The sealing performance will directly affect the operation of downhole robot under HTHP condition. Traditional analysis methods of sealing performance are that the temperature and pressure is loaded respectively. This can not really evaluate the sealing performance. Besides, the simulation process is: Step 1: pre-compress O-ring to produce contact force. According to the contact pressure, select the compression ratio to calculate the displacement of the slip ring. Step 2: load fluid pressure on the O-ring. This simulation method is to directly load pressure on the undeformed O-ring. However, the O-ring will deform after pre-compression. Therefore, this simulation method is not accurate. In order to make the simulation data more accurate, calculate the data of shape, stress, and strain of O-ring caused by pre-compression caused by assembly. Then, import the deformation body containing the real data of shape, stress, and strain into a new model. On the basis, establish the numerical simulation model of piston, piston guide rod, O-ring, and FTS-ring with HTHP loads is. Finally, calculate and analyze. When the compression ratio of the O-ring is about 14%, the sealing performance is good. What’s more, the distribution of contact stress and Von Mises of the O-ring at 8.3 mm/s of motion speed are analyzed. The results show that the foot shaped combined sealing structure can keep a good dynamic sealing performance under HTHP condition. This paper provides a theoretical basis for the analysis of the dynamic sealing performance by using HTHP coupling method. In the analysis of sealing performance: the hydraulic pressure is loaded to the real model with the real shape, stress, and strain produced by the O-ring assembly. This can more accurately evaluate the sealing performance under HTHP condition. It also provides a reference for the dynamic sealing structure design of downhole tools.

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.

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