Mud analysis logging is a direct method of detecting oil and gas in formations drilled. The factors that determine the hydrocarbon content of the drilling mud are discussed. The advantages and disadvantages of the various techniques for separation of gas from the mud and for gas detection are considered.
Examples included show field results of the latest mud logging development - the quantitative analysis of drilling mud for volatile hydrocarbons. Mud analysis provides the means for obtaining valuable formation information not otherwise available. Now, the system furnishes even greater diagnostic data for interpretation.
Mud analysis well logging is not a recent development. Even before the introduction of rotary drilling, cable tool drillers logged their hole with ballings. The early rotary drillers kept a watchful eye on the mud returns for telltale gas bubbles. They continued to examine formation fragments - this time bit cuttings - removed from the mud by screens or by settling. The "feel" of the bit told them whether they were drilling in sand or shale, and changes in rates of penetration indicated changes in subsurface rock strata. Tops and thicknesses of formations were logged on the rotary drilling reports.
Mud logging technology has come a long way since 1939 when it was first introduced commercially. While all mud logging techniques have been continually improved, the greatest strides in development have been made in Mud gas separators and mud gas detectors.
The most important diagnostic in mud analysis logging is the record of gas detected in the mud. Hydrocarbons in the formation drilled are released to the mud flow, first, at the instant the section is drilled and, second, by gas expansion in cuttings as the mud pressure decreases while the cuttings are carried to the surface. The relative concentration in mud of hydrocarbons released from the reservoir rock during drilling depends on the extent to which the rock is flushed by the mud filtrate and on the rate of mud circulation. The following three factors influence the amount of flushing.
