Prediction of Casing Failure Based on Spontaneous Potential Baseline Shift Magnitude

Casing deformation in water-flooded shale reservoirs threatens well integrity and long-term production. This study presents a quantitative method that uses the spontaneous potential (SP) baseline offset magnitude (ΔSP) to assess water invasion and predict casing failure. Multi-year logging data from the Nen-2 Formation in the Lamadian Oilfield were examined to establish an exponential relationship between ΔSP and the time since completion. The results show that ΔSP increases progressively as formation water gradually enters the shale interval. The ΔSP magnitude is mainly controlled by the degree of water flooding and the resistivity contrast between mud filtrate and formation water. Statistical analysis indicates that when the average ΔSP exceeds 4.4 mV, the probability of large-scale casing deformation increases sharply. Based on these findings, a ΔSP-time prediction model was developed to provide a practical tool for evaluating formation invasion and identifying early-warning thresholds for casing failure in low-permeability shale reservoirs. The proposed approach offers a simple and effective geophysical indicator for monitoring wellbore stability in mature water-flooded fields. It can also support reservoir management by helping operators anticipate structural failures and implement preventive measures. Overall, this study demonstrates that ΔSP serves as a sensitive and reliable parameter for tracking water-flooding processes and diagnosing casing integrity risks in shale formations.