Natural Gas Measurement, Sampling, and Analysis
- Fred G. Van Orsdal (Chevron U.S.A.)
- Document ID
- Society of Petroleum Engineers
- Journal of Petroleum Technology
- Publication Date
- June 1990
- Document Type
- Journal Paper
- 780 - 783
- 1990. Society of Petroleum Engineers
- 4.9 Facilities Operations, 4.2 Pipelines, Flowlines and Risers, 5.3.2 Multiphase Flow, 4.6 Natural Gas
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This paper addresses basic field-level problems associated with the propermeasurement, sampling, and analysis of raw and processed natural gas streams.In addition, operating suggestions, practical experience, and industry standardrequirements are reviewed with regard to primary elements and electronic andmechanical secondary devices for orifice measurement.
"In effect, gas meters are the cash registers of the natural gasindustry."
Gas processors regularly handle sour, waterwet, hydrocarbon-rich gasstreams, and pipeline-quality streams. Because of pressure decline in thefields, the quality of the rich gas is steadily worsening, at least from agas-measurement standpoint. Lower reservoir pressures allow water and heavierhydrogen-component contents to rise to the point where many pipelines becometwophase flow. In many systems, CO2, and/or H,S concentrations are alsosteadily rising. These problems create a great deal of operating expense,higher capital costs, increased risk, and increased measurementuncertainties.
As the trend toward energy measurement from the wellhead to the end usercontinues and as pipelines become common carriers, accurate measurement, evenunder difficult conditions, will become even more necessary. In the oldestsystems, gas is gathered at pressures as low as 6 psi [41 kPa] absolute, whichis well below atmospheric pressure. Air, water, dirt, and other debris canenter the pipelines if a leak develops. This gas is water saturated and so richin hydrocarbons that as much as 20 gal [0.076 M3] of product may be recoveredfrom each 1.000 scf [28 std m3] of gas from an approximately 1,900-Btu[2006-kJ] stream,
In effect, gas meters are the cash registers of the natural gas industry.Sampling and analysis are equally important; measurement, sampling, andanalysis each constitute one-third of the total equation that determines thepayments and receipts for gas that is transferred. The importance of accuratemeasurement, sampling, and analysis can be demonstrated economically. Forexample, at $2/Mcf [$2/28 std m3], or per million Btu [ 1. 06 x 10 6 kJ], a 1 %error on 50 Mcf/D [1416 std M3/d] of 1,000-Btu [ 1055-kJ] gas is worth$1,000/day. A 0. 1 % error, which is better than the capability of currenttechnology, results in either a loss or a gain of $36,500/year.
It is important to understand the measurement environment and that thesystems involved may be brand new or very old. The measurement environmentincludes wells that are not associated with oil production, such as natural gaswells, and wells that are associated with oil production, such as casingheadwells. Residue gas streams leaving manufacturing facilities are also freqentlymeasured. (Unlike casinghead wells, however, which many present a difficultmetering situation because of high concentrations of heavy hydrocarbons,residue gas streams involve relatively straightforward measurements becausethey often are highquality streams with stable flows.) Another majordistinction of measurement systems is whether the measurement if for anallocation metering system or for custody transfer. Allocation measurement maybe used to reduce the overall operating and capital costs of producing aparticular reservoir. These systems require good maintenance but do not have tobe as technically sophisticated or as frequently calibrated as custody transfersystems. Allocation measurement also requires that all producers be treatmentequally, but not necessarily with extreme accuracy. For example, instead ofhaving temperature recorders on each of several hundred-meter runs, flowingtemperatures can be fixed (by contract) at 60 deg.F (15.56MC). Then, if thevolumes of gas atribute to each producer are compared to the total volumeproducer receives its proportional share of the total products and volumes forsale from the system. A custody transfer point, on the other hand, is a directsale between two parties. Here, the measurement must be precise andaccurate.
Problems and Deficiencies. Many problems, including pulsation and variation,complicate measurement in the field. Pulsation is a variation that has afrequency of <1 cycle/sec (<1HZ). Variation has a period >1 second.Pulsation and variation usually cause measured volumes to be overstated.
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