Mud logging is vital for rig safety and gives the first indication of the potential success of a well.
In hydrocarbon exploration, hydrocarbon surface gas detectors record the level of well gas brought up in the mud. Drilling liberates gas and liquid formation fluids, and circulation of the drilling fluid carries these to the surface.
Mud logging, also known as hydrocarbon well logging or gas logging, entails gathering qualitative and semi-quantitative data from hydrocarbon gas loggers that record the level and chemical makeup of the gas brought up in the mud. Other properties such as drilling rate, mud weight, flowline temperature, oil indicators, pump pressure, pump rate, lithology (rock type) of the drilled cuttings, and other data are also recorded.
The main purpose is to identify all hydrocarbon indications from the oil and gas entrained in the drilling mud. Gas detected in the mud can be interpreted to be:
1. liberated gas
2. recycled gas
3. produced gas
4. contamination gas
5. trip gas
Only liberated gas indicates a possible prospect; the others merely confuse the analyst. This data, combined with the gas composition assists in the location of oil and gas zones as they are penetrated. The breakup of the gas shows into these categories reduces the chance of misinterpretation of a gas kick on the mud log.
Another important use of these logs is well safety, since overpressured zones, lost circulation, and
gas kicks will be recognized quickly and remedial action taken.
Total gas in the mud is measured in units of parts per million, but does not represent the actual quantity of oil or gas in the reservoir. Total gas is separated into its component parts by our Spectrometer. The most common gas component is methane (C1). Heavier hydrocarbons, such as C2 (ethane), C3 (propane), and C4 (butane) may indicate an oil or a “wet” gas zone.
Depth information is obtained from the driller’s log, which records depth versus the time of day. However, these depths cannot be used directly. We wish the mud log data to be presented at the depth of the drill bit, but the mud log measurements are made at the surface. The time it takes for the mud to move from the bit to the surface must be accounted for in positioning samples and gas kick data on the log. This time is called the lag time and depends on the velocity of the mud in the annulus between the drill pipe and the rock and the depth that the bit is at. This in turn depends on the mud pump speed and displacement, which are usually constant for reasonable periods of time.