## 3 Results and Discussion
### Effect of the Diameter of the Ball Seat on the Flow Characteristics  
According to the analysis of the flow characteristics of the drilling fluid in the differential motion assembly, the flow had a higher velocity at the inlet of the ball seat, a severe vortex was present at the outlet of the ball seat near the wall, and the pressure drop reached 1.4 MPa when the inlet flow rate was 0.04 m\({}^{3}\)/s. These characteristics will reduce the reliability and service life of the differential motion assembly. To improve these characteristics and further increase the success rate of IPP-Coring operations in deep oil and gas exploration, it is necessary to improve the local structure of the differential motion assembly. Section 3.1 shows that the change in the flow status was largest at the ball seat. Therefore, we studied the effect of the diameter of the ball seat on the flow characteristics for ball seat diameters of 30, 35, and 40 mm. Figure 9 shows the effect of the diameter of the ball seat on the flow velocity. With the increase in the ball seat diameter, the velocity of the drilling fluid in the differential motion assembly decreased remarkably. The maximum velocity in the differential motion assembly with a 40 mm ball seat diameter decreased to 36 m/s, which was only 0.55 times that of the original assembly with a 30 mm ball seat diameter. In addition, the position of the maximum velocity changed; it was no longer at the inlet of the ball seat. This meant that the erosion at the inlet of the ball seat could be  
Figure 8: Pressure contours in the differential motion assembly with different inlet flow rates (0.01–0.04 m\({}^{3}\)/s); the fluid viscosity was set as 100 mPa-s.  
improved more after the diameter of the ball seat was increased to 40 mm. In addition, in Figure 9, the axial length of the vortex area decreased to 0.85 times that of the original differential motion assembly with a 30 mm ball seat diameter when the ball seat diameter increased to 40 mm. To further study the velocity distributions in the vortex area, we extracted the velocity at the monitoring line for ball seats with different diameters, as shown in Figure 10. As the ball seat diameter increased from 30 to 40 mm, the velocity at the axis decreased from 64 to 35 m/s, and the velocity in the central jet area markedly decreased. Moreover, the distance of the vortex center (the place the velocity was near 0 m/s) with the axis increased from 29 to 33 mm. This meant that the radial length of the vortex area also decreased. Therefore, it can be concluded that the increase in the diameter of the ball seat decreased the size of the vortex area. Figure 10 also shows that when the diameter of the ball seat increased from 30 to 40 mm, the velocity near the wall in the vortex area decreased to 0.67 times that of the model with the original ball seat diameter. The decreases in the size of the vortex area and the vortex velocity near the wall would reduce erosion by drilling fluid and the risk of sticking when the differential motion assembly was triggered. In addition to velocity changes, pressure changes also affect the service performance of the differential motion assembly. The larger pressure drop not only increases the load on the drilling pump and further results in unnecessary costs but can also increase the risk of cutting the pin before the ball is dropped, which would unexpectedly trigger the differential motion assembly and cause the IPP-Coring process to fail. Therefore, we studied the effect  
Figure 10: Velocity distributions at the monitoring line for ball seats with different diameters; the fluid viscosity was set as 100 mPa-s.  
Figure 9: Effect of the diameter of the ball seat on the flow velocity; the fluid viscosity was set as 100 mPa-s.  
of the diameter of the ball seat on the pressure distribution.