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  • Spectrometry Samples Crude Oil

Photonics Spectra
May 2001
Richard Gaughan

RIDGEFIELD, Conn. -- Oil exploration requires an investment of time, money and material. But even when crude is found, questions about its composition must be answered so the type of processing it requires at the surface can be determined. A new spectroscopic technique that enables analysis down a borehole offers significant improvement for all aspects of sampling crude oil.

A downhole spectrometry technique promises to make the extraction of crude oil more economical. By sampling the gas-to-oil ratio on-site, users can determine the processing equipment that will be required.

Crude oil is a pressurized mixture of oil and dissolved gases, primarily methane. If the gas-to-oil ratio is high and the gas is to be marketed as well, gas pipelines or liquefaction facilities are necessary. Currently, samples of the crude are taken to be analyzed in off-site laboratories, which can create long delays. Moreover, the samples may be contaminated with drilling fluids.

Live crude oil, or crude oil as it exists in subsurface reservoirs with dissolved gaseous components, is a single-phase fluid. Oliver C. Mullins of Schlumberger-Doll Research and his colleagues at the company and at Oilphase of Houston were intrigued by the results achieved by near-IR spectroscopic determination of the composition and Btu content of natural gas, so they investigated the possibility of using the technique to determine the gas-to-oil ratio.

The researchers used a spectrometer to determine the absorption spectra for a variety of methane/heptane mixtures and a series of live crude oils. They attached a high-pressure, 2-mm-pathlength absorption cell to a pressurized sample bottle from Oilphase of Aberdeen, UK, and held the sample at temperatures up to 150 °C and pressures up to 20,000 psi. In the lab, they linked the high-pressure cell by fiber to the internal source and detector of the spectrometer.

They identified two distinguishing spectral features: a 1670-nm methane absorption peak and a 1725-nm peak with contributions from both methane and heptane. The ratio of the two peaks fit linearly with the gas-to-oil ratio.

'Proper application'

For field tests, the team has modified a spectrometer suspended on a wireline, which transmits electrical power and signals. The downhole spectrometer, which functions at 175 °C, has a high-pressure cell with sapphire windows, a fluid flowline and an external pressure housing. The spectrometer, located in the annular space, features a probe module with a tube extending to the wall of the borehole.

"This technique improves the efficiency of oil production by using technology that was unavailable until recently," Mullins said. One application is in deepwater oil fields, where the oil may be thousands of feet underwater. If the undersea production facilities are improperly designed, extraction may be prohibitively expensive.

"The proper application of technology," Mullins said, "can provide the information to allow these large oil fields to be economic."

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