Compact, portable, ruggedized gas sensor systems are valuable not only for making informed wellsite decisions, but also for long-term reservoir planning and development. Among these applications, measurements of isotopes ratios are used for in basin modeling reservoir design. Unfortunately, there are few portable isotope sensor systems capable of deployment at the well site, and none available for in-situ measurements.
Isotope ratios such as δ13C1 (for methane), δ13C2 (for ethane), δ13C3 (for propane) can be used to characterize source rocks, since the 12C-13C chemical bonds are more stable than 12C-12C. Indeed, hydrocarbons gases generated by thermal cracking show a decrease in 13C from, n-butane to propane to ethane to methane, while isotopic exchanges between aqueous carbonate and atmospheric CO2 results in enrichment of 13C in carbonates.
Quartz-enhanced photoacoustic spectroscopy (QEPAS) measurements can be employed to measure hydrocarbons concentration and isotopes ratios in-situ and in real time, opening the way to the prediction of production outputs, reserves estimation, and raw material quality of source rocks and reservoirs assessment. QEPAS is a trace gases detection technique exploiting a quartz tuning fork (QTF) as a sharply resonant acoustic transducer to detect weak photoacoustic excitation and allowing the use of extremely small volumes.
In this work, we report on a QEPAS sensor module for real-time monitoring of hydrocarbons, employing an interband cascade laser (ICL) emitting at l=3345 nm. The narrow spectral resolution and tunability of the employed ICL, combined with a few tens of torr operating pressures, provide selective and sensitive detection of methane, ethane and propane with ultimate sensitivity level in the part-per-billion (ppb) concentrations range. Gas mixtures containing C1, C2 and C3 species, simulating typical downhole compositions were also analyzed. The same QEPAS sensor module was also employed to detect δ13C1 in methane, by using a mid-IR quantum cascade laser emitting at 7.7 μm. The selected source allowed to reach two spectral windows: the first window is centered at 1295.65 cm-1 and includes three 13CH4 and one 12CH4 lines; the second one falls at 1298.10 cm-1 and counts just one line for each isotope. The line strengths are all comparable, around 10-21 cm/mol, allowing δ13C measurements in few ‰ range.
AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018