Three-dimensional mechanical analysis and structural parameter optimization of pre-bent BHA used in Ø311.2 mm hole in Changning shale gas block

In the development of Changning shale gas, pre-bent bottom hole assemblies (BHA) have been increasingly gaining attention owing to their cost advantages. However, its trajectory control performance is significantly affected by formation factors, BHA configuration, and drilling parameters, necessitating parameter optimization design. In this study, three-dimensional mechanical characteristics of pre-bent BHA used in Ø311.20 mm wellbore in this block are analyzed, and a practical pre-bent BHA structure optimization method based on actual drilling site conditions is proposed on the premise of ascertaining the law of bit side force variation. That is, according to the specifications of field-available positive displacement motors, stabilizers and short drill collars, the spacing between the stabilizers, the outer diameter of stabilizers and the bending angle of the positive displacement motor are optimized successively, so as to obtain reasonable steering force on the bit in dual drive drilling mode and maximize the length of drilling interval. The method has been applied in three horizontal wells on Changning 209H5 platform, and sound results have been obtained. In addition to meeting the requirements of well trajectory control, the dual drive drilling interval proportion account for 93.33%, 86.73% and 86.01%, respectively, significantly improving the operation efficiency. This method can provide robust technical support for the design of pre-bent BHAs in Changning and similar blocks.