Studies Quantify Cement Sealing Performance Under Downhole Conditions
Authors: Carpenter Chris 
Published: June 2023

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This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 212542, “Quantifying the Sealing Performance of Plug and Abandonment Cement Systems Under Downhole Conditions,” by Roel F.H. Roijmans, SPE, Timotheus K.T. Wolterbeek, and Erik K. Cornelissen, Shell, et al. The paper has not been peer reviewed.

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Reliable evaluation of cement-plug sealing performance requires testing under elevated temperature and pressure conditions representative of downhole conditions. In the complete paper, two laboratory setups designed for this purpose are described. In both setups, plugs are cured and tested under downhole conditions inside sections of steel pipe. Using this equipment, a strong positive correlation was found between the amount of interfacial 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 seepage risk after plugging and abandonment (P&A).

Importance of Cement Testing Under Representative Downhole Conditions
The authors devote the first part of the paper to making a case for including cement expansion additives (CEAs) in P&A cement slurry and explain the need for verifying the expanding properties of slurry design under elevated pressure and temperature conditions representative of downhole conditions. Cement-testing equipment currently used by the industry to determine shrinkage and expansion of cement systems includes the ring-mold test and the membrane test. However, these tests do not represent downhole conditions in terms of water access, confinement, pressure, and geometry, although test outcomes in terms of cement expansion can be influenced significantly by these factors (Table 1 of the complete paper lists factors that influence the CEA in creating interfacial stress). Two laboratory setups were designed to evaluate expanding cement slurry behavior under representative downhole conditions, namely small-scale tests on 2-in. plugs and large-scale tests on 8-in. plugs. Multiple cement slurries with varying amounts of CEA were cured and analyzed for sealing performance at representative downhole conditions, and the functionality of multiple CEAs was investigated by exposing them to heated tap water.
The complete paper provides a discussion of experimental methodologies and setups for measuring CEA reactivity and for 2-in. and 8-in. cement-plug sealing-performance-evaluation setups. It also describes a modified ultrasonic cement analyzer (UCA) vessel.

Results and Discussion
Reactivity of CEAs. The reactivity in tap water of two commercially available calcium oxide (CaO) and magnesium oxide (MgO)-based CEAs was determined. The low-temperature CEA was analyzed at 35 and 70°C. The high-temperature CEA was analyzed only at 70°C.
For the thermogravimetric analyses, the CEA was submerged in water (as opposed to cement slurry). As a result, ample water was always available with which the CEA could react. Inside a slurry, on the other hand, the reaction speed was slower because of a reduced amount of water.