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Wellbore stability analysis has been included at the well planning stage of many operating companies for its beneficial effect on improvement of drilling efficiency and cost. Such analysis can be very complex and time-consuming due to numerous factors contributing to borehole instability. As such, many wellbore stability analyses take only a few of the factors into account and are mainly based on continuum mechanics. In addition to the detrimental effect of bedding planes and fractures on borehole stability, the rock masses will become more prone to wellbore instability along fractures penetrated by mud due to reduction in the fracture friction angle. However, very few wellbore stability analyses consider the impact of friction angle reduction of fractures when they are infiltrated with mud. Analyses without considering these factors may yield significant errors and misleading predictions. In this paper, coupled numerical analyses, using UDEC, are presented to investigate the impact of mud infiltration into fractures on wellbore stability under both isotropic and anisotropic stress states. A symmetric regular fracture geometry is considered in the analyses. A simplified, but reasonably realistic, approach is developed to take into account the friction angle reduction of the fractures that occurs when infiltrated with mud. This approach evaluates the extent of mud infiltration and reduces the friction angle of fractures located in the mud-infiltrated region accordingly. The domain of mud infiltration can be either circular or elliptical around the wellbore, which covers both isotropic and general stress conditions. Introduction Maintaining a stable borehole is one of the major problems encountered in the oil and gas industry. Wellbore instability-related problems have a severe impact on drilling schedule and budget, with estimates suggesting that in excess of US$1 billion is lost each year due to the problems1. Wellbore stability analysis has therefore been included at the well planning stage of many operating companies. Numerous factors, including trajectory of the wellbore, orientation and magnitude of the in-situ stress field, rock mechanical and strength properties, in-situ and induced pore pressures, mud pressure, bedding planes and fractures, contribute to wellbore instability2. Furthermore, mud infiltrated into the fractures may interact with reactive formations/infill materials and/or lubricate the fractures which can lead to a reduction in the fracture friction angle. As a result, the rock masses are more prone to wellbore instability along the fractures. As a result of the above, wellbore stability study can be very complex and time-consuming. Consequently, many wellbore stability analyses take only a few of the factors into account and mainly based on continuum mechanics. Few records of numerical analyses to investigate wellbore behaviour in fractured rock masses can be found in the literature, especially with friction angle reduction of the fractures when they are infiltrated with mud. Santarelli et al.3, Zhang et al.4 and Zhang and Roegiers5 conducted a relatively comprehensive study of wellbore behaviour in fractured rock masses. However, no mud infiltration mechanism, which is believed to be critical in the rock masses, is considered in their studies.

In the northwest shelf, a region of growing importance for oil and gas production in Australia, drilling delays and well suspension before reaching targets have occurred frequently because of wellbore instability. As a result, the economic benefits from new drilling technologies are significantly reduced because of high additional drilling costs. To improve drilling efficiency and minimize drilling costs, it is crucial to develop and apply a practical approach for optimizing wellbore instability management.This paper initially presents guidelines, in the form of design charts, for efficient wellbore stability analysis and wellbore profile design. A comprehensive, practical approach that uses the design charts for wellbore profile design is described subsequently. The application of the design charts is demonstrated through field case studies for a range of in-situ stress regimes.Effects of In-Situ Stress Regime. The variation of wellbore stability with in-situ stress regime, which was developed from safe mud-weight windows determined for each of the in-situ stress regimes and material strength categories (lower bound, mean, and upper bound strength), is shown in Fig. 5. It can be seen that drillability from wellbore stability consideration in relation to the in-situ stress regime can be divided into three broad areas.• Area 1 represents those stress regimes in which drilling operations of any trajectory can be conducted without any instability problems if appropriate mud weights are adopted. The stress regimes in this area are close to hydrostatic; that is, the differences between v , H , and h are small.

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