Research on drilling along steep coal seam:A case study of Changji area in southern margin of the Junggar Basin

Aiming at coalbed methane(CBM) well drilling problems of high dip angle and complex stress in the south- ern margin of the Junggar Basin,the influence of the coal seam dip angle,in-situ stress and building-up (dropping) on wellbore stability were studied based on weak plane theory,and a multi-objective constraint drilling trajectory optimiza- tion model was established. The results show that ① the dip angle of coal seam in Changji area is 10°-25°,and partly reaches 25°-40°,which belongs to high steep coal seam. It has the characteristics of strong permeability anisotropy, strong stress anisotropy and strong rock mechanics anisotropy. ② When drilling horizontal well,according to the influ- ence of coal seam dip angle on wellbore stability,it can be divided into working zone,transition zone and inversion zone. It is working zone when dip angle is between 5°and 25°. Drilling along dip direction and strike direction are the safest and most dangerous orientation,respectively. When dip angle reaches 65°,it enters inversion zone,drilling along the dip direction and strike direction both become the safest orientation. The greater stress anisotropy,the greater col- lapse pressure and the smaller orientation range of safety drilling. ③ When drilling along coal seam,drilling along up- dip direction is proposed if the dip angle is less than 35°;drilling along down-dip direction is proposed if the dip angle is more than 65°;the wellbore safety is the lowest in the range of 35°-65°,then drilling along trend direction is pro- posed. It is the most dangerous building up (drop) section when the inclination angle is between 45° and 65°,build- ing-up (dropping) in coal seam should be avoided. ④ In order to reduce the collapse pressure from the landing point to the first target,S-type and J-type drilling trajectory types are proposed,the comparison results show that the trajectory de- signed in new method is shorter,less friction and torque than the trajectory designed in conventional method.