# Borehole Condition and Limit Pressure Differential Analysis in Carbonate Reservoirs
## 5 Field Application Examples  
The basic parameters of the reservoir section of Well S1-4 located in an ultra-deep carbonate reservoir are shown in Table 3. The open hole length of the reservoir section is about 730 m, which is a highly deviated well with a maximum deviation angle of about 66\({}^{\circ}\). The drilling azimuth is N270\({}^{\circ}\) E, which is located between the horizontal maximum principal stress and the horizontal minimum principal stress, and the angle with the horizontal maximum principal stress is about 60\({}^{\circ}\). According to the parameters of the reservoir intervals in Well S1-4, the volume ratio of the plastic zone and failure zone near the wellbore and the wellbore volume, as well as the maximum failure angle on the wellbore wall, were calculated under different test pressure differentials, as shown in Figure 10. Based on the three criteria for maintaining \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Value** \\ \hline Vertical depth/m & 7655–8124 \\ Measured depth/m & 7655–8385 \\ Maximum deviation angle/(\({}^{\circ}\)) & 65.85 \\ Borehole azimuth/(\({}^{\circ}\)) & 270 \\ Pore pressure/MPa & 84.2–89.4 \\ Overburden pressure/MPa & 183.5–196.2 \\ Maximum horizontal principal stress/MPa & 169.3–181.7 \\ Minimum horizontal principal stress/MPa & 126.3–135.9 \\ Effective stress coefficient & 0.6 \\ Plastic yield strength/MPa & 58.9–71.6 \\ Peak strength/MPa & 63.0–80.5 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Basic parameter Table of reservoir section of Well S1-4. Figure 9: Variation in the maximum failure angle on the wellbore with the test pressure differential. wellbore stability during testing, the volume ratio of the failure zone to the wellbore volume should not exceed 10%, with a test pressure differential below 23.1 MPa. The volume ratio of the plastic zone to the wellbore volume should not exceed 20%, with a test pressure differential below 17.1 MPa. To ensure that the maximum failure angle on the wellbore wall does not exceed 90\({}^{\circ}\), the test pressure differential should be below 15.6 MPa. Based on comprehensive judgment, the maximum test pressure differential for the reservoir interval in Well S1-4 is 15.6 MPa. Well S1-4 was completed by an open hole with a hole diameter of 149.2 mm. To fully release the production capacity, the maximum test pressure differential reaches 30 MPa, which is significantly higher than the calculated maximum test pressure differential of 15.6 MPa. During the actual testing process, a mixture of cuttings and fluids blocked the oil choke, causing the pressure gauge reading to drop from the initial 40 MPa to 0. When the testing string was pulled out, it was discovered that five oil tubes at the bottom were blocked by debris, indicating wellbore collapse and instability, consistent with the engineering situation and model calculation results. In the subsequent operation of the reservoir, the same model was used to theoretically analyze the ultimate test pressure differential of Well S5-6. The main parameters of the well are shown in Table 4. The length of the open-hole section is about 406 m; the maximum deviation angle is 44.5\({}^{\circ}\), and the drilling azimuth is located at N40\({}^{\circ}\)-45\({}^{\circ}\) E, which is close to the direction of the horizontal maximum principal stress. Figure 10: The variation in the failure zone around the wellbore, the volume ratio of the plastic zone to the wellbore, and the failure angle of the wellbore with the test pressure differential in the reservoir section of Well S1-4. Figure 11 shows the variations in the volume ratio of the plastic zone and failure zone near the wellbore and the maximum failure angle on the wellbore wall with the test pressure differential in the reservoir interval of Well S5-6. According to the calculation results, to maintain the volume ratio of the failure zone to the wellbore volume below 10%, the test pressure differential should be below 26.0 MPa.