Extending the reach of drilling: better wellbore trajectory and torque & drag models
Extended reach [ER] drilling refers to the practice of directionally drilling to a given geological target located at a significant horizontal distance [horizontal departure] from the drilling rig. The ability to perform complex ER operations has become increasingly important in recent years, as the average length of the wellbore horizontal section continue to be extended. Drilling these complex wells, whether it is to reach reserves far from existing facilities or to expose reservoir sections for production and reservoir management advantages, requires accurate planning to reduce the borehole friction and ensure reaching desired targets within defined accuracy. This dissertation aims to extend the horizontal section and push the drilling capabilities near their limits without taking undue risks with the drilling system. These risks include stuck pipe, drillstring wear, and fatigue. This objective will be achieved by [1] calculating accurate measurements of borehole positions [true vertical depth, northing and easting], [2] calculating truer measures of wellbore tortuosity and geometric torsion that define the shape and the undulation of the borehole, and [3] predicting accurate measurements of drillstring to borehole friction forces from torque and drag [T&D] models. The novelty of the proposed models is the ability to estimate more realistic bending effects, accurately predict the contact forces between the drillstring and the wellbore, and solve T&D parameters from surface to total depth in reasonable time using standard engineering computers. The current 3D borehole trajectory model based on the minimum curvature method [MCM] tend to mathematically smoothen the wellpath. This is due to the assumption that the borehole trajectories are composed of constant curvature arcs. This assumption creates an artificial low tortuosity expressed as dogleg severity [DLS] which leads to the miscalculation of borehole positions, and generating unreliable predictions of T&D. Today's T&D models are either based on the assumption of continuous drillstring to wellbore contact [the soft-string model] or intermittent contact due to drillstring stiffness [the stiff-string model]. In both cases, the bending parameter, the change in the rate of curvature and the geometric torsion in the T&D equilibrium equations are nil, because the wellbore trajectory is based on the MCM. In the scope of this study, a non-constant curvature trajectory model, the Advanced Spline-Curves [ASC] borehole trajectory model, has been developed to overcome these limitations. The principal method proposed using the spline-curves does not make the potentially unrealistic assumption of a constant curvature arc between survey measurements. It provides realistic results and accurately calculates the spatial course of the wellpath. This model is straight-forward and has the robustness and flexibility to calculate complex 3D wellbore trajectories. Based on this non-constant curvature 3D borehole trajectory model, an extended stiff-string 3D T&D model [the ASC 3D T&D model] is developed. This model includes the geometric torsion, the wellbore curvature, the change in the rate of wellbore curvature and the drillstring bending stiffness in its equilibrium equations. Various applications of the ASC borehole trajectory model and the ASC 3D T&D model are presented, and their results have been validated using field cases with real-time data. The ASC borehole trajectory model has been validated using two methodologies: [a] five horizontal wells with actual survey data recorded at one survey per foot using the high resolution continuous gyroscopic [HRCG] survey tool and [b] two synthetic wellbore examples of known trajectory values. The ASC 3D T&D model has been validated using field cases with real-time forces that define T&D measured at the surface by comparing two methodologies: [a] a semi-analytical approach – pseudo-high resolution [PHR] survey data generation from the ASC borehole trajectory model at one survey per foot as an input in current industry-approved T&D software, and [b] the ASC 3D T&D model compared to actual drilling data. The calculated borehole trajectory utilizing the ASC model guarantees curvature continuity along the entire wellpath with significant improvement in wellbore positioning and tortuosity as compared to MCM. This allows the introduction of the change in the rate of curvature and the geometric torsion measurements. The calculated T&D outputs from the ASC 3D T&D model provide a more accurate view of the drilling conditions downhole, including the downhole weight and torque on bit. The use of these models yields the best-known solution for the industry to design longer horizontal section. It provides multiple advantages to drilling efficiency and borehole quality. In terms of drilling efficiency, it helps to evaluate drilling equipment and procedures while drilling ER and complex horizontal wells. This allows drilling engineers to update the driller with surface weight on bit and torque parameters to improve the drilling performance. In terms of borehole quality, it increases the calculated borehole position accuracy and calculates truer measures of tortuosity without increasing the number of surveys taken. Thus, the ASC borehole trajectory model and the ASC 3D T&D model are not only an alternative approach to accurately approximate wellbore trajectories and model downhole T&D parameters to be used in real-time operation centers [RTOC]; it can also serve as a step toward drilling automation.
dogleg severity, torque and drag, wellbore trajectory, extended reach drilling, directional drilling, tortuosity
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Colorado School of Mines. Arthur Lakes Library