Evaluating the Sealing Performance of Plug and Abandonment Cement Systems Under Downhole Conditions

Abstract

Summary Microannuli are an important cause of post-abandonment seepage along abandonment plugs set in wells. Autogenous cement shrinkage, occurring when the cement sets, is a contributing factor to the formation of such flaws. Cement shrinkage and its potentially deleterious effects can be mitigated by using a properly designed and placed expanding cement system. Reliable evaluation of cement plugs’ sealing performance requires testing under elevated temperature and pressure conditions representative for downhole conditions. In this contribution, two laboratory setups designed for this purpose are described, namely (i) small-scale tests on 2-in. plugs and (ii) large-scale tests on 8-in. plugs. In both setups, plugs are cured and tested under downhole conditions inside sections of steel pipes. These pipes are equipped with lateral pressure-temperature sensors, which allow the recording of stresses that develop on the cement-steel interface when the hydrating cement plug expands. Using this bespoke cement testing equipment, a strong positive correlation was found between the amount of interface stress buildup and the sealing performance of the set cement plug against gas flow. This demonstrates that cement systems that expand effectively under downhole conditions can reduce post-plug and abandonmen (P&A) seepage risks. While providing valuable insight, the 2-in. and 8-in. plug setups are labor intensive to operate and hence impractical to use in field laboratories for cement slurry verification. Unfortunately, standard industry methodologies to verify the effectiveness of the cement expansion additives (CEAs) do not take into account in-situ well conditions. To allow the industry to still evaluate cement plug sealing performance with low complexity while maintaining reliability, a conventional ultrasonic cement analyzer (UCA) was modified to enable simultaneous measurement of interface stress development. Similar to the 2-in. and 8-in. plug sealing evaluation setups, this modified UCA vessel was equipped with a lateral pressure-temperature sensor, to monitor interface stress between the cement and vessel wall with time. For this contribution, multiple cement slurries were cured and evaluated in the 2-in. and 8-in. plug evaluation setups, as well as in this newly developed modified UCA vessel. Strong correlations were found among the stress buildups in all three setups. This enables the modified UCA test results to be used as a proxy for sealing performance of P&A cement plugs under downhole conditions.