Approach Integrates Mechanical, Hydraulic Simulations for Efficient Well Planning

_ This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 227998, “Pushing the Limits of ERD: Integrating Mechanical and Hydraulic Simulations for Efficient Well Planning,” by Gergana Vanova, SPE, and Florian Mercier, SPE, Helmerich & Payne. The paper has not been peer-reviewed. _ A drilling campaign in a mature field presents challenges beyond extended-reach drilling (ERD) profiles, with water-based mud and depleted reservoirs introducing added complexities such as abnormally high friction factors and narrow drilling windows. Iterative torque and drag (T&D) and buckling (stiff-string model-based) and hydraulic simulations were conducted for multiple tapered string combinations. The results, combined with operational constraints, guided the selection of a string configuration that allowed planned total depths (TDs) to be deemed feasible. Methodology The industry has used conventional models to simulate T&D in the vast majority of cases, subsequently combining them with hydraulic simulations to cross-check and validate equivalent-circulating-density (ECD) evolution. However, conventional T&D models are in essence soft-string models of the tubulars (assuming that the drillstring is a soft and malleable assembly always on the low side of a deviated borehole). While these models provide a good approximation of forces and contact loads in perfectly smooth well trajectories, they become completely inappropriate in realistic scenarios where trajectories become more complex or where tortuosity (micro or macro) and local doglegs are in play. A subsequent advancement enabled the development of a more-accurate pseudostiff-string T&D model that assumes the drillstring is a rigid body and yielded better results, yet it is still not realistic enough and fails to reproduce measured T&D data from the field mainly because of the fact that in pseudostiff-string model, the locations of the contact points between the drillstring and the casing or wellbore are not computed but still assumed. The latest research in drillstring mechanics modeling has, however, managed to develop a more-precise tool to properly assess T&D and buckling phenomena occurring within the well: the stiff-string model. As a step up from the pseudostiff-string model, this latest development takes into account the stiffness of the drillstring while also computing the position of the drillstring and wellbore contact points, as opposed to merely assuming their location. Since its development, the model has been used in the planning, design, and optimization of a wide variety of well trajectories and calibrated with field data, consistently delivering predictions that are a close match to reality. Thus, the application of the model has unlocked the truthful assessment of the stresses, loads, and forces all along the drillstring, and enabled on one hand better well planning and design, and on the other hand better execution through precise directional bottomhole-assembly (BHA) performance analysis (T&D and buckling for the overall bit/BHA/drillstring system). Despite multiple successful cases wherein the stiff-string model has optimized drilling planning and performance while correctly managing mechanical limitations, it is still an underused tool. Nevertheless, with an exponential rise in trajectory complexity, exemplified by U-shaped profiles drilled in US shale gas reservoirs and geothermal advanced closed-loops, and the drive to extend horizontal sections and push the ERD envelope, its use becomes essential.