Study of well tractor conveyance technology for ultra-long horizontal well in shale gas reservoir

As one of the most convenient conveyance tools in shale gas horizontal wells, the well tractor plays a crucial role in conveying downhole tools. With the emergence of ultra-long horizontal sections in shale gas wells in recent years, traditional tractors may face challenges related to insufficient pull force. As the well inclination increases and the length of horizontal well section increases, the required pull force also increases. Research on the force characteristics of tractor conveyance revealed that when the well inclination reaches 120° and the section length reaches 4500 m, the required pull force can reach 10555 N, far exceeding the pull force provided by traditional tractors. To address this situation, this study investigated two aspects to improve pull force: improving the friction of drive wheels and optimizing the tractor's power system. The research shows that the friction force can be changed by controlling the normal force through the hydraulic system, which can effectively improve the pull force. When the normal force is between 0 and 10 MPa, pull force steadily increases, reaching a maximum of nearly 10000 N. When the normal pressure exceeds 10 MPa, pull force begins to decline. Therefore, setting the hydraulic pressure (normal force) provided by the hydraulic system to a continuously adjustable range between 0 and 10 MPa ensures efficient pull force output. Second, replacing the triangular-shaped teeth drive wheel with a conical tooth drive wheel can increase pull force by 12 %–14.9 %. Furthermore, with respect to the drive power system, the efficiency of the tractor can be improved by using reducers with varying reduction ratios. The higher the reduction ratio, the greater the tractor's power output. Finally, the feasibility of the proposed methods is validated through case studies, offering valuable insights and references for the development of tractor conveyance technologies for horizontal wells.