Study On Drilling String Dynamics Of Concentrated Drilling And Drilling In Side Drilling

As the development of TaHe oilfield is approaching the production tail, the water content in some old wells is rising and water surrounding production wells is growing. To meet the geological needs of waterproof, re-entry from carboniferous system BaChu formation or Ordovician system SangTaMu formation which are mudstone section was applied. In order to stablize the borelone of mudstone and guarantee150m waterproof distance, a new casing string was used. But there are some problems that regular casing may reduce the borehole diameter of production zone and the cementing quality is poor for slim hole. In order to solve such drilling and completion problems, reaming while drilling (RWD) method in kick-off section is an effective means.Based on the Elasticity and FEM method, considering the force applied on the drill string in curved section, structural characteristics of the drill string as well as WOB and torque distribution on the reamer bit, the models of concentric re-entry RWD drill string dynamics were built. Then the boundary conditions were clarified, after that, the modality of re-entry RWD drill string was conducted, modes of drill string vibration were calculated. Based on the modal analyses, considering the effects of loads on drilling string and reamer bit, this paper analyzed the transient dynamics of concentric re-entry RWD drill string, the equivalent stress, axial acceleration and torque in different cross sections in the process of Rotating drilling were simulated. The drill string dynamic influential factors such as WOB, rotary speed, DLS, inclination and the reaming tools, drill string structure were studied.Concentric re-entry RWD drill string transient dynamics simulation results show that the drill string tends to vibrate along the high and low side of borehole because of the gravity effects in deep well re-entry. At different locations of the well trajectory, farther the distance away from the bit, the greater the severity of drill string vibration, especially the lateral vibration. The equivalent stress at PMD and drill collar location is high, the equivalent stress decreases and axial acceleration increases with the distance from the bit, Lateral acceleration in drill collar location is high and lateral acceleration in wellhead location is low. Rotary speed is the main factor affecting drill string torsional angular velocity, torque, drill pipe stress and wellhead axial force, large WOB will aggravate the drill string lateral vibration, high rotary speed may cause the drill string torsional vibration intensified, proper WOB and rotary speed will weaken the drill string vibration. Drill string tends to rotate within low side of borehole with increase of DLS and inclination, and drill string vibration tends to weaken, but the drill string withstands high bending stress, and the torque and drag will increase too. Under identical drilling parameter conditions, this paper analyzed the vibration characteristics of concentric reaming tools and eccentric reaming tools, simulation results confirmed that concentric reaming bit is more stable than bi-center bit.The recommend RWD drilling parameter:WOB is controlled within30-60kN, rotary speed should keep between30-60r/min. The recommend RWD drill string structure is listed as follow:PDC bit+NBR+Φ120mmPDM+Φ88.9mmNMDC×1-2joints+Φ88.9mmDP×15～30joints+Φ88.9mm HWDP X15joints+Φ88.9mm DP.Research results in this paper could provide the theoretical support for ascertaining proper concentric reaming while drilling parameter as well as the drill string structure.