# Research on Pore Pressure Detection While Drilling Based on Mechanical Specific Energy
## 3 Results  
To demonstrate the applicability of the new technique in deep and complicated lithological formations, we took well L and well M in a region in western Sichuan as case studies. In this paper, all depths were referenced to the true vertical depth (TVD) below therotary table. The marine strata start from the Leikoupo Formation at about 3850 m in this region, and the strata above the Leikoupo Formation are dominated by elastic rocks such as mud, shale, and sandstone, while limestone and dolomite gradually become the main lithology in the following strata. According to the actual pore pressure measurements in the overpressure intervals of the offset wells, the pore pressure gradient is about 1.70 sg in this region near 4925 m. Therefore, the interval in well L near this depth was selected as the calibration interval for the Eaton exponent. As shown in Figure 2, the average Eaton exponent was fitted to 0.226. The well M is a sidetracking well, 6510 m deep, and was sidetracked from 5195 m with a maximum inclination of 20.12 degrees. In this well, PDM was used extensively for compound drilling, so both the influence of wellbore friction and the contribution of PDM to the total MSE should be considered. The most realistic downhole parameters can be obtained using MWD tools, but they are costly. Here, the downhole torque was computed from surface measurements (hook load and wellhead torque), and the results are plotted in the Figure 3b. The overall compliant trend of downhole torque is lower than the wellhead torque due to the friction of the drill string with the wellbore wall. Moreover, as the depth increases, the friction gradually increases. In the absence of reliable torque measurements, some studies have estimated torque from WOB, bit coefficient of sliding friction and bit size. The same operation was made here. The sliding friction coefficient of the bit is related to the type of bit, lithology, rock strength, mud weight and bit wear. To minimize the error in calculating MSE, it is reasonable to take 0.25 for roller-cone bits and 0.5 for poly-crystalline-diamond-compact (PDC) bits [45]. Since the friction coefficient between the drill string and the well wall was not sufficiently known in well M, the WOB was calculated using the data from the field. The WOB recorded in the field used the total weight of the drill string minus the hook load measured while drilling, which may have some error with the real WOB; we simply take a trend of variation. In fact, the contribution of WOB to MSE is rather limited compared to torque, especially in deep intervals where PDC bits are mainly used, and rocks are largely broken by the shearing of the bit. Figure 2: Estimation of the Eaton exponent from well L.  
The section from 1950 m to 6125 m was under compound drilling conditions. Based on the type of PDM and flow rate used in each section, the torque and rotary speed of the PDM were calculated. In contrast to the rotary table, the PDM drive is the main power for the bit during compound drilling. Combining the torque and rotary speed of the rotary table, the total torque and total rotary speed were calculated separately, as plotted in Figure 3.  
Taking the sidetrack section in the well M as an example, the MSE was computed by using the wellhead torque, the downhole torque, and the torque estimated from WOB, respectively. Figure 4 illustrates the comparison of the pore pressure detection results for these three cases. The differences in the comparison of the MSE plotted in Figure 4b are striking, especially since the MSE computed from the torque estimated by the WOB is significantly lower than in the other two cases, indicating the high sensitivity of the MSE to torque. Typically, the mud weight (MW) has a safety adder of 0.07 to 0.15 sg compared to pore pressure. In Figure 4c, the MW was reduced by 0.15 sg to serve as a rough reference for the comparison of pore pressure detection values.