Improving Sub Salt Wellbore Stability Predictions Using 3D Geomechanical Simulations
- Wouter Van Der Zee (Baker Hughes Inc.) | Jack Taylor (Noble Energy Inc.) | Martin Brudy (Baker Hughes Inc.)
- Document ID
- Society of Petroleum Engineers
- Abu Dhabi International Petroleum Conference and Exhibition, 11-14 November , Abu Dhabi, UAE
- Publication Date
- Document Type
- Conference Paper
- 2012. Society of Petroleum Engineers
- 5.1.7 Seismic Processing and Interpretation, 5.1.8 Seismic Modelling, 1.7 Pressure Management, 5.1.5 Geologic Modeling, 5.1.10 Reservoir Geomechanics, 5.5 Reservoir Simulation, 4.1.5 Processing Equipment, 5.3.4 Integration of geomechanics in models, 4.1.2 Separation and Treating, 3.3.2 Borehole Imaging and Wellbore Seismic, 5.1.1 Exploration, Development, Structural Geology, 1.6 Drilling Operations, 1.12.6 Drilling Data Management and Standards, 1.2.2 Geomechanics, 1.1 Well Planning
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Some of the most active and high profile hydrocarbon plays currently being explored and developed around the world lie below a salt canopy. Drilling through a thick salt canopy has the potential to provide a faster route to reach a sub-salt objective rather than drilling through the overpressured sedimentary section in a supra-salt mini-basin. Unfortunately, numerous geological factors can complicate the drilling leading to expensive sidetracking or casing operations. Wellbore stability problems, such as unexpected low fracture gradient, are relatively common while drilling close and out of salt structures. Significant savings on drilling costs can be made if potential wellbore stability problems could be identified and avoided in the well planning process. In this paper we present a workflow to improve wellbore stability predictions for drilling through and near salt structures.
Common assumptions in wellbore stability studies on stress magnitude and orientation are not valid while drilling close to a salt body as salt structures create, due to their shape and rheological behavior, a perturbation of the stress field with strong spatial variation of the principal stress magnitudes and orientations. To provide realistic stress input data for wellbore stability predictions, the stress fields around salt structures are simulated using non-linear materials and realistic 3D geometries. The workflow presented in this paper provides an efficient way to create realistic 3D finite-element based geomechanical simulations from these complicated structural data.
The workflow allows for a detailed simulation of the stress field around salt bodies that is new to the hydrocarbon industry and helps to significantly reduce the risk for wellbore failures of increasingly costly wells drilled to exploit, e.g., sub-salt plays in the Gulf of Mexico and offshore Brazil.
Many major exploration and new field development efforts around the world such as Gulf of Mexico and Campos Basin (Brazil) take place in near- or sub-salt fields. In these fields the salt canopies have variable depths, geometries, and thicknesses. Drilling through these thick salt canopies can provide a more effective way to reach the target reservoir rather than drilling through the surrounding overpressured sediments. Unfortunately, wellbore stability problems, such as unexpected low fracture gradient, are relatively common while drilling through and out of these salt structures. Significant drilling costs could be eliminated if these hazards could be identified and avoided in the well planning process. Since the in-situ stress and temperature fields are critical and major input parameters for wellbore stability prediction studies, it is of utmost importance to have a good understanding of the stress and the temperature fields near and within salt structures.
The stress state near a salt structure is heavily influenced by the fact that salt cannot sustain shear stresses. As soon as shear stress develops in a salt structure it will creep in response to the applied deviatoric stress to minimize the shear stress. As a result, the stress state within a salt structure would be nearly isotropic (though not necessarily lithostatic (Fredrich et al., 2007)). This creeping behavior of the salt causes the principal stresses in the surrounding formations to re-orientate from their far field orientations to be almost parallel and perpendicular to the salt-formation interfaces. The ‘fluid-like' behavior of the salt means that the salt-clastic interface can act as a ‘free surface', with no oblique principle stresses associated with it. This also means that the principal stress orientations are not necessarily vertical and horizontal around salt structures. Moreover, the stress magnitudes in the formation are altered close to salt structures. In addition, the presences of salt canopies also alter the far- field temperature gradients around and within the salt structures. This is due to the fact that the thermal conductivity of salt is 2-3 times greater than that of surrounding formations. Thus, salt structures act as channels for heat transport and cause thermal anomalies around these canopies (Yu et al., 1992). This is particularly important if rate of wellbore closure within salt is of interest since the creep rate of salt strongly depends on temperature (Willson et al., 2003).
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