Before a well is drilled, the forces and stresses acting within a formation are in equilibrium. However, as the wellbore is drilled and a cylindrical volume of rock is removed, the stresses originally exerted on that volume must instead be transferred to the surrounding formation. The cylinder of rock is essentially replaced by a cylinder of drilling fluid, with its hydraulic pressure substituting for the mechanical support of the rock being removed. The hydrostatic pressure from the drilling fluid is uniform in all directions, and it cannot replicate any directional shear stresses that exist within the formation.

Formation stresses redistribute around the borehole wall, and if they exceed the rock strength, the borehole will start to deform. If the borehole wall itself begins to fail, the resulting problems could include stuck pipe, borehole wall breakout, swelling shales, and unintentional hydraulic fracturing.

The ability to predict and implement corrective action prior to any borehole problem occurring greatly reduces operational risk and ultimately increases operational efficiency. This is accomplished through the development of a geomechanical model which combines local geological and geomechanical field knowledge with comprehensive drilling and evaluation data from offset wells.

The geomechanical model provides the ability to predict and simulate wellbore stability of a planned well. Fields exhibiting considerable lateral heterogeneity and depositional variation can be challenging, but prediction and planning become more accurate as additional wells are drilled.

The well discussed in this paper was drilled with a bottomhole assembly (BHA) which included an azimuthal acoustic tool that enabled real-time wellbore stability analysis and equivalent circulating density (ECD) measurements to maintain an accurate mud-weight window. Real-time monitoring allowed predicted model responses to be confirmed, and/or the model to be updated.

The well was drilled to its final depth with no adverse borehole events, resulting in a quality borehole and a casing run with zero nonproductive time (NPT).

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