Wellbore integrity is a principal concern for safe and efficient underground gas storage activities, including carbon dioxide (CO2) disposal and hydrogen (H2) accumulation, in aquifers, depleted hydrocarbon reservoirs and salt caverns. In particular, defects in the well hydraulic barrier (i.e., cement sheath) cannot guarantee the confinement of the stored species and can be the cause of undesired groundwater contamination. This study aims at evaluating potential CO2 and H2 migrations through possible leakages at the casing-cement interface by means of a quantitative use of sonic and ultrasonic logs integrated into an analytical flow model. The outcomes are critical information to assess the overall integrity of the storage project under investigation.

Among the long-term factors responsible for possible CO2 and H2 leakages, the proposed method considers cracks and wet microannuli (small fluid-filled gaps between casing and cement). This is because 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 from literature, an ad-hoc relationship between acoustic impedance (by ultrasonic measurements) and microannulus thickness has been established through a generic 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.

The defined analytical model leads to the quantification of the order of magnitude of possible leak flow rates behind casing (including quantitative information from cement placement scenario, completion environment and pressure/temperature regimes). This represents a fundamental input for subsequent wellbore integrity studies and/or remedial job activities. However, the current leakage modeling does not consider frictions, turbulences, and other complex dynamic phenomena. In this sense, the estimated flow rate should be viewed as an upper limit and investigations are planned to further strengthen the current methodology.

Although several approaches exist to aid leak rate quantification behind casing, the microannulus thickness estimation downhole from ultrasonic logging is relatively new and opens the way to critical operative applications in CO2 and H2 storage projects.

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