Analysis Of The Contact Problem Between The Steerable Rib Of The Static Push-the-bit RSS And Wellbore Wall

Rotary steerable drilling system(RSS)has greatly improved the technical level of directional drilling.Now the static-push-the-bit RSS has been developed in China and is prepared for large-scale field test and application.However,the field practice has showed that the contact issue between the steerable rib of the static-push-the-bit RSS and the wellbore wall may lead to the occurrence of the complicated downhole situation.At present,the problem has not been solved effectively at home and abroad.In order to ensure the build-up rate and the accuracy of wellpath control and prevent the self-lock of steerable tools and the yield failure of the wellbore wall,it is necessary to study the contact problem between the steerable rib and the wellbore wall,and finally to realize the improvement of the steerable rib's structure and the optimization of the drilling parameters.On the basis of understanding the structure and working mode of the existing steerable rib,the contact mechanics model between the steerable rib of the push-the-bit RSS and the wellbore wall is established by using the finite element method.The main influencing factors of contact pressure and the degree of influence are determined by orthogonal test.And then the process of wellbore wall's failure and the influences of various factors are further studied.Based on the above analysis and aimed to avoid the wellbore wall's failure,the improvement measures are put forward from two aspects which include the control scheme of the steering force and the structure of the steerable rib,and then the results are verified by simulation analysis.The results have showed that:(1)the contact pressure between the steerable rib and wellbore wall is mainly related to the wellbore wall's elastic modulus,the steering force and the curvature radius of the steerable rib,and it increases with the increase of wellbore wall's elastic modulus and the steering force,and decreases with the increase of the steerable rib's curvature radius;(2)while drilling through soft formation,the steerable rib will gradually press into the stratum,and the pressing speed will be obviously accelerated when the rock mechanical parameters of the wellbore wall and the radius of curvature of the steerable rib are decreased and the push force is increased;(3)reducing the push force of the single rib under the premise of not changing the steering force and changing the structure of the steerable rib from inclined push into flat push can effectively prevent the damage of the wellbore wall and the steerable rib pressing into the stratum.