Analysis of the main factors affecting bottom hole assembly Re-entry into main hole in forward drilling of fishbone wells

Abstract

Wellbore re-entry is one of the critical technologies in fishbone well drilling. In the sequence of forward drilling, the main hole is designed as a low wellbore and the branched hole is designed as a high wellbore to ensure that the bottom hole assembly (BHA) can re-enter the main wellbore. However, there is no quantitative theoretical analysis regarding the reliability of wellbore re-entry at present. In order to analyze the reliability of the main hole re-entry, an analysis method was provided in this paper. Firstly, a detailed description of the wellbore model around the sidetracking point was built, which can consider the different build-up rate and bending direction of the main and branched hole. Secondly, a mechanical model based on the elastic rod theory was established to calculate the force and deformation of drill strings in the wellbore and a re-entry coefficient was introduced to evaluate the reliability of the main hole re-entry. Moreover, the effects of tool face angle, build-up rate of wellbore, junction wall geometry, and motor structure on the re-entry coefficient are analyzed. Finally, the main controlling factors were analyzed to distinguish the influence degree of different parameters on the re-entry coefficient. From the results, the angle between the bending direction of the branched hole and the tool face of the BHA is the decisive factor in the process of the main hole re-entry. The BHA should be controlled so that the angle is greater than 90° as much as possible. If such angle is less than 90°, the change of wellbore build-up rate, junction wall geometry, and motor structure can be used to increase the re-entry coefficient, such as increasing the build-up rate of the branched hole, wellbore enlargement rate, depth of the slot in the junction wall, and ridge width of the junction wall. Meanwhile, decrease the angle of bent housing, distance between the bending point and the top of bit, and outer diameter of the body stabilizer of the motor can also increase the re-entry coefficient. Among the effects of re-entry coefficient, the main controlling factors are the bend point position and the angle of bent housing.