thus, a nitrate other than water will not change the connate-water saturation of 

 the core. Oil or oil-base mud will displace interstitial water to the extent that 

 a transition zone or the actual water table might appear oil productive when 

 interpreted on the basis of laboratory results. For this reason, the use of such 

 coring fluids is not advisable when the oil-water contact is in question. 



The basic data necessary for reservoir information are contained in the 

 routine measurements of porosity and permeability, and the special measurement 

 of connate water. Of further value to the reservoir engineer are studies such as 

 relative permeability, water-flooding tests, displacement tests, pore-space dis- 

 tribution, and many others. 



CORE-ANALYSIS Core-analysis results (fig. 12-4) are reported 



REPORTING in a variety of forms of tabular and graphic 



arrangement. First and foremost are the tab- 

 ular results of the analysis on an individual footage basis which can be expanded 

 to include drill-stem-test data, lithology, perforated zones, and any other in- 

 formation the individual operator deems most essential. The same material is 

 often graphically portrayed in such a manner that producible zones can be 

 selected at a glance. Color adds to the picture, and attention to detail and color 

 selection often results in a very artistic and easily interpreted graph. 



The next step is to group the tabular results into producible sections or 

 zones with a weighted summary for each. This can be expanded, if connate- 

 water and formation-volume factor data are known, to include stock-tank oil in 

 place and estimates of recoverable oil under the different types of drive that 

 might be present in the reservoir. 



The movement of oil or gas from the reservoir into the well bore is a 

 function of permeability, among other factors. It is believed that the oil moves 

 first from and through the more permeable channels; thus the flush or initial 

 production will depend upon the total footage of higher permeability, irrespective 

 of where or at what particular depths this footage occurs. Sections of lower 

 permeability will progressively produce as the oil is exhausted in the higher 

 permeability zones. One can assume, then, percentages of depletion in the 

 permeability zones, a greater percentage of the oil in place being produced 

 from the higher permeability zones down to only a small percentage of the 

 oil in place in the extremely tight sections. 



It is thus advantageous to segregate the net pay thickness into ranges of 

 permeability, regardless of position in the lithologic column. These ranges can 

 be empirically selected for a particular field or area, depending upon fluid char- 

 acteristics; for example, a cutoff permeability of 5 millidarcys is used, and the 

 selected ranges are: below 5 md, 5-25 md, 25-100 md, 100-500 md, 500 md and 



247 



