LabNotes - December 2013

Foamed Cement Can Seal Tricky Oil and Gas Wells

In oil and gas well drilling, a steel casing is inserted into the wellbore and cemented in place to create a barrier between well fluids (oil or gas) being produced from the well and shallower rock formations, isolating the fluids from the environment. The cement, which is pumped into the space between the steel casing and the rock, also supports and stabilizes the casing. If the well is drilled through weak, porous, or highly fractured rock, cement can leak into the rock before it solidifies, which can lead not only to a loss of cement, but to a situation where the cement may crack or collapse, creating a path through which the oil and gas can easily flow to the surface, into drinking water aquifers, and even into the ocean in deepwater drilling environments. In rare but extreme cases, a blowout of high-pressure gas or oil can result causing injuries, environmental damage, and property losses.

Low-density, foamed cements—cements containing microscopic bubbles of nitrogen or air—have been developed for wells where the rock is too weak to support conventional cement. Foamed cement is better able to withstand the temperature- and pressure-induced stresses common in these wells, and provides a more reliable seal. To have the greatest strength and effectiveness, the gas in the cement must be evenly distributed.

CT scanner composite video of a cube of foamed cement containing 10% gas by volume. The cube, which measures 10.4 millimeters (mm) on each side, contains more than 160,000 bubbles. In the video, the largest bubbles (> 0/01 mm3) are shown in red and the smallest (<0.00005 mm3) are green

The size and placement of gas bubbles in foamed cements generated in the laboratory under atmospheric conditions can be readily characterized. However, due to pressure effects, it is challenging to characterize foamed cements generated under in situ (in place) wellbore conditions. To do this, foamed cement would need to be generated and collected under relevant wellbore pressures, and it would need to remain under those pressures during characterization. In the wellbore, the gas pressure in the foam cement bubbles is equal to the surrounding pressure; and the solidified cement is stable as long as it remains in the well. However, if brought back to atmospheric pressure, depressurization may damage the bubble structure and ruin any possibility of understanding what the cement was like while underground.

NETL researchers are studying the physical properties of foamed cement under wellbore conditions to understand how they can be modified to lower the risk of spills. Dr. Barbara Kutchko, who leads the research team, is using a computed tomography (CT) scanner to generate data and images of cements containing various amounts of air or nitrogen, at atmospheric and wellbore pressures. Just as in a medical procedure, the CT scanner produces and combines cross-sectional x-ray images to create a 3D view of the cements. The tests will help scientists and oil and gas companies understand the effects that cement production and placement have on the effectiveness of isolation of well fluids (oil and gas) from the environment.

Contacts: Dr. Barbara Kutchko (Foamed Cement), 412-386-5149; Dustin Crandall (CT Scanner Lab), 304-285-1384.

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