Fid gas detector
Flame ionization detector
I - Introduction
The Flame Ionization Detector (FID) is a relatively new type of gas detector to find its way into the oil field. For many years the only type of gas detector available was a thermal conductivity (TC) or Hot-Wire type detector. These detectors were troubled by a drift lack of sensitivity , difficulty in calibration, short filament life and a very non-linear output. The FID addresses many of these problems and provides an accurate measurement of the amount of gas in the trap sample.
The Total Hydrocarbon Analyser (THA) is an FID gas detector manufactured for Sperry-Sun by Baseline Industries Inc, of Lyons Colorado. Sperry-Sun has incorporated this instrument into the MWD system (LS-2000) along with a Total Hydrocarbon Conditioner (THC) to control the system flows and provide the logger with a flexible, highly accurate, easy to operate and easy to calibrate gas detector. Sperry-Sun also uses a Baseline Industries FID gas chromatograph together with the THA and a good correlation of data between the tow instruments is achieved.
The following section is designed to acquaint the logger or technician with the THA and THC.
II - Operating Principle
When many organic compounds are burned in a hydrogen flame, charged particles or ions are given off. Hydrocarbons from the formation during drilling are among these organic compounds. The common hydrocarbons that are detected from the sample gas are methane (CH4), ethane (C2H6) propane (C3H8) normal & iso-butane (C4H10) and pentanes (C6H12).
The gas to be sampled is drawn into the FID gas detector from the trap motor. Only about 1 % of the sample gas is actually burnt in the flame whereas the remainder is vented.
The gas which is going to the flame is mixed with air as a make-up and hydrogen as a carrier. Another flow stream of combustion air is routed to the flame separately to help support the combustion process.
As the sample is burnt in a clear hydrogen flame the released ions or free electrons are forced to travel as a small currant into an amplifier called the electrometer.
The current flow is extremely small on the order of 10-9 to 10-12 amperes and the electrometer produces a voltage proportional to this current.
In this way the voltage generated is proportional to the amount of hydrocarbons present in the sample.
Advantages of the FID
it is about 1000 times as sensitive as the thermal conductivity detector for measuring hydrocarbons.
It has mush more linear output than the thermal conductivity detector.
it is not sensitive to carbon dioxide, hydrogen sulfide, water vapour or hydrogen.
In mud logging applications the FID is only secretive to the hydrocarbons which are released from the formation.
Figure 1 shows the FID assembly.
III - Output from an FID
Output from an FID is similar to the TC in that the total output is the sum of the responses from the different gases contained in a sample. An advantage of the FID is that the output are linear as the gas concentration in air increases and the output can be easily converted to give the true amount of hydrocarbon present. For instance suppose that an FID gas detector is calibrated to give a residing of 1.0 % on the digital meter for a one percent concentration of methane in air. Heavier gases give proportionately greater responses.
1 % methane (CH4) in air reading = 1.0
1 % ethane (C2H6) in air reading = 1.5
1 % propane (C3H8) in air reading = 2.0
1 % n-butane (C4H10) in air reading = 2.5
1 % iso-butane (C4H10) in air reading = 2.5
A one percent mixture of all the above gases would give a reading of 9.5 or the sum of the individual gases. Ethane yields 1.5 times the response of methane because 6 hydrogen ions are released during combustion i.e 1.5 the number of electrons. Propane and butane respond in alike fashion yielding 2 and 2.5 times the free elections. So, the output of an FID is described as “percent equivalent methane” . Using the example, a one percent mix of the five gases in air would be known as 9.5 percent equivalent methane.
This is the same output that a 9.5 percent concentration of methane in air only would give. A chromatograph is necessary to provide the actual analysis of the sample gas.
Other advantages of the FID
Using both an FID gas detector and chromatograph is useful because the data from the two instruments will easily correlate. The logger will then have an easier time making sure that both instruments are operating correctly.
The THA or gas detector can pick up rapid changes in the volume or ratio of gases coming from the hole. A sudden change in the THA output would flag the logger that a volume or ratio change had occurred and he should run a chromatogram. Since the contributions of the various gases in a TC detector are more equal than in a FID, The ratio or volume change wouldn’t be picked up as fast. The logger wouldn’t have as good or as fast data with a TC detector telling him that a significant change had occurred.