Experimental investigation of the interfacial debonding strength of class G cement and the implications to well integrity

Abstract

For the repurposing of plug and abandoned (P&A) wells for the Carbon Capture and Sequestration (CCS) project, it is essential to know the condition of the well plugs. These plugs serve as a barrier that restricts the movement of unwanted fluids to the surface. Therefore, the integrity of the cement plug becomes essential. Thus, this study focuses on the interfacial debonding strength of the cement, which is a crucial parameter that must be quantified to find the integrity of the plug. The methodology used for this testing consisted of the novel apparatus established at the University of Oklahoma to find the debonding strength of the cement. The cement used for these experiments consisted of neat Class G mixed according to the API standard. The samples were cured for seven days in the pipe, which had a diameter of 2″ and a height of 6″. Different test variations were conducted, including thermal cyclic loading, transient, elevated, and room temperature tests. The samples were heated with the help of a thermal jacket, whereas the water for the hydraulic debonding test consisted of room and elevated temperature (95 °C). The experiments found that before the complete debonding of the cement plug, the leakage of water (wetting phase) on the sample surface was observed, which happened at a lower pressure than the interfacial debonding pressure. This wetting phase starts at a low pressure, i.e., around 500 psi, compared to debonding pressure, which mainly was above 1,000 psi. It was also noted that samples exposed to temperature testing (elevated temperature, transient, and cyclic loading) had lower interfacial debonding strength than those tested at room temperature. The final failure mode of these samples was due to the shear debonding, which was facilitated by the development of the microannuli in the testing samples, especially when the cement was exposed to temperature testing. Therefore, care should be taken, and proper calculations should be performed when using the P&A wells that are exposed to high-temperature conditions for the CCS project.