Wellbore friction represents one of the biggest limitations for drilling and completion of long 3-dimensional wells. Traditionally, wellbore friction forces calculation is performed using soft-string torque and drag models, which assume tubular to be in contact with the wellbore at any point along its length. However, precise results are needed for wells with complex geometry and high doglegs.
This paper presents a novel way regarding wellbore friction forces calculation, which takes into account both wellbore deviation and wellbore tortuosity. To locate contact points of the string and the wellbore, a Dogleg Severity filter, or DLS-filter is proposed. The DLS-filter is integrated into soft-string torque and drag models by taking into account dogleg, wellbore geometry and depth. Such simple implementation of DLS-filter makes it applicable for any case only if survey data is available.
Fundamental understanding of drillstring mechanics and drilling fluids properties is essentially required in planning phase and drilling operations. To enhance the accuracy of torque and drag calculation, thermal effects on buoyancy forces and viscous forces have been studied. Experiments using one oil-based mud (OBM) recipe and one water-based mud (WBM) recipe have been conducted to measure viscosity and density of fluids in different pressure and temperature conditions. Based on the obtained results, viscosity model and density model as functions of pressure and temperature have been developed for better model interpretation of fluids thermal effects in HPHT conditions. Friction factor is a critical parameter to affect wellbore friction, which depends on fluids composition, contact surface, rotary speed, temperature, etc. Conventionally it is set constant for friction forces calculation. Experimental results show that the friction factor is heavily dependent on the temperature. In this study, friction factor was assumed to increase linearly with temperature for torque and drag calculation.
The new approach provides more correct values for torque and drag, and gives a better understanding of the downhole environment, as cuttings transport and drillstring dynamics. The study can be further used for the evaluation and recommendation of drilling muds for HPHT wells. Such analysis will aid in the design of appropriate drilling mud in the integrated well planning phase.
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