Transient Behavior of Annular-Pressure Buildup in HP/HT Wells

Abstract

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 88735, "Transient Behavior of Annular-Pressure Buildup in HP/HT Wells," by P. Oudeman, SPE, and M. Kerem, SPE, Shell Intl. E&P, prepared for the 2004 SPE Abu Dhabi International Petroleum Exhibition and Conference, Abu Dhabi, UAE, 10-13 October. Pressure buildup resulting from the thermal expansion of fluid in the sealed annuli of high-pressure/high-temperature (HP/HT) wells can have serious consequences such as casing failure or tubing collapse. To determine if mitigation was required for an HP/HT development, annular pressures in an appraisal well were studied with a dedicated field test that 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. Introduction Pressure buildup in tubing/casing or casing/casing annuli is usually undesirable. Although casing design should consider high pressures at the casinghead that could result from leakage or thermal expansion of annular fluids, high pressure differences always present the risk of casing burst or collapse at weak points. Most operating companies have some form of annular-pressure management for onshore and platform wells that calls for bleeding off pressure through the wellhead once a predetermined pressure is reached. Such schemes cannot be used in subsea wells because these wells usually will not be equipped with the option to bleed off the annular pressure and to divert the released liquids. In HP/HT wells, the high temperature in the well during prolonged production periods aggravates pressure buildup in the annuli because the thermal expansion of liquids tends to increase at higher temperatures. Because casing-string design must consider those pressures, accurate prediction of annular-pressure buildup is important. Theoretical models to predict thermal annular-pressure buildup have been proposed by a number of authors. Usually, these models take into account fluid thermal expansion, ballooning and compression of the casings, annular fluid leakoff, and formation fluid influx. In a first attempt to validate the theoretical models, a field test was set up to study pressure development in the casing/casing annulus of an offshore platform well. However, in this particular well, the inner (production) casing was not cemented into the shoe of the outer casing. The test demonstrated that in this case, annular-fluid leakoff completely dominated the pressure buildup. The test did not allow any conclusion to be drawn about the overall validity of the theoretical models for pressure buildup. Hence, a second test in the sealed annulus of an offshore HP/HT gas well was planned and executed.