The Road Through Microannuli: Advanced Ultrasonic Log Analysis and Mechanistic Modeling for Leak Rate Quantification

Wellbore integrity is a principal concern for safe and efficient hydrocarbon production activities, as well as for fluid injection and storage purposes. In particular, defects in the well cement sheath cannot guarantee the hydraulic confinement of the desired fluid (e.g., reservoir hydrocarbons/water and injection/storage fluids) and can be the cause of undesired leakages and/or contaminations. This study aims to evaluate potential migrations of those species through possible pathways at the casing-cement interface by means of a quantitative use of sonic and ultrasonic logs integrated into an analytical flow model. The proposed method considers cracks and wet microannuli (small fluid-filled gaps between casing and cement). They can represent the main and sneaky preferential leak pathways in case of such wellbore integrity issues. The first step is the identification of the presence and location of a wet microannulus from sonic and ultrasonic log responses. Then, by exploiting different experiments/simulations found in selected literature, an ad-hoc relationship between acoustic impedance (by ultrasonic measurements) and microannulus thickness has been established through a dedicated analytical model suitable for various completions and cement property scenarios. This allows the proper estimation of microannulus thickness downhole using ultrasonic log data. Next, the obtained value is used as input to Poiseuille’s equation for the final behind-casing flow rate estimation. The model also relies on the pressure drop along the considered vertical portion. It is corrected for gravitational effects and strongly depends on the properties and behaviors of the leaking fluids. Several sensitivity analyses according to the referred input have been performed to evaluate the main model dependencies and, as a consequence, the associated uncertainty. The defined analytical model leads to the quantification of the order of magnitude of possible leak flow rates behind the casing (together with quantitative information from cement placement scenario, completion environment, and pressure/temperature regimes). This represents a fundamental input for subsequent wellbore integrity studies, remedial job activities, and/or proper mitigation actions. To date, the model has been applied to dozens of scenarios, including flow rate quantification of undesired water movement behind casing, gas leakages through casing shoe, and stimulation efficiency evaluation in the presence of possible communication between perforations. Selected case histories are presented to highlight the versatility of the developed methodology. Applications to underground storage projects (e.g., carbon dioxide) are also foreseen. The novelty of the approach relies on the downhole application of the microannulus thickness estimation from ultrasonic logging. This allows an effective leak rate quantification behind casing, and it opens the way to critical operative applications.