Torque and drag modelling for Redhill South-1 in the Northern Perth Basin, Australia

Abstract

Drilling deviated and horizontal wells is commonly used in the oil and gas industry for different purposes. Particularly in unconventional reservoirs such as gas shales or tight formations, horizontal wellbores provide a larger exposure to the formation, which enhances the production from such tight formations. The increase in torque and drag forces downhole in deviated borehole trajectories is one of the technical challenges that needs to be carefully studied during the design phase. There have been a number of different approaches to the way that torque and drag has been modelled in the industry. These include the soft string and stiff string approach and accounting for the effects of viscous fluid flow. The soft string model treats the drill string as a cable and assumes that it lies against the low side of the wellbore, meaning that the stiffness of the drill string is not accounted for. On the other hand, stiff string models take into account the stiffness and bending moment in the drill string and the radial clearance in the wellbore. Fluid flow during drilling results in the loss of the normal component of fluid pressure on the drill string as the flowing condition becomes dynamic. There is also an additional tangential component caused by viscous drag on the drill string due to the fluid flow. In this paper we will present the findings of a study aimed at determining the most appropriate type of torque and drag modelling approach that should be applied for Norwest Energy’s Redhill South-1 well and future wells of similar nature. Redhill South-1 was directionally drilled to test the gas potential of the Permian sands in a fault dependent structural closure. Both soft and stiff string approaches will be studied. The necessity to account for viscous drag effects will also be analysed.