Tortuosity Factors for Highly Tortuous Wells: A Practical Approach

Abstract Tortuosity is one of the critical factors to be considered for complex directional well trajectories, complicated build rates, precise steering in thin reservoirs and extended reach wells. When planning a well, wellpath modeling commonly generates smooth curves, whereas an actual well contains severe doglegs and other irregularities. Determining the tortuosity factors to apply in the well path is always a challenge during the planning phase. Presently no quantification method is available and field tests have shown that the present calculations gravely underestimate the drillstring torque and drag values for highly deviated, tortuous deep wells. Problems mostly occur when the drillstring lies in a strongly curved wellpath. In the long transitional region that covers most of the operational range in field applications, the conventional equations used for torque and drag calculation results in underprediction. Due to increased extended and steered wells the torque and drag calculations need correction to take care of the tortuous effects of the wellbore. This paper addresses the adverse impact of the torque and drag calculations in highly tortuous wells and presents an approach to calibrate the tortuosity factors whereby the accuracy of the calculations is enhanced. The methodology uses actual survey data and known surface hook loads for different operating conditions, such as tripping in, tripping out, and rotating off bottom, to arrive at appropriate friction factor values that can be used further in torque and drag calculations. Once the friction factors for both cased hole and open hole are determined, sensitivity analyses can be done on the planned survey. Appropriate tortuosity values are derived at by comparing predicted results with the field driller's data. This paper also describes four methods, Sine Wave, Random-Azimuth, Random-Inclination-Azimuth and helix used to calibrate the tortuosity factors that better represent the actual curved wellbore. The simple guidelines are presented along with two field examples from Gulf of Mexico. The simple, pragmatic approach provides a comprehensive framework for calibrating tortuosity values for complicated tortuous wellpaths.