Titanium drill pipe have promising application prospects in ultra-deep drilling, but the nonlinear dynamic characteristics of composite drill strings are very complex. It is very important to use titanium drill pipe safely, economically, and effectively in the drilling process. In this paper, different schemes of the titanium/steel drill pipe composite drill string was designed, and the statics and dynamics characteristics of these composite drill strings were analyzed. The design scheme of titanium/steel composite drill strings were optimized by using whirling characteristics, dynamic stress and vibration acceleration. The results show that the titanium drill pipe can effectively improve the tensile allowance of the wellhead and reduce the whirling velocity, dynamic stress and vibration acceleration of the drill string. By comparing and analyzing the dynamic characteristics of 3 kinds of composite drill strings, the use of titanium drill pipe for the lower 2000 m of the 4′′ drill pipe have the smaller dynamic stress and lateral vibration, and less collision with the wellbore. Based on the analysis of vibration and dynamic characteristics, an optimal scheme of drilling operation parameters for the titanium/steel composite drill string was formed finally, which can provide guidance for the use of the drill string composed of titanium/steel drill pipe in natural gas drilling of ultra-deep well.

In the drilling process of horizontal and highly deviated well sections, cuttings are deposited on the lower side of annulus under the action of gravity during its transport, which affects the normal drilling operation. A variety of cuttings removal tools have emerged and achieved good results, but they are only effective when the drill string is rotating, while it is difficult to ensure the removal effect of cuttings in sliding drilling. In the drilling process of directional well and horizontal well, rotary drilling is sometimes difficult to meet the requirements of wellbore control, and sliding drilling is often used at this time. Therefore, there is an urgent need for a tool that can effectively remove cuttings in sliding drilling. In the present work, an efficient vortexing cuttings removal tool (VCRT) has been developed and the Computational Fluid Dynamics (CFD) simulation technology was used to study the cuttings removal mechanism of VCRT. The cuttings removal effect under the action of VCRT was compared with that of the conventional cuttings removal tool (CCRT) and the conventional drill pipe. The results show that VCRT has good cuttings removal effect in both sliding and rotary drilling conditions. The average cuttings volume fraction under the action of VCRT during sliding drilling is 59% lower than that of the conventional drill pipe, and 26% lower than that of CCRT. The average cuttings volume fraction under the action of VCRT during rotary drilling is 78% lower than that of the conventional drill pipe, the removal efficiency is similar to CCRT, but VCRT can send more cuttings into the annulus. The research reveals the mechanical mechanism of VCRT which provides a scientific basis for the cuttings removal mechanism of VCRT.