Most borehole stability problems occur during drilling in the overburden shale and mudstone. Typically borehole instabilities are associated with high pore pressures just above the hydrocarbon reservoir. Worldwide there is a tendency going towards deeper HPHT reservoirs with increased pressure and temperature. Consequently the drilling window margins are reduced and more emphasis is put on the borehole stability predictions. In order to prevent water influx the mud weight is kept above pore pressure and the collapse pressure. Further to avoid fracturing and tensile cracks the mud weight is kept below the fracture gradient. In order to predict the upper and lower limits for mud weight densities a proper description of the formation is needed. It is well known that the shale exhibits anisotropic behaviour due to sedimentation processes. The anisotropy is apparent both with respect to deformation and strength.

This paper presents the borehole stability simulations carried out for an HPHT field offshore Norway. An anisotropic shale model is implemented and calibrated against laboratory test results. Both drained and partially undrained stress paths are compared. In total three triaxial tests with different sample orientations are required to fully describe the anisotropic properties. The borehole stability calculations are carried out in the finite element code Abaqus, where the anisotropic model is implemented as a user routine. The simulation results are presented for different borehole inclinations, where emphasis is put on predicting the allowable underpressure.


1.1. GeneralA borehole stability study is carried out for a planned HPHT (High Pressure High Temperature) field development offshore Norway. The field is planned with several production and injection wells. Some wells are drilled prior to production, while others are planned to be drilled after production startup. Due to the potential problems related to drilling in a depleted reservoir, the plan is to start with the deepest hydrocarbon layers. Top reservoir is situated at about 4000m TVD MSL and consists of sandstone from Late Jurassic age. Typical downhole reservoir pressure and temperature is 785 bar and 137 degrees Celsius. The overburden consists of shale formations from the Shetland group and Viking group, where the shale just above the reservoir has high organic content.

A narrow borehole stability window is predicted just above the reservoir. This is because of a high overpressure at top reservoir. In addition the collapse pressure is limited by the predicted fracture gradient. The mud weight should be designed to lie between these lower and upper boundaries. However, there are some uncertainties regarding the overpressure in the shale section. To be conservative the maximum pressure at top reservoir in the sandstone section is used.

In order to predict the allowable underpressure in the shale section both analytical and numerical borehole stability analyses were carried out. A one meter shale core was recovered and a comprehensive test program performed. From the test results a proper material design basis was then established.

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