Abstract
Bent-housing positive displacement motors (PDM) keep the leading position among tools for drilling directional wells. The efficiency of drilling deviated wells can be greatly enhanced by the method of Targeted Bit Speed (TBS). This method enables well deviation with the continuous drillstring rotation by means of amplitude modulation of PDM's rpm, which is provided by controlling the fluid flow oscillations in its working chamber. In those cases when the required dog-leg severity cannot be achieved with TBS technology, the system's operation can be suspended without retrieving the BHA to the surface with further directional drilling by conventional PDM orientation method.
In order to control and direct a well trajectory in an efficient manner, it is essential to be able to assess effect of the system parameters on deviation. The paper proposes a numerical model of a BHA equipped by TBS. The model is based on the universal 3-D dynamic drillstring-in-hole model DYNTUB, which has been previously developed by the authors. For implementing TBS technology the latter model has been refined to integrate the modulation of motor rpm with the specified amplitude setting the rate of inclination angle increase/decrease and with the phase determining horizontal angle. The deflection increment of the drilling vector against the previous position is determined by averaging the inclination and horizontal angles in one revolution of the drillstring. The model accounts for the dependence between the ROP and the bit rpm composed of constant drillstring rpm and PDM rpm, which value is subject to periodic changes during rotation.
The developed program was used to analyze the build-up rate of wells drilled by TBS and oriented assembly. The analysis covered straight wellbores with different orientation in the initial state (vertical and horizontal), various types of near-bit section to kink, as well as the effect of the PDM rpm modulation amplitude and the lateral bit milling capacity. Despite of the fact that in most instances the build-up rate with TBS was slightly lower than when drilling with an oriented assembly, this was compensated by a far more valuable advantage of a significant reduction in drag force during drillstring rotation. Examples of calculating the drilling directional control of S-shaped well sections at Vostochno-Urengoysky area in Yamalo-Nenets Autonomous District drilled by the TBS method and their comparison with the planned and actual trajectories are presented. The ways to improve the efficiency of well drilling by the TBS method are provided.
