F = io/<:-^, (1) 



302 L. A. Galkin 



at low temperatvire for slightly saline fluids than for less viscous and more 

 saline fluids, and for high temperature fluids. 



It is obvious that, in order to allow for the degree of degasification of the 

 degasifier used, and to exclude the eSect of drilHng fluid properties on the 

 results of the gas logging, it is essential to perform a calibration — a deter- 

 mination by experimental means of the dependence of gas readings on the 

 gas content of the drilHng fluid. Such a caHbration should be carried out 

 before each operation and on every occasion in case of changes in the prop- 

 erties of the drilling fluid. 



In calibration, a sample of drilhng fluid is taken, it is degasified by means 

 of a TVD instrument, and its content of hydrocarbon gases is determined. 

 Then the gas saturation of the driUing fluid in cm^/1. is determined from 

 the formula 



C_ 



Q' 



where K is amount of gas-air mixture in the gas container of the TVD 

 degasifier in cm^; C is the concentration of combustible gases in the gas- 

 air mixture in per cent; Q is volume of the degasified solution in cm^. 



At the same time as the sample is taken, the readings of the gas analyser 

 of the station are noted. 



The results of the calibration are plotted on a graph with percentage gas 

 indications as abscissae and the gas saturation i% obtained by the method 

 described above, as ordinate (both after allowance has been made for back- 

 ground values). By drawing a straight line through the origin of coordinates 

 and the point obtained on the graph, we obtain the calibration curve. By 

 making use of this curve it is possible to arrive from the gas indications in 

 per cent, obtained in the gas logging (after allowance for background values), 

 at the actual gas saturation of the solution. 



In practice it is recommended to choose a calibration curve from those 

 drawn on Fig. 1 according to the results of comparison of gas readings with 

 data on the actual gas saturation of the drilling fluid, obtained by the method 

 described above. 



Using data on the actual gas saturation of the fluid one can make a quaU- 

 tative estimation of the gas contained in a bed, which is marked on the gas 

 log by high gas readings. The gas content of a bed can be expressed by the 

 gas factor— the content of gas in 1 m^ per volume of porous space in 1 m^. 



The mechanism whereby hydrocarbon gases enter the drilling fluid has 

 hitherto not been studied. Various points of view are held on this problem. 

 Most investigators consider that gas and oil are transferred to the drilling 

 fluid principally from the pores of rock which has been drilled through; 



