Trapped Annular Pressure: A Spacer Fluid That Shrinks

Abstract

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 104698, "Trapped Annular Pressure - A Spacer Fluid That Shrinks," by B. Bloys, SPE, and M. Gonzalez, SPE, Chevron; R. Hermes, Los Alamos Natl. Laboratory; R. Bland, R. Foley, SPE, and R. Tijerina, Baker Hughes Drilling Fluids; J. Davis and T. Cassel, Baker Oil Tools; J. Daniel, I. Robinson, and F. Billings, Lucite Intl.; and R. Eley, ICI, prepared for the 2007 SPE/IADC Drilling Conference, Amsterdam, 20–22 February. The paper has not been peer reviewed. In subsea-completed wells, fluids commonly are trapped in casing annuli above the top of cement and below the wellhead. When these trapped fluids are heated by the passage of warm produced oil and gas, thermal expansion can create very high pressures and cause the collapse of casing and tubing strings. A successful midscale field trial has been conducted in a 500-ft test well to gauge the ability of a liquid monomer that shrinks 20% upon experiencing heat-triggered polymerization to mitigate trapped annular pressure (TAP). A method was devised to add the initiator on the fly as the spacer is pumped downhole. Introduction TAP, also called annular-pressure buildup, is caused by the thermal expansion of fluids trapped in casing annuli between the top of cement and the wellhead. The pressure buildup usually is a result of heat transfer from the produced fluids or from hot drilling fluids circulated while drilling a high-pressure/high-temperature (HP/HT) well. The pressure can exceed the collapse strength of the casing and production tubing. In land wells, the pressure is relieved easily by bleeding off some fluid through a casinghead valve. In subsea-completed wells, wellheads are much less accessible and generally not fitted with the necessary valves. One of the best-documented cases involved the Marlin field, where the production casing and tubing of the first production well collapsed after only a few days of production. A wide range of mitigation techniques has been used, including vacuum-insulated tubing (VIT), leaving the previous casing shoe uncemented, burst disks in casing, nitrogen-based spacers, and crushable urethane foam. Perhaps the most successful mitigation approach has been use of VIT. This technique generally has been successful in keeping the annular-fluid temperatures within an acceptable range. However, with the advent of deeper and hotter wells in deep water, the limits of the protection provided by VIT are being approached in two ways. First, deeper wells have much higher hanging weights that are reaching the stress limits of VIT designs. Second, the greater depths also are producing at higher temperatures. Even with the insulating effects of VIT, temperatures are sufficiently high that pressures are predicted to increase to dangerous levels.