Abstract
Since the initial recognition of the hazards associated with gas migration, it has remained one of the most challenging technical issues in cementing under complex conditions. Gas migration hinders the formation of an effective permanent zonal isolation, resulting in interzonal gas migration that contaminates hydrocarbon-bearing formations and severely impairs production of oil and gas wells. Consequently, to ensure both optimal well performance and cementing quality, it is imperative to effectively mitigate gas migration. In this study, a high-temperature, high-pressure gas migration evaluation apparatus that enables the investigation of all potential gas migration pathways within the annulus has designed and fabricated. Utilizing the apparatus, we conducted a series of gas migration evaluation experiments on conventional anti-gas migration cement systems, modified with latex and resin, under varying gas pressures and temperatures to elucidate the pathways and conditions that lead to gas migration. The experimental results indicate that under the tested conditions, gas usually does not infiltrate the cement matrix; rather, the cement interface represents a vulnerable zone that readily serves as a pathway for gas migration. Gas ingress into the interface occurs only when the gas pressure exceeds the cement radial stress. Moreover, gas pressure plays a pivotal role in the migration phenomenon: higher gas pressures result in increased gas flow rates. In all interface experiments, the maximum gas flow rate exceeded 0.5 ml/min, with the highest reaching over 4 ml/min. Although the incorporation of latex and resin can enhance the interfacial properties of the cement, the performance of latex-modified cement is highly dependent on the mixing procedure, and the simple addition of these materials is insufficient to effectively prevent gas migration along the interface. Additionally, high temperature accelerates the cement hydration reaction, expediting the decline in cement pressure and leading to an earlier onset of gas migration. These findings clearly delineate the location and mechanism of gas migration, providing a robust framework for its prediction and prevention in cementing operations.
