Development of Improved Testing Procedure for Wells with Sustained Casing Pressure

Abstract

Abstract In the US, the Bureau of Safety and Environmental Enforcement (BSEE) regulations require pressure bleed-down/buildup (B/B) testing of wells with annular Sustained Casing Pressure (SCP) if the casing head pressure is greater than 100 psig. The test involves bleeding the pressure with a needle valve followed with up to 24-hour monitoring pressure return after the valve is closed. As the test description is limited to its principle, the way it is actually performed varies considerably among operators leading to different results. Analysis of B/B tests described in the API standards is merely qualitative and limited to the finding if pressure could be bled to zero and if it would re-build to its initial value before the test. Needless to say, the two findings might depend upon the way needle valve has been operated and also they do not provide any quantitative information about gas migration in the leaking cement. Quantitative analyses methods and models have been already proposed, but – as shown in the paper – they either produce ambiguous results or they have oversimplifying assumptions that lead to widely-spread estimations of the cement leak size by orders of magnitude. The presented work is a study of the B/B testing as a source of useful information for SCP problem assessment and remediation. Simulation software based on improved mathematical model of SCP well is used for theoretical parametric study of the testing process. Analysis of variance and multivariate regression method consider four unknown system parameters to be determined from the test (length and permeability of the leaking cement, and length and compressibility of the annular fluid above the cement). The study qualifies significant system parameters controlling each of the three stages of the B-B test: pressure bleed-down, stabilization and buildup. The results show that significance of these parameters varies between the stages. The finding indicates that B-B test should involve three stages – instead of two stages presently practiced. Also, the results show merit of a stage-by-stage analysis of the test instead of trying to find all system parameters by matching the whole test. In addition, three operational parameters of the B-B test (bleeding rate and time, and pressure recording time step) are evaluated to see how they affect test results in view of possible improvements in the testing procedure. It is shown that the operational parameters have significant effect on well response and may cause misinterpretation of test results. Early closure the bleed valve prior to gas flow stabilization would not indicate the true bleed-down pressure and produce pressure buildup that misrepresents the cement leak size.