Study On Structure Of Reaming While Drilling Tools And Design Of Suitable Bottom-Hole Assembly

As a new technology used in optimization of casing program, improvement of well cementation, increase of penetration rate, and decrease of drilling cost, reaming while drilling (RWD) still can not be widely used because of the insufficient acknowledgement of the structure, the bottomhole hole assembly (BHA) design and the rock characteristics of the ladder borehole. Even if the rate of penetration and drilling cost reach the present level, the technology also can bring much benefit and progress in drilling. So it is of great practical value and significance to study the structure of RWD tools and design of suitable BHA.The force and deformation of RWD model was analyzed by finite element method (FEM), which was the base of the BHA model of the RWD. An equation calculating the critical length between the pilot bit and the reamer was set up if the RWD tool could be taken as a rigid body. A calculation method of parameters for BHA with RWD tools was setup by the beam-column theory, and an optimization method of parameters design for BHA with RWD tools was established by the bit and rock anisotropy theory.Based on the elastic mechanics and rock mechanics theory, a calculation method of forces and deformation for the anisotropic formation around the ladder borehole and rock failure criteria was setup. The research showed that the rock around the ladder borehole is closer to the yield condition than the rock around the pilot hole, and is apt to fall into pieces. According to the rule of the strain energy density for rock failure, the method of calculating the ladder effect factor was derived. And a calculation method of ratio for the weight on bit (WOB) was established by the ladder effect factor and the geometry parameters of the borehole.By the theory of structural dynamics, a lateral vibration model and an axial vibration model of BHA with RWD tools were setup. The results showed the lateral displacement and the axial displacement resulting from both the pilot bit and the reamer were much larger than those of BHA with normal bits. There was a point where the axial displacement is zero when the frequency of the axial vibration is constant. And the point could be an optional place suitable for a reamer.The experiments simulating the dynamics of the drillstring with RWD tools showed the ratio of WOB was smaller than what was calculated by the geometry method, and both the lateral and axial displacement were larger than those of BHA with normal bits, which coincides the results by the theory method. By using the pendulum theory and off-center theory, many kinds of BHA with RWD tools were analyzed and evaluated by the optimization method of parameters design for BHA. The rules of effects for different parameters on the deviation were gained, which could be used to estimate the rationality of the BHA design and optimize the operation parameters.