Raman Spectroscopy and WellDog

Raman spectroscopy and coalbed reservoir analysis

Raman spectroscopy is a well-established laboratory chemical analysis technique. Raman spectroscopy was invented after the discovery of the Raman Effect in 1928, for which Sir Chandrasekhra Venkata Raman won the Nobel Prize in Physics in 1930.

The Raman Effect is when light scatters from a molecule with a slightly changed energy, or color, due to excitation of the molecule’s chemical bonds. The change in energy, or color, is representative of the energy of the bond or bonds that were excited. As a result, observing the colors of light scattered from a material indicates which molecules make up the material. Raman spectroscopy observes these colors by collecting the scattered light and then separating and detecting the colors that make up the light.

A challenge in using Raman spectroscopy is that only one photon in about one million are changed when scattering from a material. The rest of the photons remain unchanged in energy. In order to increase the number of changed photons, researchers have increased the number of incident photons by employing lasers.

A strength of Raman spectroscopy is that water molecules do not change many photons that scatter from it. As a result, in contrast to infrared systems Raman spectroscopy is not overly sensitive to water — an advantage when analyzing materials in systems such as coalbed reservoirs that contain water.

Fingerprinting Coalbed Natural Gas

In its 80-year history, Raman spectroscopy has been used to analyze many chemicals and chemical systems. Because of its sensitivity to the molecular structure of chemicals, Raman spectroscopy is called a "fingerprint" technique — every chemical give a unique Raman signal, or spectrum. More than 300 books have been published relating the fingerprints of various chemical systems.

WellDog has pioneered the use of Raman spectroscopy to analyze coalbed reservoir systems. Over the past five years, WellDog has collected hundreds of thousands of spectra on simulated and real-world coal seams, both in the laboratory and down hole.

The figure at right shows two Raman "peaks" of photons collected at the energies, or colors, characteristic of methane gas and methane dissolved in water. (The gaseous methane peak at a wavenumber of 2917 cm-1 is broadened and reduced in energy slightly to a wavenumber of 2912 cm-1 due to interactions with the water solvent.)

Raman spectra at 1,400 psi of methane production gas (red line) and methane solution gas (blue line). Solvation of the methane causes a shift to lower energy for the solution gas peak.

Quantifying Solution Gas

In addition to providing chemical fingerprints, Raman spectroscopy allows direct quantification of chemicals. For example, a series of peaks for methane dissolved in water are shown in the figure at bottom left. The size of those peaks can be used to build a quantitative calibration between instrument response and methane partial pressure, as shown in the figure at bottom right. As the partial pressure is increased, the instrument response increases linearly. Through careful instrument design and maintenance, this instrument response can be calibrated to concentration or partial pressure of the methane. As a result, WellDog is able to quantitatively determine solution gas levels in coalbed reservoirs.

Raman spectra at 1,400 psi of methane production gas (red line) and methane solution gas (blue line). Solvation of the methane causes a shift to lower energy for the solution gas peak.

Example correlation between WellDog instrument response and methane partial pressure.

Technology You Can Trust in the Wellbore

Reliably performing these measurements in a coalbed wellbore is not trivial. Most Raman spectrometers are bulky, difficult to operate, and require substantial power — in many cases occupying entire benches in the laboratory.

Over the past five years, WellDog has worked to miniaturize and ruggedize a Raman spectrometer for use in the wellbore. The conditions in a typical coalbed natural gas wellbore can include pressures as high as 3,000 psi, temperatures as high as 45 °C (113 °F), and harsh reservoir or drilling fluids. WellDog has successfully addressed these conditions by designing, building, and testing the smallest and most rugged high performance Raman spectrometers in the world and packaging them into a four-conductor wireline tool form factor.

These spectrometers are encased in corrosion-resistant stainless steel housing built to withstand reservoir pressures and temperatures. WellDog spectrometers can be operated on any standard four-conductor wireline drawworks.

In addition, WellDog has ensured that data quality and reproducibility remains high throughout a service call. Company spectrometers undergo rigorous system checks before being released for commercial deployment. Spectrometer performance is verified before, during and after a service call. WellDog instruments automatically notify field operators if performance degrades while downhole. Coalbed reservoir data collected downhole is transmitted in real-time to the wellhead via the wireline and to the WellDog Data Analysis and Interpretation Center via satellite uplink. Optical spectroscopists and reservoir experts validate and interpret reservoir data while the company’s field crew is still at the well.

Successfully addressing these engineering and scientific challenges has allowed WellDog to reliably perform coalbed reservoir analysis using Raman spectroscopy. WellDog has collected more than 30,000 Raman spectra in coalbed wells. The results are highly consistent with laboratory results.