Analysis of drilling difficulty of extended-reach wells based on drilling limit theory

Abstract

With the continuous increase in vertical depths and horizontal displacements of directional wells, the difficulties of drilling operations continue to increase, and more accurate methods of drilling difficulty evaluation are needed. In this paper, a drilling difficulty evaluation method is built by combining drilling limit model and expert evaluation. Firstly, the concept of drilling difficulty index is introduced, and the method to calculate drilling difficulty index is established. Next, the meanings of five drilling difficulty levels are explained and the optimization design method with drilling difficulty as the target is built. At last, the theoretical model is applied to the extended-reach drilling of the Liuhua oilfield in the South China Sea, in which drilling difficulties are evaluated and the relationship between drilling difficulty and development control radius is revealed. The results indicate that extended-reach drilling in the Liuhua oilfield is on the “normal” difficulty level on average, rotary drilling in 8 1 / 2 -in. section is the most difficult, and the main constraint conditions are excessive torque and high friction. Through technology upgradation, the drilling difficulties are decreased, the development control radius increases from 6.6 to 11.4 km, and the maximum horizontal-to-vertical ratio increases from 5.3 to 8.7. Then, the development wells in marginal oilfields and adjustment wells in old oilfields can be drilled on “normal” difficulty level. Therefore, technology upgradation, especially drilling rig upgradation, is the most important development direction for extended-reach drilling in the South China Sea.