Sustained Annulus Pressure (SAP) is a common production constraint in the oil and gas industry, it is usually caused by impaired seal Integrity within the wellbore system resulting in barrier failures. In peculiar scenarios the thermal expansion creates pressure build-up in the annulus as well which can equally impair the integrity of the wellbore. In this paper the results of downhole and surface pressure monitoring surveys are presented, the objectives aim at determination of both downhole leaks and verification the influence of thermal expansion into a wellbore system integrity in a field located onshore Niger Delta. SAP in a producing well was earlier recorded during routine annular pressure monitoring in 2017 during the production rate increase by changing the bean size from 18/64" to 24/64". Initial diagnostics observed pointed towards SAP resulting from a possible downhole seal integrity issue leading to a leak to the surface. While putting the well on stream with current bean size and the pressure regime for both THP and CHP was observed. Pressure with time analysis showed annulus pressure builds up rapidly while flowing and bleeds off within 30 min from 700 psi to 0 psi when well shut in. Downhole logging and sensitive passive acoustic monitoring was conducted, the survey aimed to detect barrier failures by capturing its acoustic leak patterns under shut-in and bleeding off condition. Considering the suspected leak behaviour, the data acquisition included the procedure to build up the annulus pressure by flowing the well and monitoring the annulus discharge. Integrity logs survey and passive acoustic monitoring confirmed there were no downhole failures and after several bleed-offs when Tubing choke was beaned down to 18/64" no annulus pressure build-up was observed from the Well head gauge on the Casing head confirming the source of the sustained annulus pressure is driven by the temperature expansion of the annulus fluid. Remedial action and recommendation after Simulation were to de-risk the well at a controlled bean size to mitigate SAP and optimally flow the well.