304 L. A. Galkin 



is a high rate of circulation of the drilhng fluid at rates of boring not exceed- 

 ing 5 m/hr. However, this point of view is incorrect ; this follows if only 

 from the fact that in regions of the Ural-Volga province productive beds 

 of the Naryshevskian Beds and sediments of the Upper Givetian substage, 

 despite the occurrence of the favourable conditions indicated above, are 

 quite often not shown up by gas sampling. Neither is confirmation obtained 

 for the possibility of creating the pressure difference— necessary for filtra- 

 tion—between the bed and the bore-hole during rotation of the bit. 



Assuming that gas and oil enter the drilling fluid from the rock that has 

 been drilled out, the following formula can be written for the gas factor a: 



Vnm VnTn 

 where: m is the coefficient of porosity; 

 Q^ is the pump capacity; 



V^ is the volume of rock drilled out in the time t\ 

 Q is the consumption of drilling fluid in the time t. 



This formula can be used to determine the gas factor. 



In calculating the gas factor it is convenient to use Table 1, which has 

 been constructed from formula (2) for the most probable values of effective 

 porosity {m = 0.2) and well diameter (llf in.). 



In order to find the gas factor, using Table 1, for a selected portion of 

 the gas log showing high gas readings, the consumption Q of drilHng fluid 

 corresponding to the drilling time for an interval of 0.5 m is determined; 

 against the value found for Q and the actvial gas saturation of the drilling 

 fluid, found as described above, the gas factor is obtained from the top 

 Une*. 



If the consumption of drilling fluid is kno^vn for a penetration of 0.25 or 1 m, 

 then the same course is followed as in the previous case, but the result ob- 

 tained is respectively multiplied or divided by two. 



If the bit diameter is different from llf in. (the diameter for which the 

 table is calculated) then we obtain the gas factor by multiplying the value 

 obtained from the table by a coefficient K, which has the following value '^. 



* In calculations the coefficient of oil and gas saturation is taken as unity, and therefore 

 the gas factor obtained must be multiplied by the coefficient of oil saturation. 



+ The correction factor for a core drill is not derived: it is the same as for an ordinary 

 drill. 



