The drilling of extended-reach wells is an increasingly common practice for improving the recovery from mature fields, and for producing distant oil and gas resources using existing infrastructure. From a geomechanical point of view, the drilling and completion of extended-reach wells may become technically very challenging, as these wells often have long sections drilled at high inclinations that can be prone to borehole instability problems such as pack-offs or collapse of the wellbore wall. The case of an extended-reach well drilled in the Ekofisk field in the North Sea where borehole stability issues were observed and eventually resulted in the loss of the well is presented. A wellbore stability assessment is performed with well-specific stress and formation strength data that explores the possible failures that may have resulted in the loss of the well. In particular, a plane of weakness model is used to model possible shear failure along the bedding of the overburden shale formations. The uncertainty in the rock matrix strength is accounted for, as well as the cohesion, friction factor and orientation of the bedding plane, on the mud window using a Monte Carlo approach. This paper focuses in particular on how the properties of the bedding plane affect the minimum required mud weight, and compare to the actual mud weight used in operation. The generated mud window acknowledging failure along the weakness planes suggests that this type of failure was a relevant failure mechanism over the 13 1/2-in section of the well, as the mud weight employed was not high enough to avoid it. Accounting for uncertainty and the failure along the weakness planes in extended-reach wells to be drilled in the Ekofisk area may generate safer mud windows that in turn may reduce the occurrence of wellbore instability in the field.
The mechanical stability of wellbores is governed by formation stresses, the local pore pressure and strength properties of the rock. These formation and rock properties are often associated with uncertainty that in turn affects the confidence of collapse and fracture prognoses. We use formation stress and well-specific rock strength data to perform a mechanical wellbore stability analysis for a deep, anisotropic shale formation in the North Sea. Uncertainty in the available data and stress prognoses is accounted for and reflected in the safe mud weight window for the formation. We base the wellbore stability analysis on predictions of shear and tensile failure at the borehole wall for different wellbore trajectories through the shale formation above the reservoir. We utilize in-situ stress and pore pressure prognoses in combination with results from rock mechanical tests of 11 oriented core specimens from the actual shale formation, including unconfined and triaxial tests with different bedding plane orientations relative to the specimen axis. We propagate uncertainty in both rock strength and anisotropy, and formation properties to generate operational mud weight windows in terms of fracture and collapse pressures using a Monte Carlo approach. The formation stress prognosis for the actual shale formation suggested the largest in-situ stress to be the vertical overburden, while the two horizontal stresses are of nearly equal magnitude. The wellbore stability analysis shows a relatively minor sensitivity in the predicted mud weight window to the azimuthal direction of the trajectory and improved stability for near-vertical compared to highly inclined trajectories. The observation is in line with established field practice of limiting the inclination angle drilled through the shale. Due to the relatively low values of cohesion in the shale, the formation collapse pressure is sensitive to the internal friction angle of the shale which in turn varies with bedding plane orientation. A safe mud weight window for varying trajectories through a deep shale formation in the North Sea has been presented. The study demonstrates how uncertainty in formation and rock properties can be accounted for to determine the confidence level of the mud weight window. The resulting operational pressure window is essential for avoiding drilling problems and optimizing casing setting depths.
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