The Gulf of Mexico (GoM) is home to more than 50,000 oil and gas wells with approximately 30,000 wells that are plugged and abandoned leading to concerns of oil and gas leakage where currently, little to no monitoring is performed. The cement used when completing and eventually plugging wells are subject to harsh conditions leading to failure of the cement due to debonding of the cement to the formation and/or casing, shrinkage of the cement, and chemical degradation in the cement. Due to the complicated mechanical and chemical nature of cement, researchers have turned to numerical simulations to model cement failure and predict potential leakage rates. However, the numerical models in previous studies either predict leakage mechanisms but fail to provide comprehensive quantification of the fracture magnitudes. Or the models assume a fracture value and quantify leakage assuming water as the leaking fluid. The goal of this study is to determine if leakage is occurring through the cement sheath in GoM wells. This study develops a realistic finite element analysis (FEA) model coupled with fluid flow to determine if hydraulic propagation occurs providing a continuous leakage pathway. An analytical gas flow model is developed and used in conjunction with the FEA fracture volume to provide accurate fluid leakage rates within the pathways. The results of this work show that FEA models coupled with fluid flow can accurately quantify microannuli magnitude and predict if the microannuli propagate up the wellbore to the environment. The fluid flow models show that assuming wellbore water leakage versus gas leakage has an order of magnitude difference in leakage rates, cement shrinkage occurs in conventional cements causing continuous leakage pathways, and cement additives that change the mechanical properties of the cement are not required to achieve wellbore integrity.