Calculation and Analysis of Drag and Torque for Drill String Considering the Effect of Cuttings in Extended-Reach Drilling

In order to promote the exploration and development of oil and gas resources in complex areas such as mountains, lakes and oceans, extended reach wells have been widely used. However, extended reach drilling generally faces challenges like high drag and torque, buckling of drill string, inadequate hole cleaning, etc., which lead to high drilling risk. Therefore, it is of great significance to accurately predict the drag and torque and extension limit of drill string for safe drilling. In this paper, the pipe-cuttings contact stress is introduced to characterize the effect of cuttings on pipe load, and the mathematical expression of the pipe-cuttings contact stress is obtained through numerical simulations and non-linear regression method. Secondly, the modified drag & torque model of drill string is established through introducing the pipe-cuttings contact stress into the conventional drag & torque model, and it is applied to a case study of an extended-reach well. The results show that the change of flow rate, rate of penetration (ROP) and rotational speed affects the hole cleaning and further affects the torque loss caused by cuttings and the ground torque in extended-reach drilling. This explains the abnormal phenomenon that the ground torque shows a decrease trend with the increase of well depth when increasing the flow rate, rotational speed, or reducing the ROP. Furthermore, the modified drag & torque model can reveal the above synchronous change between flow rate, ROP and ground torque.