The Theory And Testing Study Of Two-stage PDC Bit

For hole sections that the size is equal or more than 311mm is increased along with increasing of deep wells and ultra-deep wells, improving their ROP already became an important problem that be in dire need of resolving. Two-stage PDC bits advance ROP effectively, because a section of smaller hole drilled first released the stress of formations and formed a large free face. This paper has immediate significance to research two-stage PDC bit completely.①On the Base of theory and method of rock excavating and finite element, the finite element models of the bottoms of a two-stage PDC bit and a regular PDC bit are set up. It is bring forward three conditions that are before spudding, during process of drilling, and after the bottom formed are simulated more truly by three time steps. A finite element analyses program is developed. A series of bottom fields of bits are obtained by program. The mechanics mechanism of heightening ROP is studied through comparing the stress field of two-stage PDC bit with that of regular PDC bit. The optimum pilot's diameter is obtained through comparing the stress fields of various two-stage PDC bits with a different size pilot.②Using the theory of non-Newtonian fluid mechanics, drill-stem hydraulics, and bit hydraulics, system of equations on flow rate distributing of two stages and the simplified formula of parameter influencing are derived, a program of distributing flux of two stages computing and nozzle sets design is designed. The regular pattern and affect curves that distributing flux of the pilot is decreasing with increasing of the equivalent diameters of nozzle assembly in pilot, and increasing with increasing of the equivalent diameters of nozzle assembly in enlarger, and decreasing with lengthening of section length between two stages, and ranges of other parameters affecting are achieved, through analyzing by changing parameters. Further the basic principle of flow distributing of two stages is put forward.③Using finite element method and gap element method, using the whole well stem as research object, putting forward hypothesis of bit-formation contact is elastic contact, and the idea of using modern multi-direction gap contact theory to realize gap element, the finite element models of BHA of a two-stage PDC bit and a regular PDC bit is set up. Through analyzing performance of real PDC bits and curve fitting, and then thinking about bit-formation friction, PDC bits'torque formula is obtained. It is bring forward that hook load under zero weight-indicator WOB and WOB under given weight-indicator WOB are calculated by iterative process, which simulated the process of WOB-torque answerbacking, steadying, and oscillating truly in drilling. Taking the boundary conditions of mud buoyancy, mud resistance, stem-wall friction and torque, and torque on bit into account, a finite element analyses program is designed, the feature of controlling well deviation of BHA conjugating two-stage PDC bit is researched. Analyzing of tower BHA and tower pendulum BHA conjugating two-stage PDC bit by the model shows that they has good anti-deviation effect under low well deviation and larger borehole enlargement factor.④On the base of design theory and method of regular PDC bits, the design process, flow and method of two-stage PDC bits is established, a optimum design software of two-stage PDC bits is developed.⑤The validity of conclusion of theory study and the rationality of bit design are verified through field test.