of fluids within the core takes place. In particular, the solution gas in the oil 

 still remaining in the core expands and expels both oil and mud filtrate. 



The gross effect of the coring operation is to submit the core to a gas 

 drive and a water drive, though in reverse order. Thus, the assumptions: (1) 

 the core contains the irreducible minimum oil saturation, oil that cannot be 

 produced by any method short of mining; and (2) the difference between 100 

 percent and the sum of the percentages of residual oil and connate water is 

 theoretically recoverable reservoir oil. 



The usual analysis of cores taken with water-base mud as the coring medium 

 will indicate a residual oil content that is interpreted as irreducible, and will 

 indicate a water content that includes both connate water and an unknown 

 quantity of mud nitrate. The use of any other coring medium will also affect 

 the residual fluids. Crude oil, for example, will infiltrate the core just as mud 

 filtrate does, and the resulting analysis will show a high residual oil percentage 

 and a water content that is interpreted as connate. Table 12-11 gives an example 

 of a Wyoming sandstone cored with water base mud and the same sandstone 

 in a different well cored with crude oil. The connate water saturation of this 

 sand averages 20 percent. 



The interpretation of core-analysis data is influenced by such factors as 

 the coring medium, size of core, permeability, fracturing, lithology, and others 

 not under the control of the analyst. Far more important, though, than any 

 of the above is the previous core history of the field or area and the analyst's 

 experience in the interpretative phase of core analysis. Adherence to generalized 

 rules is inadvisable, but the following suggestions can be applied cautiously to 

 hard-rock areas: 



244 



