PROBLEMS OF GAS LOGGING ESTIMATION 305 



If the actual value of the gas saturation exceeds those given in Table 1, 

 then it must be reduced by a factor of five or ten and the result obtained 

 must be increased by the same factor. 



Example —The consumption Q of drilling fluid during the time taken to 

 penetrate 0.5 m through the bed is 40,800 1. ; the gas saturation of the drilling 

 fluid is 21 cm^/1.; the porosity of the bed is 20 per cent; the diameter of the 

 drill bit is llf in. It is required to find the gas factor for the bed. 



Because a gas saturation of 21 cm^/1. does not appear in Table 1, we reduce 

 it ten times; for a gas saturation of 2.1 cm^/1. and the nearest value of Q 

 to the actual consumption of 40,800 1. the gas factor will be between 10 and 

 15 (more precisely 12.4) ; thus the gas factor of the bed will be 124 m^/m^. 



Table 1 is calculated for a bed porosity of 20 per cent. If the porosity of 

 the bed has some other value, the gas factor is calculated from the formida 



jna 



where: a is the gas factor obtained from the tables; 

 % is the required gas factor; 

 m is the porosity (per cent) of the rocks for the given bed. 



Conclusions as to the nature of the bed are reached by comparison of 

 the gas factor obtained with values of the gas factor for loiown beds of the 

 given and neighbouring localities. 



Usually gas factors below 3 m^/m^ correspond to waterbearing strata, 

 containing dissolved hydrocarbon gases; beds with residual gas and oil 

 saturation have gas factors roughly three to four times less than productive 

 beds. The gas factor for gas-bearing beds is numerically equal to the bed 

 pressure. 



In practice it is not rare to find cases where waterbearing beds with resid- 

 ual gas and oil saturation have the same gas factor as productive strata. 

 In order to avoid errors in such cases, it is necessary to know the composi- 

 tion of the gas or the quality of the bitumen. 



On the basis of the foregoing it can be concluded that interpretation 

 using the calibration curves and Table 1 permits the effect of physicochem- 

 ical properties of the drilling fluid, and also the effect of drilling condi- 

 tions on the results of gas logging to be excluded; it also allows a more cor- 

 rect result for the gas saturation of the rocks which are being drilled. 



GAS ENRICHMENT AND DEGASIFICATION OF THE DRILLING FLUID 



The gas saturation of the drilling fluid during boring of a productive bed 

 depends very much on the rate of boring. Table 2 shows rates of boring 



Applied geophysics 20 



