Cement performance impacts integrity of oil and gas wells
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Energy Sector Analysis
The debate around the environmental risks of hydraulic fracturing reveals a need for more research about how cement seals behave in the field.
Cement performance impacts integrity of oil and gas wells By Melissae Fellet Feature Editors Michael Marder and Tad Patzek
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he ability to retrieve fossil fuels from deep shale formations has sparked a revolution in oil and gas development. It has also triggered a complicated political debate involving energy, economics, and the environment. Concerns about groundwater contamination and stray methane leaking from compromised wells grow with the increasing number of unconventional wells in production. The process of preparing an unconventional well, commonly called “fracking,” involves two technological advances: horizontal drilling through shale deposits and hydraulic fracturing through the well casing by inj ecting pressurized fluid into the well. The result is a network of interconnected cracks in the shale, similar to a web of cracks in a broken windshield, through which oil and gas can flow. Preparing a well for production involves rigorous engineering and materials design to accommodate the sub-surface conditions at each unique site. Cement fills the space between a well’s casing and the surrounding formations. But placing those seals becomes increasingly challenging as bores travel deeper underground, turn horizontal, and face high reservoir pressures and temperatures. As geoscientists, environmental engineers, and researchers in the petroleum industry formulate new cements in the lab, they seek to test the materials under the varying temperatures and pressures found inside a wellbore. Maintaining well integrity to prevent stray gas migration has been an issue for the industry since the earliest gas wells were drilled in the United States in the 19th century. Today, industry experts, academic researchers, and environmentalists agree that properly constructed wells can mitigate the environmental risks of oil and gas production, which increasingly comes from unconventional shales. The basic barriers that insulate a gas well from surrounding rock are steel pipes and cement seals. After drilling a wellbore, operators center a steel pipe in the opening, leaving about an inch of space between the outside diameter of the pipe and the inside diameter of the bore. Then they pump cement into the pipe. When the cement reaches the bottom of the bore, perhaps a mile underground, it travels into the annulus—the gap between the pipe and the rock—and returns to the surface. For some wells, equipment at the bottom of the well contains a one-way valve
Michael Marder and Tad Patzek, University of Texas at Austin, USA Melissae Fellet, [email protected]
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MRS BULLETIN
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VOLUME 39 • DECEMBER 2014
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www.mrs.org/bulletin • Energy Quarterly
to prevent backflow of cement into the center of the steel pipe. Modern wells can have multiple cement and steel pipe barriers, especially through critical portions of the bore. “Getting a solid sheath of cemen
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