Cement's properties depend strongly on the mix's molecular-level composition and its reactions with water to form a hydrated solid. The strength of the cement develops with hydration reactions that promote molecular cross-linking, Chmelka says. "Given the complexity of the mixture, we thought it would be an unresolvable distribution of components that we couldn't identify, but that's not the case," he adds. The research team reported in its first study, published in May in the Journal of the American Chemical Society (JACS), its use of solid-state nuclear magnetic resonance (NMR) spectroscopy to distinguish among different molecular rearrangements that occur as a result of cement hydration and setting.
Still, the molecular compositions, structures and changes that occur as a result of hydration are poorly understood, because cement has a composition that is partially crystalline, partially disordered, and holds multiple components that also change with time, Chmelka says.
Chmelka and his colleagues are trying to gain an understanding of cement at the molecular level to determine properties such as how calcium and magnesium ions are affected by and participate in the hydration process as well as how cement setting can be slowed or better controlled. For their next paper the researchers are specifically studying retardants—how they inhibit solidification and how sugars bind to and affect the different components used to make cement, Chmelka says.
The results of this research could be put to good use, helping companies like BP and Halliburton better design a reliable cement for sealing oil wells deep beneath the ocean and, hopefully, avoid catastrophes such as occurred in the Gulf this year. Cement used at underwater drilling sites is designed to flow down the center of a well bore so that it does not set on the way down. In the case of the Deepwater Horizon, Halliburton added nitrogen gas to the mix to improve the cement's ability to flow. Once at the bottom, the cement needed to overcome pressure from the well bore and flow up the annulus (the space between the casing and the sides of the bore hole), where it should have set and blocked any oil or gas from escaping.
The commission obtained samples from Halliburton of the cement recipe used on the failed well, including the same proportion of nitrogen gas used as a leavening agent, along with a number of chemicals used to stabilize the mixture, according to an October 28 letter (pdf) from Fred Bartlit, the commission's lead investigator. The foamy cement mixture was sent to a laboratory owned by BP-rival Chevron for independent testing. Chevron conducted a series of postmortem tests and reported being unable to generate stable foam cement (one that would set properly in the annulus) in the laboratory using the materials specified by Halliburton and available design information regarding the cement used at the Macondo well, according to the commission. Chevron conducted nine separate stability tests intended to reproduce conditions at the BP well, and the cement failed them all.
Halliburton defended itself by pointing out that "significant differences" between its internal cement tests and the commission's test results may be due to differences in the cement materials tested, according to a statement posted on the company's Web site. The commission's tests were not an apples-to-apples comparison because they used off-the-shelf cement and additives, whereas Halliburton tested the unique blend of cement and additives that existed on the rig at the time the latter's tests were conducted, the company claims.