Gas Determination from the Drilling Mud

 Gas Determination from the Drilling Mud

As the drill bit breaks loose the formation, cuttings and gas in the formation are
transferred to and entrained in the drilling mud and transported to the surface.
With this in mind, the surface data logger hypothesizes the existence of a direct
relationship between the kind and amount of gas and/or oil in the drilling mud
arriving at the surface, and the gas and/or oil that was in place in the formation
being drilled at the time that particular mud was passing by the bit at the bottom of
the hole. It may be an over simplification at this point, but this represents the
situation in general and the description of the gas parameter.
The gases, if present, are assumed to be released by the formation and cuttings
into the mud stream to be entrained in solution in the mud. The concentrations or
amount of this entrainment normally encountered is on the low order of less
than 5%. This entrainment is also influenced by pressure, temperature, etc.

All that remains now is to convert this parameter to a meaningful representation of
the character of the formation before being disturbed by the bit. This conversion is
accomplished by three parts of the gas detector and related equipment. These parts
are:
1. The gas trap, which is the device for removing gases from the drilling mud.
2. The transporting equipment, consisting of a sample pump, pumping the
gas-air mixture to the gas detector, the hoses, the plumbing, and flow
regulation equipment.
3. The gas detectors proper (Total Hydrocarbon Analyzer and Gas
Chromatograph). These detectors are the Flame Ionization Detectors, see the
Sperry-Sun Drilling Services Gas Systems Manual for detailed information
on these detectors and flow conditioners

Aeration Gas Trap
The gas readings from the drilling mud as related to fluids and gases in-place in
the formation must be interpreted with the following consideration in mind:
The extraction of this gas from the drilling mud must be done in a manner that is
independent of variables such as density, viscosity and gel strength of the mud; in
a manner independent of the flow rate of the mud through the whole mud system;
in a manner so that all the gases as completely possible may be extracted even
from a high gel strength mud, and in a manner which would be considered reliable
around drilling rig conditions which tend to be destructive of sensitive equipment.
Sperry-Sun currently uses two types of gas traps. These are air powered and
electrically powered.
In operation, the bottom of the trap lies submerged about two inches under the
surface of the returning mud stream. The mud, tending to seek its own level, flows
in the inlet in the bottom of the trap canister. Rotation of the motor-driven impeller
blade causes this mud to whirled around rapidly. The centrifugal force of this
whirling action causes the level of the mud to be raised around its periphery inside
the canister until it flows out the discharge on the side of the trap.
The depth to which the trap is lowered into the mud should be adjusted to give a
continuous sample of 3 gallons per minute (1 quart in 5 seconds) of mud flowing
through the trap

Simultaneously, as the sample of mud is being pumped through the trap, the
whirling action of the impeller whips air from the atmosphere inside the canister
into the mud. These bubbles of air tend to become united with the tiny molecules
of gas entrained in the mud and being much larger, develop a size having a surface
tension sufficiently low to be released from bondage by the drilling mud. These
bubbles of air thus serve as a carrier by going into the mud, uniting with the gas 

 and carrying the gas out of the mud into the atmosphere of the canister

Hints and Precautions:
1. The trap should be located as near as possible to the discharge of the flowline,
or at least in a way so that it will have immediate access to the mud returning
to the surface.
2. The trap should be located in an open atmosphere. The trap discharge must
have immediate access to fresh air. This may not always be possible on some
rigs.
3. If difficulty is experienced with lost circulation material, removing the flange
on the trap bottom sometimes helps maintain a steady flow of mud.
4. Keep the locking screw on the adjustment jack tight after adjustments are
made.
5. Mud should not be discharging from the trap in an intermittent manner but
should exit in a continuous flowing manner

Transportation Equipment
After the mud has been sampled and the gases removed from it, these gases must
be transported to the gas detectors in the logging unit. This is accomplished by a
small motor-driven compressor which is connected to the trap by a length of
rubber hose. The compressor pulls a continuous stream of fresh air in through the
discharge of the trap. As the gases, if present, are being continuously extracted
from the mud in the trap, they will be continuously mixed with this stream of air
and carried with the air into the logging unit through the connecting hose. There,
the flow of air, or air-gas mixture, passes through additional flow regulation
equipment, plumbing and instruments and finally arrives at the detector element
for continuous detection and monitoring

In the logging unit, the flow is kept constant by the Total Hydrocarbon
Conditioner, and the flow rate is read on the flowmeter. The flow is split either
two or three ways, and the majority of the flow is discharged back to the
atmosphere.
The optimum flow of air from the trap is 6 to 8 cubic feet per hour (cfh). The total
flow meter is a small flowrater, calibrated to read directly in cfh of 0 to 10 cfh.
The flow is adjusted by an adjustment knob in the flowrater and should be kept at
6 to 8 cfh which will be maintained at this constant by the regulator. More volume
of the air-gas mixture than is necessary for detection is drawn from the trap in
order to minimize the lag time between the arrival of a show at the surface and its
detection.
It now becomes apparent that a reliable gas detecting technique demands that the
number of influencing variables be kept in control. Ideally, only the amount of gas
in the mud and its corresponding reading should be variable. For this reason, it is
important that the trap consistently pump a constant volume of mud, that the
amount of air drawn through the trap be maintained at a constant, that there be no
leaks or restrictions in the flow system

Gas Detection Equipment
See the Sperry-Sun Drilling Services, Surface Logging Systems, Gas Systems
Manual for the detailed operation of the Total Hydrocarbon Conditioner, Total
Hydrocarbon Analyzer, and Gas Chromatograph and the peripheral equipment
related to these instruments