Some Recent Developments in Mud-analysis Logging

Abstract

Abstract Method As the drilling of a well progresses, the drilling bit dislodges anddisintegrates a cylindrical section of the formation. If oil or gas iscontained in the pore spaces of the cylinder, some of the contents of theseporous spaces will be entrained in the drilling fluid. In wall logging by mudanalysis, the mud is continuously examined for oil and gas on its return to thesurface, and the results are correlated back to the actual depth of the hole atthe time the mud was clearing the eyes of the bit. The test for gas is made by diverting a portion of the circulating mud from theflow line to a separator or gas trap, where the mud is thoroughly mixed withair and a portion of the gas entrained in the mud is removed. A stream of airis drawn countercurrent to the flow of mud in the gas trap, thus materiallyassisting the separation of the gas from the mud. The air-gas mixture is thendrawn into a "hot-wire" gas-detector instrument where the percentage ofthe combustible gas is determined. The presence of oil in the formation drilled is detected by a physicalexamination of the drilling mud under ultraviolet light. A sample of the mud istreated to reduce the surface tension and gel strength, after which it isplaced in a viewing box. Mud-analysis logging provides identification of the fluid content of formationsdrilled by the rotary method and permits the accurate correlation of oil andgas shows with the depth. Mud-analysis logging eliminates much coring and hasan especially important application in drilling programs when coring isparticularly hazardous or the electric log is difficult to interpret correctly.An appreciation of the factors that influence the results of the mud analysis, such as rate of penetration, mud properties, coring, and characteristics ofreservoir rock as related to the "flushing effect," permits a betterunderstanding of the mud-analysis log. Recent studies of the relationship of the two gas readings obtained by thegas-detector instrument have proved significant. The difference between thetotal combustible gas reading and the higher molecular weight gas readingindicates the presence of methane. With few exceptions, zones not showing anincrease of methane probably can be condemned. Mud-analysis logging furnishesvaluable information, which generally cannot be obtained by othermethods. Introduction Logging by mud analysis of wells drilled with rotary tools is a direct methodof locating oil-and-gas-bearing formations. The use of this method of logginghas increased each year until now more than 20'Field units are being used in 13oil-producing states and in some foreign countries. Several papers have beenpublished in the petroleum trade journals describing in detail the equipmentand the application of mud-analysis logging. This paper will discuss brieflythe method and review its application and some of the late developments. T.P. 2026