Construction of Ultralong Wells Using a Catenary Well Profile

Abstract

Abstract This paper presents analysis and design of ultra-long wells using a catenary well profile. Ultra long wells have a considerable economic potential as remote reservoir pockets can be drained from existing platforms. There is however, a high economic risk for these wells, many drilling issues must be resolved. The paper presents results from a project undertaken to explore the limits and the potential for ultra-long wells. Wellbore friction is an important issue, and to analyze this a complete catenary model for design of wellpath, drill pipe loads and torque and drag forces are developed. The new model is analytically exact. Standard build profile is compared with a catenary profile. It is shown that the catenary is not as favourable as expected due to friction in the entrance to the catenary, except that friction reduction measures can be applied over a smaller area. Application of light weight drill pipes is more important to reduce load and torque. Both 10 km and 15 km horizontal departure wells are compared. The study shows that a 15 km horizontal departure well can be drilled with existing equipment and modern drilling units. Introduction Well reach has increased significantly the past decade. In the eighties 3 km stepout was common. This increased in the 1990s, and, one targeted 10 km as a likely stepout. Ryan et. al.[1] perform in 1995 a review of technology and conclude that 10 km stepout is likely. This was proven a few years later on Wytch Farm[2], where it was also proven that shallow extended reach wells can be drilled. Mason and Judzis[3] also perform a comprehensive review and identify technological constraints for reach up to 18 km. Although such reach will require fit-for-purpose equipment, it will be possible in a few years. The present paper will mainly investigate the potential for a catenary build section to reach these goals. The catenary profile was first introduced to the oil industry by McClendon and Anders in 1985[4]. Several later attempts have been used, but the method has had limited application. Aadnoy and Andersen5,6 derived the general catenary solution which is applicable for any inclination. Ma et al.[7] presented field cases for catenary wells drilled in China. Aadnoy and Huusgaard[12] showed that by building angle with a constant BHA, the DLS will increase with inclination and an approximate catenary will result. Torque and drag are critical issues for all types of extended-reach wells, and the reader is referred to references 5, 6, 11 and 12 for details on other well geometries. The catenary consept has a high potential, but it requires detailed analyses and follow-up to be sucessful. With reference to Fig. A1, a catenary shape is simply the natural shape of a rope, chain or a drillstring, due to its own weight. The idea it to drill a wellbore with this shape. The drillstring will then (theoretically) hang freely inside the wellbore. Ideally there should be no contact between the hole and the string, and hence no friction. Torque and drag should be minimized this way. In reality there are a few concerns: