borehole geometry on borehole stability and hydraulic fracture initiation, a parametric study has been conducted for a set of in-situ stress, borehole pressure and borehole breakout geometry parameters. Borehole stability analysis has further demonstrated the effect of breakout width allowed on the lower bound of the stable mud weight window.

1. Introduction

Openhole instability during drilling and production remains a significant problem in the oil and gas industry (e.g., Aadnoy and Ong, 2003; Zoback, 2012). It is well known that drilling a borehole disturbs the local state of stress and leads to stress concentration on the borehole wall. When the induced state of stress exceeds the strength of the rock, rock failure or breakouts can take place, resulting in a noncircular borehole. Illustrated in Figure 1 are typical borehole cross-sections observed from laboratory simulation of borehole breakouts conducted at CSIRO.

Although borehole instability or breakouts can be prevented with a higher mud weight, a high mud weight can have undesirable ramifications, such as a lower rate of penetration and a higher risk of drilling fluid loss and formation damage. Furthermore, a certain amount of breakout can be tolerated as long as they are stable and don't have a significant impact on subsequent drilling and completion operations, and the function of the well (e.g., Zoback et al., 2012; Tan et al., 2019).

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