Measuring and testing – Gas content of a liquid or a solid
Reexamination Certificate
1999-04-29
2001-08-21
Larkin, Daniel S. (Department: 2856)
Measuring and testing
Gas content of a liquid or a solid
C073S019090, C073S152040, C073S152420
Reexamination Certificate
active
06276190
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to gas detection of hydrocarbons extracted from mud while drilling. In particular, two or more rare-earth sensors are used simultaneously as gas sensors.
BACKGROUND OF THE INVENTION
During the drilling of a well, mud is circulated downhole to carry away drill cuttings. Should gas be encountered, it becomes incorporated with the mud and is conveyed to the surface. In an active mud system, the mud is circulated in a loop; pumped from the mud tank, downhole to the bit, up to the surface, and back to the mud tank.
As the mud flows to the mud tank, an agitator, placed in the mud stream, causes contained gas to be liberated from the mud.
The liberated gas is directed past a gas sensor. One type of gas sensor is gas chromatography which produces a record of the constituents of the gas. Unfortunately, chromatography apparatus and methods of using same obtains only discrete analyses of gas in batches. A gas sample is occasionally selected and tested by the chromatograph. By the time the chromatograph is ready for the next sample, the drilling may have travelled a further ten feet or so and passed through and beyond a formation of interest. When the subsequent sample is obtained, the formation may then be uninteresting.
For producing a continuous gas trace, it is generally known to use a catalytic, rare earth or hot wire gas sensor. The sensor detects the presence of combustible gases. These devices are also called explosimeters and indicate the relative fraction of volatile hydrocarbons in a gas steam. Often these apparatus are used to determine if a gas mixture may be explosive.
The conventional gas sensor is a rare earth (hot-wire) sensor. An electrical current is passed though the sensor. The sensor heats up and dissipates energy dependent upon its ability to exchange energy with the surrounding environment. In these applications it is the gas flow and gas composition which affects the heat dissipation. Heat or power dissipation results in a change in the resistance of the sensor.
The sensor is epoxy coated for limiting the sensor from thermal effects and for excluding chemical interaction with the sensor's rare-earth portion.
The sensor output is recorded as a trace on a strip chart recorder or digitally on a computer and output for viewing on a screen. The presence of combustible gas shows up as an analog voltage output.
The difficulty with the prior art predominately lies in the interpretation of the continuous gas sensor output. This output responds to a high concentration of a predominantly methane gas with an output similar to a lesser amount of a heavier hydrocarbon.
There is therefore a demonstrated need for a real-time system which is capable of distinguishing heavier hydrocarbons (indicative of oil) from lighter hydrocarbons (representing coal gas or methane) while drilling, thereby affording the drilling operator an onsite ability to assess the value of the well.
SUMMARY OF THE INVENTION
The present invention is based upon a discovery that rare earth sensors are more usefully applied to gas detection, and more generally, fluid identification, if stripped of their epoxy coating. Without the epoxy coating, the rare earth oxides of the sensor are subject to absorption and electrochemical interactions with the measured fluid, in addition to the thermal effects. Stripped of their coatings, individual sensors have individual responses. By carefully selecting certain sensors which respond differently and predictably to known ranges of hydrocarbons, more useful analyses of the relative concentrations within gases can be made.
According to one embodiment of the present invention, two rare earth sensors are provided. Each sensor is sensitive to different ranges of hydrocarbons in sampled gases. Changes in relative concentration of the selected hydrocarbon in the sampled gas results in a change in the output of the corresponding sensor. Thus, where the sampled gas is a mixture of light and heavy hydrocarbon gases, the two sensors generally respond differently as the relative concentrations in the mixture change. The different response can be accentuated by obtaining the difference of the two signals. So, as drilling progresses through subterranean zones having different qualities of gases, the gas sensors provide distinctive output dependent upon whether they detect light or heavy hydrocarbons. For the first time, these different gas qualities are distinguishable, whereas previously, one only knew that volatile hydrocarbons merely existed in determinable relative concentrations.
Accordingly, in a broad aspect, a novel process is provided for distinguishing the quality of hydrocarbons extracted from gas encountered while drilling, comprising the steps of:
providing a first rare-earth metal oxide gas sensor which is sensitive to the concentration of a first group of components in a hydrocarbon mixture;
providing a second rare-earth metal oxide gas sensor which is sensitive to the concentration of a second group of components;
exposing the metal oxide of the first sensor to the extracted gas and outputting a first signal indicative of the concentration of the first group of components in the gas, preferably proportional to the relative concentration of light hydrocarbons;
exposing the metal oxide of the second sensor to the extracted gas and outputting a second signal indicative of the concentration of the second group of components in the gas, preferably inversely proportional to the relative concentration of heavy hydrocarbons; and
obtaining the difference between the first and second signals for establishing a differential third signal which is demonstrative of the quality of the gas extracted from the well.
Preferably, the first sensor is sensitive to light hydrocarbons (like methane), but characteristically also responds to any hydrocarbons (total-gas) in the gas sample. The second sensor is sensitive to heavier hydrocarbons (such as ethane through pentane). Further, the first sensor preferably produces an increasing signal at increasing light hydrocarbon content and the second sensor produces a decreasing signal with increasing heavier hydrocarbon content. Accordingly, the difference in quality becomes even more marked as the hydrocarbon content increases. The resultant difference accentuates the quality characteristics of the gas sample rather than speaking merely of quantity or concentration.
The apparatus and methods disclosed in the present invention now enables a log analyst to easily visualise, detect and distinguish the distinct nature of a downhole gas event, whether it be the crossing and detection of a coal seam producing light gas, or the crossing of an interface of gas (light hydrocarbons), oil (heavier hydrocarbons), or water (no hydrocarbons).
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BetaTHERM NTC Thermistor Products Catalog 1994—95, pp. 8-10.
Goodwin Sean W.
Larkin Daniel S.
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