Changes

==Gas extraction methods==

==Gas extraction methods==

 

[[file:mudlogging-gas-extraction-and-monitoring_fig1.png|left|thumb|{{figure number|1}}Mud system. (From <ref name=pt03r51>Whittaker, A., 1991, Mud logging handbook: Englewood Cliffs, NJ, Prentice-Hall.</ref>.)]]

mudlogging-gas-extraction-and-monitoring_fig1.png|{{figure number|1}}Mud system.<ref name=pt03r51>Whittaker, A., 1991, Mud logging handbook: Englewood Cliffs, NJ, Prentice-Hall.</ref>

mudlogging-gas-extraction-and-monitoring_fig2.png|{{figure number|2}}Location of gas trap in possum belly.<ref name=pt03r51 />

Samples of gas must generally be extracted from the drilling mud by the gas trap. The gas trap is a metal box immersed in the shale shaker possum belly, preferably in a location of maximum mudflow rate ([[:file:mudlogging-gas-extraction-and-monitoring_fig1.png|Figure 1]]). Ports in the lower part of the trap allow mud to enter and leave the trap. An agitator provides both pumping and degassing of mud passing through the trap. Gas evolved from the mud is mixed with ambient air in the upper part of the trap and is then drawn through a vacuum line to the logging unit for analysis ([[:file:mudlogging-gas-extraction-and-monitoring_fig2.png|Figure 2]]).

Samples of gas must generally be extracted from the drilling mud by the gas trap. The gas trap is a metal box immersed in the shale shaker possum belly, preferably in a location of maximum mudflow rate ([[:file:mudlogging-gas-extraction-and-monitoring_fig1.png|Figure 1]]). Ports in the lower part of the trap allow mud to enter and leave the trap. An agitator provides both pumping and degassing of mud passing through the trap. Gas evolved from the mud is mixed with ambient air in the upper part of the trap and is then drawn through a vacuum line to the logging unit for analysis ([[:file:mudlogging-gas-extraction-and-monitoring_fig2.png|Figure 2]]).

Inside the logging unit, the gas-air mixture is drawn through a sample chamber containing a heated platinum filament forming one arm of a Wheatstone bridge circuit. Catalytic combustion of hydrocarbon gases at the filament (the so-called hot wire) further heats the filament, causing an increase in its electrical resistance. This change in resistance unbalances the bridge, causing an electrical current to flow in proportion to the resistance change and a resulting concentration of gas. The current is displayed by deflection on a milliammeter or may be used to scale the meter or recorder in terms of percentage or “gas units” by calibrating the hot wire periodically with a gas-air mixture of known concentration. A second hot wire detector, set at a lower temperature, allows discrimination of ''total gas'' (all combustible hydrocarbon gases) from ''[[petroleum]] vapors'' (combustible hydrocarbon gases other than methane). This aids in identifying the types of hydrocarbons in place (that is, oil or gas).

 

Inside the logging unit, the gas-air mixture is drawn through a sample chamber containing a heated platinum filament forming one arm of a Wheatstone bridge circuit. Catalytic combustion of hydrocarbon gases at the filament (the so-called hot wire) further heats the filament, causing an increase in its electrical resistance. This change in resistance unbalances the bridge, causing an electrical current to flow in proportion to the resistance change and a resulting concentration of gas. The current is displayed by deflection on a milliammeter or may be used to scale the meter or recorder in terms of percentage or “gas units” by calibrating the hot wire periodically with a gas-air mixture of known concentration. A second hot wire detector, set at a lower temperature, allows discrimination of ''total gas'' (all combustible hydrocarbon gases) from ''petroleum vapors'' (combustible hydrocarbon gases other than methane). This aids in identifying the types of hydrocarbons in place (that is, oil or gas).

==Extraction efficiency==

==Extraction efficiency==

An alternative to the conventional gas trap is the steam or vacuum mud still. In this device, a small sample of drilling mud is collected at the ditch and distilled under vacuum. The method provides a relatively high and uniform extraction efficiency for all hydrocarbons. It is, however, a time-consuming manual process.

An alternative to the conventional gas trap is the steam or vacuum mud still. In this device, a small sample of drilling mud is collected at the ditch and distilled under vacuum. The method provides a relatively high and uniform extraction efficiency for all hydrocarbons. It is, however, a time-consuming manual process.

[[file:mudlogging-gas-extraction-and-monitoring_fig3.png|left|thumb|{{figure number|3}}Gas chromatograph system.<ref name=pt03r51 />.)]]

==Gas chromatography==

==Gas chromatography==

[[file:mudlogging-gas-extraction-and-monitoring_fig3.png|300px|thumb|{{figure number|3}}Gas chromatograph system.<ref name=pt03r51 />]]

Most logging units contain a gas chromatograph (see [[Mudlogging: equipment, services, and personnel]]). The hydrocarbon gas chromatograph is probably the most accurate and most consistent of the data recorded on the mudlog.

Most logging units contain a gas chromatograph (see [[Mudlogging: equipment, services, and personnel]]). The hydrocarbon gas chromatograph is probably the most accurate and most consistent of the data recorded on the mudlog.

In gas chromatography, a fixed volume gas sample is carried through a separating column by a carrier gas, usually air ([[:file:mudlogging-gas-extraction-and-monitoring_fig3.png|Figure 3]]). The column contains liquid solvent surface or a fine molecular sieve solid. By difference in gas solubility or by differential diffusion, the gas mixture becomes separated by molecular weight into its components, the lightest traveling quickly through the column and the heaviest more slowly. Common alkanes detected within a reasonable time frame include

In [[gas chromatography]], a fixed volume gas sample is carried through a separating column by a carrier gas, usually air ([[:file:mudlogging-gas-extraction-and-monitoring_fig3.png|Figure 3]]). The column contains liquid solvent surface or a fine molecular sieve solid. By difference in gas solubility or by differential diffusion, the gas mixture becomes separated by molecular weight into its components, the lightest traveling quickly through the column and the heaviest more slowly. Common alkanes detected within a reasonable time frame include

[[file:mudlogging-gas-extraction-and-monitoring_fig4.png|thumb|{{figure number|4}}Peak heights from chromatographic analysis. (From <ref name=pt03r51 />.)]]

[[file:mudlogging-gas-extraction-and-monitoring_fig4.png|thumb|300px|{{figure number|4}}Peak heights from chromatographic analysis.<ref name=pt03r51 />]]

* Methane (C1)

* Methane (C1)

[[Category:Wellsite methods]]

[[Category:Wellsite methods]]