Measuring and testing – Sampler – sample handling – etc. – Automatic control
Reexamination Certificate
2002-07-01
2003-04-15
Raevis, Robert (Department: 2856)
Measuring and testing
Sampler, sample handling, etc.
Automatic control
Reexamination Certificate
active
06546818
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a system for the analysis and measurement of selected gases and, more particularly, for measurement of combustible gases such as hydrocarbon gases contained in a gaseous sample to be analysed.
BACKGROUND OF THE INVENTION
There is a demand for information indicating the hydrocarbon content of gaseous mixtures. For example, the return flow drilling mud material discharged from an oil or gas exploration well can contain entrained hydrocarbon gases. Detection and measurement of the hydrocarbon gas content of the well return material can be used to give an indication of when a certain zone is being penetrated in the well drilling process. Such data can provide information to the geology personnel on the drilling project to enable them to form an assessment or provide an indication as to whether the well drilling has hit a producing zone. In oil and gas exploration, the primary hydrocarbon gas of interest is generally methane, although, under certain drilling conditions, there is also interest in information relating to some of the other hydrocarbon gases that may be present.
The current state of the art uses a variety of apparatus and methods to quantify and qualify the hydrocarbon content of a gas sample, that is, to perform analysis of the sample. The simplest types of apparatus to perform analysis of a gas sample, are systems that use a “thermal conductivity detector” (TCD). Thermal conductivity detectors are suitable when the gas to be analyzed by the detector contains a known gas in a known carrier gas. This is often referred to as binary analysis of gas. Every gas has a unique thermal conductivity as one of its properties. Thermal conductivity detection works best when the carrier gas and the sample gas have very different thermal conductivities. Typically, the TCD detector has a Wheatstone bridge arrangement where the detector element manifests a decrease in resistance with increasing thermal conductivity of the sample gas. By way of example, U.S. Pat. No. 3,683,671 to Van Swcray entitled Measuring System Including Thermal Conductivity Detector Means provides an electrical circuit bridge excited at one power node, by a clamped square wave arm at another power node by a feedback circuit. The output of the circuit bridge is fed to a demodulator to generate an output signal representative of the sample being sensed.
A Wheatstone bridge and visual indicator in the form of light emitting diodes in a gas analyser arrangement is disclosed in U.S. Pat. No. 4,028,057 to Nelson. These detectors are used in gas chromatography where a carrier gas that has a very high thermal conductivity, such as helium, is used. When a sample that has a much lower thermal conductivity than helium is introduced into the carrier gas the output of the detector will show a change relative to the amount of sample contained within the carrier. A thermal conductivity detector can be confused, that is produce erroneous output, if more than one type of sample gas is introduced into the carrier gas. That is if the thermal conductivity detector is used to analyse a gas mixture of multiple sample gases. For example, if one of the sample gases has a higher thermal conductivity than the carrier gas and the second sample gas has a lower thermal conductivity than the carrier gas, then the detector output may not even change for varying constituent gas compositions or mixtures.
Thus, a thermal conductivity detector is not well suited to analysis of hydrocarbon gases entrained in well returns for a number of reasons. First, it is not feasible to transport large tanks full of helium to the well site. Consequently, the carrier gas that is generally used is air. Air has a thermal conductivity of 1.00 and methane a thermal conductivity of 1.3. This means there is not a very good signal to noise ratio between the air carrier and the gas of interest, which makes a thermal conductivity detector based instrument prone to drifting. Notwithstanding their drawbacks, such thermal conductivity detectors are in use in analyzers used in the oil well drilling industry. However, because of the inherent limitations of using TCD detectors in these environments, it is not uncommon to need to zero the baseline of a TCD based system on an hourly basis. Automated baseline adjustment apparatus have been proposed to compensate for temperature changes in such systems. For example, the arrangement proposed by Hagen in U.S. Pat. No. 4,817,414.
Also, thermal conductivity detectors are, by their nature, sensitive to ambient temperature. Even a 1 degree shift in ambient temperature will cause a noticeable shift in the baseline of a thermal conductivity detector operating in this low signal to noise ratio configuration.
Another, somewhat more sophisticated detection apparatus employs a catalytic combustion detector (CCD) to detect the presence of hydrocarbons. For example, U.S. Pat. No. 3,607,084 to Mackey for Combustible Gas Measurement describes passing a stream of gas containing the combustible gas analytes over a conductive metal wire coated with a think catalytic coating which is at a temperature at which oxidation of the gases is initiated. Numerous other arrangements of CCD apparatus are known for example, U.S. Pat. No. 4,045,177 to McNally, U.S. Pat. No. 4,072,467 to Jones, U.S. Pat. No. 4,111,658 to Firth et al, U.S. Pat. No. 4,123,225 to Jones et al, and U.S. Pat. No. 4,313,907 to McNally are examples of such CCD detectors. CCD's are sensitive to anything that is combustible and in an oil and gas well drilling environment, hydrocarbon gases are the combustible gases that would be encountered. This means a CCD can be used as to provide a measurement of the total hydrocarbon content of a gas without regard to the particular type of hydrocarbon gas. While a CCD will respond to combustible compounds other than hydrocarbons, it is the gaseous hydrocarbon compounds that will be of interest in the sample gases recovered from the drilling mud in a well drilling environment. A major problem with CCD's is their limited range. If a CCD is subjected to explosive combustible gas concentrations, that is concentrations between the upper and lower explosive limits of that compound, they are destroyed as the gas actually combusts and coats the detector surface with carbon, rendering it ineffective after that point. For methane the lower explosive limit is 5% in air. An air mixture containing methane gas concentrations greater than the 5% lower explosive limit will result in a mixture that becomes explosive.
To obtain the benefit of a stable baseline and wider range of methane concentrations in a sample, two detector systems have been produced. Current state of the art two-detector apparatus uses a CCD sensor to around 4% concentration in the mixture. Above that point, the sensor apparatus control turns off the CCD sensor and passes the sensing over to a thermal conductivity sensor. A thermal conductivity sensor, of course, has all of the problems as described above. However, a major advantage of a two-detector analyser is a more stable baseline.
A combined CCD and thermal conductivity analyzer has some major drawbacks if a gas other than methane is present in the sample to be analyzed. For instance, if C
2
is the gas being presented to the CCD, the CCD will detect its presence very nicely. However, when the analyzer switches over to the thermal conductivity detector, the C
2
gas may not be detected at all. The system will respond by switching back to the CCD which ultimately causes the system to keep switching back and forth between the two sensors and can result in the destruction of the CCD due to exposure to explosive levels of C
2
gas in the sample. An example of a two-detector system is shown, for example, in U.S. Pat. No. 4,804,632 to Schuck et al which switches from one sensor to another based on set sample temperatures and holding the sensing devices to a preset temperature.
Another gas detection system using a CCD detector, operates by diluting the sample with air when it exceeds 4% as s
Skupinski Wiktor
Taylor Brian
Blake Cassels & Graydon LLP
Leier Terry L.
Pason Systems Corp.
Raevis Robert
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