Prediction of the Maximum Core Length of Well Section over 10,000 m in Depth Based on the Analysis of Drillstring Dynamics

Summary The SDTK-1 well is the first ultradeep well exceeding 10,000 m in China and Asia, undertaking the dual tasks of deep-Earth scientific exploration and oil and gas discovery. Ensuring the success of coring operations at a depth exceeding 10,000 m is of utmost importance. Due to its length exceeding 10,000 m, the coring drillstring has complex and dynamic characteristics, which made designing the coring drillstring and the predicting the maximum core length significantly challenging. In this paper, we present a maximum core length prediction method based on the dynamic analysis of the drillstring. First, we establish a dynamic model of the drillstring, and solve the dynamic characteristics of the drillstring with lengths exceeding 10,000 m by combining the Newman method and the node iteration method. Then, we study the dynamic stability of the coring bottomhole assembly (BHA) and the contact interaction between the inner and outer barrels of the coring tool. Finally, we predict the maximum core length, and on this basis, discuss the rationality of the coring BHA’s structure. The results show that the core sampling ability at depths exceeding 10,000 m is closely related to the structure of the coring BHA, as well as the whirl trajectory and the maximum dynamic displacement of the outer barrel axis of the coring tool. The maximum core length mainly depends on the position of the contact points between the inner and outer barrels. The actual core length is 6.20 m, while the predicted result is 6.12 m. The coincidence rate reaches 98.7%, which proves the correctness of the method for predicting the core retrieval length by considering the dynamic contact relationship between the inner and outer barrels of the coring tool. By comparing the maximum core lengths corresponding to the three coring BHAs, it was found that the designed coring BHA structure has a more reasonable structure.