Chapter 12 



SOME PROBLEMS OF GAS LOGGING ESTIMATION 

 OF GAS SATURATION OF ROCKS 



L. A. Calkin 



The object of gas logging is the detection of productive beds in the section 

 ■of a well. However, the currently used methods of gas logging do not permit 

 rehable evaluation of a bed. This is associated with the fact that, as a resuh 

 of gas logging, one determines the content of hydrocarbon gases in a gas- 

 air mixture, obtained by degasifying the drihing fluid. This content largely 

 depends on the method of degasification and the properties of the drilling 

 fluid being degasified. Thus, the degree of extraction of hydrocarbon gases 

 from the drilhng fluid will oscillate within wide limits, depending on the 

 viscosity, static shear stress, temperature and salinity of the fluid. There- 

 fore a direct relationship is not always observed between the gas measure- 

 ments on the gas log and the concentration of hydrocarbon gases in the 

 fluid. In its turn, the gas saturation of the drilling fluid depends not only 

 on the gas content of the bed but also on many other factors, the main 

 ones being rate of boring, rate of circulation of the drilling fluid and the 

 nature of the bed. 



It is quite clear that exact evaluation of the bed from the gas log data 

 can be given only when the above factors are taken into consideration. 



In order to establish the relationship bet^veen the readings obtained in 

 gas logging and the actual gas saturation* of the drilling fluid, we analysed 

 the results of gas logging conducted in various regions, by the simultaneous 

 appUcation of a floating degasifier and a TVD degasifier, whose degree of 

 degasification is about 100 per cent. It was established that in most cases 

 a direct correspondence is observed between the actual gas saturation of the 

 drilling fluid in cm^/1. (data of TVD instrument) and the readings of a sta- 

 tion, the reading being obtained in working with a floating degasifier. How- 

 ever, in some cases this regularity is not maintained, and quite different 

 readings are obtained for the same gas saturation, the readings being lower 



* By actual gas saturation we understand the amount of gas (in cm^) contained in 11. of 

 drilling fluid. 



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