Oil and gas wells that require to be shut off forever, after depleting their reserves, need to be plugged and abandoned. Plug and Abandonment (P&A) operations induce many arduous challenges worldwide. The aim of P&A is to isolate and prevent fluid leakage in the wellbore in such a way that all fluids are contained in their formation for an undefined time. Failure of P&A in isolating and preventing fluid leakage can jeopardize the well integrity. Cement plugs that are used in this operation play a crucial role in maintaining the well integrity.
Cement is considered as a porous medium that has an ultra-low permeability that can be achieved when some additives are used in the cement slurry to reduce its permeability and pore space. The cement plug may deteriorate with time under harsh downhole conditions, such as high pressure and temperature and exposure to different fluids. Cement plug deterioration will result in increasing the cement permeability or the overall permeability by creating channels or microannuli.
In this study, several scenarios are presented for gas leakage through cement plugs. In these leakage scenarios, the differential pressure across the cement plug was varied. The aim of generating these scenarios is to investigate the current required cement plug length. In each scenario, four different permeability values were used to assess the risk associated with each value. In addition, the cement plug length was varied to investigate how the cement plug length is going to help ensure good well integrity.
The leakage scenarios presented revealed that longer cement plugs have a longer leakage time. In addition, the results show an increase of leakage time as microannulus gap permeability decreases. Differential pressure exerted on the cement plug have a strong effect on the leakage time. To achieve a long term well integrity in P&A phase, an ultra-low permeable cement plug with excellent bonding, longer cement plug, and a lower differential pressure across the cement must be considered.