Study On The Motion States Of Bottom Hole Assembly And Its Master Mechanism

With the development of exploration and development of complex oil and gas resources,complex structure wells are widely used.Especially,with the application of various advanced downhole devices,trajectory control is more accurate and efficient.However,because the precision measurement and control devices are more sensitive to the severe vibration,it is necessary to understand the motion states of BHA and its master mechanism.Thus,the BHA motion states and its master mechanism are systematically studied,and a reliable analysis method of drillstring dynamic is strived to form,which can provide theoretical bases for the development of drilling theory and technology.Firstly,because of the problem of wellbore misplacement and drillstring with bending angle in the existing dynamic experimental setup,a new setup of BHA motion states with higher accuracy and more comprehensive consideration was developed.Then,the experimental studies on BHA motion states and the motion rules were carried out.The results show that well inclinations,rotation speeds and friction coefficients have significant effects on the motion states.There are four dominating factors of BHA motion states,including radial component of gravity,centrifugal force,tangential friction and internal friction of material.The radial component of gravity can promote circular arc swing,but hinder complete revolution of BHA.The centrifugal force can promote forward whirl before drillstring contacts wellbore,and promote backward whirl after drillstring contacts wellbore.The tangential friction can promote backward whirl,but hinder forward whirl.The internal friction can promote forward whirl,but hinder backward whirl.Meanwhile,combined with the advantages of beam theory,rotor dynamics and finite element method,a finite element dynamic model with three-dimensional beam elements was established,and the four dominating factors of BHA motion states were considered in the model.A processing method of drillstring initial deformation in curved wellbore was given,so as to conduct dynamic analysis in three-dimensional wellbore.And the second-order accuracy HHT-α implicit solution method was used to solve the dynamic model.Then,the rationality and practicability of the dynamic model were verified by indoor experiment and field application.Moreover,based on the results of laboratory experiment and numerical simulation,five types of BHA motion states were summarized,and the physical characteristics,generation conditions and recognition methods of these motion states were given.The results show that the motion states can be divided into forward whirl,irregular motion and backward whirl in the straight section and small inclined section.The forward whirl generally occurs at low speeds,the lateral acceleration is small and the fluctuation frequency is low.The irregular motion generally occurs at medium and low speeds,and there is no fixed rule of lateral acceleration.The backward whirl generally occurs at medium and high speeds,the lateral acceleration is large and the fluctuation frequency is high.The motion states can be divided into circular arc swing and dot-like circular motion in the inclined section and horizontal section.The circular arc swing is the most important motion state,which occurs in most areas of BHA without disturbance of other downhole tools,and the dot-like circular motion occurs in the narrow position,which is near the full-hole downhole tools such as bit or stabilizer.Furthermore,experimental and numerical simulation study on the master mechanism of BHA motion states were conducted.The principle of least energy consumption and the principle of least power consumption were used to systematically clarify the master mechanism of BHA motion states.The results show that BHA will choose the motion state with the least output power of the system at any time,and will choose the motion state with the least input power of external force as its stable motion state.In particular,if BHA wants to form backward whirl,it needs to cross the peak point of external power dissipation firstly,which proves that the conditions of forming backward whirl are stricter than those of maintaining backward whirl.Therefore,the backward whirl is a stable motion state,and is difficult to stop once it occurs.Finally,for the UPC-VDS vertical drilling system,a dynamic analysis method of the vertical drilling system was formed,the influences of drilling parameters and drillstring structures on its dynamic characteristics and dynamic drop-off ability were analyzed,the optimization methods of drilling parameters and drillstring structures were obtained,and the field application was analyzed.The results show that there is an optimal rotation speed when using the tool for vertical drilling.If stick-slip vibration is serious,it is recommended to increase rotation speeds appropriately.If lateral vibration is serious or the tool has obvious wear phenomenon,it is recommended to reduce rotation speeds appropriately.Under the condition of no backward whirl or helical buckling,it is recommended to increase WOB appropriately,which can effectively reduce lateral vibration and improve ROP appropriately.For singlestabilizer BHA,the stabilizer has an optimal position.If the position is moderate,the whirling radius of drill string is small,the lateral acceleration is minimum,and it has a good drop-off ability,thus the UPC-VDS can have good motion stability and high drop-off ability.Combined with laboratory experiments,numerical simulations,theoretical analyses and field applications,the BHA motion states and its master mechanism were systematically studied,and a reliable analysis model of drillstring dynamic was formed,which can provide theoretical guidances for the prediction and control of BHA motion states and provide a powerful analysis method for the researches of drillstring dynamics.The researches have both theoretical and practical value.