ABSTRACT: Design and Operation of Mudlogging Equipment
CEARLEY, J. DOUGLAS
Hydrocarbon well log analysis instrumentation typically consists of a hotwire, or total gas detector, and a gas chromatograph. Commercial brands of logging systems include Analytical, Aztec, Core Lab, Lee, Roach, and Midland Instruments. These systems share a basic flow system design and most utilize a filament type gas sensor. Gas contained in the porosity of the rock is entrained in the mud flow along with the drill cuttings. Only in the case of underbalance does other formation gas enter the annulus; this represents a control problem. Some extremely permeable or fractured formations may instantly flush, if overbalanced, and yield only a very mild gas increase, although they might produce a flowing drill-stem test.
Gas is separated from the mud at the shaker by an agitator. The eggbeater action of this device is retarded by high mud viscosity, and less gas may be removed from the mud stream. Flow from the trap to the instruments is typically through 1/4" outside diameter poly tubing; use of 3/8" tubing greatly facilitates reliability.
A new type of ceramic bead filament allows measurement of gas at levels which previously demanded air dilution of the sample. This filament has a very long working life.
Hotwires are calibrated with 100 percent methane test gas. Gas from a BIC brand cigarette lighter, which is 100 percent isobutane, can substitute if calculations for the difference in BTU (about 3 to 1) are applied.
AAPG Search and Discovery Article #90947©1997 AAPG Southwest Section Meeting, San Angelo, Texas