Transient Behavior of Annular Pressure Build-up in HP/HT Wells

Abstract

Summary Pressure buildup, caused by fluid thermal expansion in sealed annuli of high-presure/high-temperature (HP/HT) wells, can have serious consequences such as casing failure or tubing collapse. To determine whether mitigation was required for a HP/HT development, annular pressures in an appraisal well were studied with a dedicated field test, which consisted of running a pressure/temperature memory gauge in a casing/casing annulus of a well, and testing the well several times during a 3-month period, after which the gauge was retrieved, and the data were read out. First of all, comparison of the magnitude of the observed annular pressures with the burst and collapse ratings of the casings shows that annular pressure buildup is a serious consideration in casing design. Such design is to be based on theoretical models for annular pressure buildup. The data acquired with the test serve to validate these models. The data demonstrate that, in general, the theoretical models overpredicted pressure buildup in the annulus. This overprediction was more pronounced at higher temperatures (and pressures) than at lower temperatures, which could not be explained by mechanical factors such as casing ballooning. The influence of these factors was quantified by analyzing the transient pressure response of the annulus. Neither could permanent leakoff of completion fluids explain the discrepancy between theory and test. Leakoff of the annular fluids, which was seen to dominate pressure development during a previous test in a well with a cement shortfall between casings, does not play a significant role in this fully cemented and sealed annulus. This left (1) the properties of the completion fluids differing from the properties of the base fluid (water), and (2) temporary leakoff to near-wellbore fracture systems through the microannuli between cement sheaths and casings as explanations for the observed overprediction. Therefore, estimates on the basis of pure water properties, considering the annulus to be a perfectly pressure-tight vessel, can be considered a worst-case estimate for pressure buildup and a safe basis for design.