Pressure buildup test analysis in wells with sustained casing pressure

Abstract

When the well's casing head pressure cannot be permanently bled off with a needle valve, the casing is said to exhibit Sustained Casing Pressure (SCP). Rebuilding of surface pressure (pressure buildup – after closing the valve) results from migration of gas in the leaking cement sheath of the well's annulus. Problem of sustained casing pressure (SCP) has been widespread in the Gulf of Mexico and also reported in Canada, Norway, and other places. Regulations require 24-hr testing of SCP wells comprising pressure bleed-down followed with pressure buildup. In these tests, the rate of pressure buildup is indicative of the size of cement leak – prompt buildup implies larger leak on a relative scale with no quantification. Presented here is the first mathematical model for quantitative analysis of pressure buildup in SCP wells. The model simplifies transient gas flow in cement and ignores migration time of gas in the annular fluid column above the cement top. The simplification allows finding the size of cement leak and the depth and value of gas pressure source formation.Also presented is validation of the model with actual field data from testing SCP wells. Matching the model to field data gives acceptable estimates of the gas-source formation depth and pressure, cement leak size, and expected maximum casing pressure value. The results also reveal a correlation between the pressure buildup-stabilization pattern and well parameters - cement leak size controls the pressure buildup rate, while the gas-source formation pressure controls the stabilized pressure value. Quantitative analysis of SCP buildup with the new model could be extremely useful as it provides values of three parameters (cement leak size, gas source formation depth and pressure) that are critically important for designing remedial treatment.