Transient Pressure Unloading - A Model of Hydrostatic Pressure Loss in Wells after Cement Placement

Document Type

Conference Proceeding

Publication Date

1-1-2002

Abstract

Hydrostatic pressure loss (unloading) after cement placement is widely considered a reason for early gas migration in wells. In the presence of high-pressure zone, the unloading leads to pressure underbalance causing gas invasion and flow in the cemented annulus. The work, reported here, is an attempt to analyze and model pressure unloading as a propagation of pressure transient resulting from downhole fluid loss - a primary mechanism of slurry volume reduction. (Experiments, discussed here, had shown that another mechanism of volume reduction - volumetric shrinkage is not at work early enough to contribute to the pressure unloading process.) The pressure-unloading model has been validated using data from field experiments in the instrumented wells. The theoretical results matched experimental data showing exponential trend of the downhole pressure loss. Also, the model was used in a sensitivity study to identify parameters controlling the process. The study revealed that large annuli with high water loss would give rapid and large reduction of hydrostatic pressure in the cement column and more likely intrusion of gas. The observation implies that surface holes should be more sensitive to gas migration than deep holes and the cement slurry filtration should be minimized in surface holes. Another finding showed that compressible annular cement systems would be more tolerant to fluid loss in terms of losing hydrostatic pressure than the low-compressibility systems. The observation emphasizes the importance of annular system compressibility control.

Publication Source (Journal or Book title)

Proceedings - SPE Annual Technical Conference and Exhibition

First Page

3609

Last Page

3618

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