Borehole instability mechanisms have been studied in oilwell drilling for many years. Rock behavior has been important because drilling complexity and risk increase accordingly, leading to the need of more complete risk assessments in well design. A new need for risk assessment arises in South Mexico where Pemex and Schlumberger have encountered very challenging areas: a depositional HPHT environment with salt presence, faulted formations, and depleted reservoirs resulting in high-risk wellbores.

In strong dipping formations, wellbore instability occurred as a result of planes of weakness. Several events that resulted in stuck pipe due to packing off, losing wellbore sections and downhole tools translated into severe financial impact. Planes of weakness are directly related to the well's trajectory. Developing a method was needed to predict the attack angle - during the design phase. An erroneous appreciation of the instability mechanism could lead to wrong actions, like increasing the mud density without additional considerations, making problems worse.

Using surface seismic readings and attributes resulted in successfully predicting wellbore instability caused by planes of weakness. The signal-to-noise ratio of a seismic survey was enhanced. Attributes were applied to get an improved structural continuity with reliable dip/azimuth results. An innovative methodology to calculate the attack angle from dip/azimuth from seismic and associated risks of wellbore instability was developed. Three wells drilled in a complex structure affected by a compressive salt environment were evaluated and compared to well logs, getting a good fit.

A complete risk assessment includes the calculation of the attack angle, fault mapping, and rock strength. Knowing these allows for a more complete trajectory planning while predicting wellbore instability. Thus, the output provides a valuable tool to predict failure caused by planes of weakness in the design phase, allowing trajectory modification and operational prevention/mitigation measures to avoid catastrophic stuck pipe incidents and achieve a hazard-free well construction.

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