Experimental Investigation of Well Cement Integrity in Abandoned Oil and Gas Wells

Abstract This research is focused on testing the limits of cement abandonment plugs, evaluating their isolating capabilities under severe conditions. The results will be used to understand possible failure modes and ensure long term effectiveness of wellbore abandonment operations of different cement systems. In this study, small-scale bulk cement and abandonment plug (i.e. cement in casing) samples were optimally cured under 50°C and 1000 psi for a period of 7 days and tested for their permeability to understand the basic behavior of the cement. Water and Nitrogen permeability tests for all small-scale samples were conducted using a typical core flow setup. A set of permeability experiments were also carried out using a physical wellbore simulator, which allowed cement curing and subsequent permeability testing under elevated pressure and temperature. The setup is designed to simulate abandoned wellbore section with variable fluid injection pressure, confining pressure and temperature conditions. Cement shear bonding strength was also determined after each experiment. From the small-scale experiments, it was observed that bulk cement on its own had exceptionally low permeability. The small-scale abandonment plug exhibited relatively higher permeability than the bulk cement. Nitrogen flow was observed in the small-scale abandonment plug experiments, possibly through the cement-casing interface. Two different cement blends were tested using the wellbore simulator; Neat G cement without any additives and a Class G cement with additives that has been engineered to enhance its sealing performance. The wellbore simulator experiments had shown that the Class G plus additives cement plug outperformed Neat G cement blend in every testing scenario. Although both cement systems were tested in severe conditions, both performed adequately. The class G plus additives cement blend also resisted the formation of a micro annulus better than the Neat G cement. Results also indicate an inverse relationship between the shear bond strength of the cement plug and the permeability of the cement system, indicating that stronger mechanical bonding also delivered improved resistance to gas flow. Data measured from these experiments will form the baseline for future experiments where other crucial parameters are to be investigated.