Combined Control Mechanism of Weight on Bit and Rate of Penetration with a Downhole Robot in the Coiled-Tubing Drilling Process

Abstract

Summary Downhole robots were used to solve the problem of downhole tool transportation in an oil/gas horizontal well. However, current downhole robots do not control the weight on bit (WOB) and rate of penetration (ROP). This paper proposes the combined control method of WOB and ROP using an electric proportional overflow valve (EPOV) and an electric proportional throttle valve (EPTV). First, the mathematical model of the electrohydraulic control of the downhole robot is established. It is found that when the maximum pressure of the EPOV is greater than the differential pressure between the inner and outer of the downhole robot, the control parameters are drilling-fluid displacement and circulation area of the EPTV. When the maximum pressure of the EPOV is less than the differential pressure between the inner and outer of the downhole robot, the control parameters are drilling-fluid displacement, circulation area of the EPTV, and pressure of the EPOV. Moreover, it is found that the relationship of WOB and ROP in the combined control method is a surface rather than a line in a 2D coordinate. Therefore, the downhole robot can be adjusted while drilling at a stable ROP or a stable WOB. Finally, the combined control method of WOB and ROP with the downhole robot proposed in this paper was verified with an experiment. According to the experimental data, it is further found that an EPOV cannot only control WOB and ROP, but also can control the upper limit of WOB fluctuation. Thus, the control of WOB fluctuation can protect the bit from damage and prolong the life of the bit. This paper presents a foundation for the control of WOB and ROP with downhole robots. It has scientific and engineering significance for promoting downhole robots in drilling engineering.