Local-Integral Coupling Model of Tubular Strings with Connectors and Its Application in Periodic Sticking Analyses

Summary As tubular structures and wellbore conditions in drilling engineering become more complex, the integral mechanical problem of tubular strings with connectors becomes more complicated, and it exceeds the scope of conventional tubular mechanics. To solve this problem, the concept of local-integral coupling of tubular strings is presented, and a local-integral coupling model is built. The coupling model is obtained by introducing the amended factors from local model into the integral model. Next, the coupling model is applied to extended-reach drilling in the South China Sea. The main reason for a common problem in drilling 12¼-in. sections in X oilfield, named periodic sticking, is studied from qualitative and quantitative aspects. The mechanical drags on connectors when periodic sticking occurs and mechanical extending limits considering mechanical drags are analyzed. The results indicate that openhole keyseat is the main reason for periodic sticking in long openhole drilling in X oilfield. Openhole keyseats are formed by drillstring connectors grinding the borehole wall under rotary drilling mode. Reducing or eliminating openhole keyseats is very important for safe and efficient extended-reach drilling.