A mobile gas chromatograph (GC) allows complete well-compliance testing, required by contract every 3 or 6 months, at the well site.
Mounted in a truck (Fig. 1), the GC provides hydrocarbon analysis and BTU measurement through C6+, according to Western Gas Processors Ltd.
When the truck arrives at a well site, the operator can check meter calibration, orifice plates, meter tubes, and analyze line gas with a self-calibrating BTU chromatograph which includes chromatogram printout.
All work is completed before moving to the next well. Furthermore, analytical data obtained can be sent via modem link over the nearest phone line to the central off ice.
Before Western Gas Processors inaugurated the new mobile lab, gas analysis was made with a "bomb" (cylinder) sample which had to be transported to the company's lab for analysis. Up to a week often passed between taking the field sample and getting a lab report.
Furthermore, various physical parameters modified gas properties between sampling time and lab work, and these had to be taken into account to determine data for reports. Variances often appeared whose causes were not easily identified.
The on-line/on-site chromatograph eliminates sampling Variances and human error to provide a true picture of gas constituents and heating values.
H2S COMPLICATES TESTING
Wells currently in the test rotation for the new mobile unit usually have significant H2S concentrations, up to 20%.
And expansion of testing operations will include wells with even higher concentrations.
Pertinent standards do not permit "bomb lab samples" for H2S analysis. Stain-tube tests are typically run, but wide variations in ambient temperatures in the North Dakota region, where the new truck is primarily assigned, greatly complicate testing, particularly when ambient is below freezing.
For example, iodine-starch titration tests such as the Tutweiler procedure are awkward, and results are often suspect with extremely low temperatures.
The chromatograph from Daniel Flow Products provides repeatability of 0.5 BTU per 1,000 over a gas temperature range of 0 - 130 F.
MOBILE LAB
A complete on-site chromatogram is obtained in only 10 min. The time can be shortened to 6 min, but the slightly longer period yields maximum reliability and accuracy.
The analysis spectrum can be increased to hexanes (Ca), heptanes (C7), and octanes (Ca) with just 9 min additional analysis time.
Columns are warranted for 2 years; sample, backflush, and dual-column valves for 5 years.
The mobile GC is identical to standard "Danalyzer" units except that it has been "squeezed" and packaged for mobile use.
It can be quickly and easily moved from one vehicle to another.
Analysis reports (Table 1) are printed in 80-column format. A related calibration report accompanies each analysis.
Clean and reliable field power comes from an uninterruptable power supply that will operate the system long enough to complete chromatogram runs in case of primary power failure.
As the mobile testing program becomes routine, work is being done to make a direct feed of field data via telephone-modem to the company's prime mainframe computer.
Copyright 1990 Oil & Gas Journal. All Rights Reserved.
