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Oilfield Glossary
Oilfield Glossary
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carbide lag test

1. n. [Drilling]
A test performed by the mudlogger or wellsite geologist, used to calculate sample lag. The lag period can be measured as a function of time or pump strokes. Acetylene is commonly used as a tracer gas for this purpose. This gas is generated by calcium carbide, a man-made product that reacts with water. Usually, a small paper packet containing calcium carbide is inserted into the drillstring when the kelly is unscrewed from the pipe to make a connection, and the time is noted, along with the pump-stroke count on the mud pump. Once the connection is made and drilling resumes, the packet is pumped downhole with the drilling fluid. Along the way, the drilling fluid breaks down the paper and reacts with the calcium carbide. The resulting acetylene gas circulates with the drilling fluid until it reaches the surface, where it is detected at the gas trap, causing a rapid increase or spike in gas readings. The time and pump-stroke count are again noted, and the cuttings sample lag interval is calculated.