On Quantitative Assessment of Effective Cement Bonding to Guarantee Wellbore Integrity

Methane leakage due to compromised wellbore cement integrity may result in operational complications and environmental contaminations in oil and gas wells. In this work, the problem of fluid-driven fracture propagation at the cement interfaces is revisited by a thorough and comprehensive consideration of the non-uniform cement bonding to the formation along the wellbore. While previous works were mainly focused on discharge without attention to mechanical failure or mechanical failure without ties to seepage rate; here, we couple these two analyses to provide a practical aspect of this approach. As revealed by cement evaluation logs, the quality of the cement behind the casing varies and may include flaws in the form of channels or pockets of mud residuals. A novel methodology, initiated with laboratory-scale cement bonding properties using the push-out test, is introduced to estimate the cohesive properties of the cement interface, considering mud removal and mud residuals in the rock. Then, the measured cohesive properties are applied to a field-scale numerical model with an embedded cohesive layer between cement and formation to evaluate the susceptibility of the wellbore to develop cement debonding. The excessive fluid pressure at the casing shoe is assumed to be the source for the fracture initiation. The proposed numerical model has been tested against actual sustained casing pressure (SCP) field tests for validation purposes. This model may estimate the geometry of leakage pathways and predict leakage flowrate within acceptable ranges. The effect of several key factors in the development of SCP due to the cement debonding is investigated. The results show that the early stage of SCP buildup is controlled by the cohesive properties of the cement interfaces (i.e., cement properties), but the cohesive properties have minor effects on the stabilized pressure. The method proposed herein presents a method to evaluate the cement bond quantitatively to be further integrated into cement design.