304 Subsurface Geologic Methods 



volume, percentage of the pore space, and barrels per acre-foot of for- 

 mation (figs. 127 and 131). 



Residual Core Water 



In high-pressure, flush reservoirs cored with water-base mud, the 

 amount of residual or total water in the core at the time of analysis is 

 the sum of the connate plus any drilling water that may have been forced 

 into the pores of the sand while the core was being cut. 



If the formation contains minerals characterized by a high chemically 

 bound water content, this water will also be recovered when high-temper- 

 ature-retort methods are used, and corrections for such water of crystal- 

 lization should be made. Low-temperature extraction methods seldom 

 recover water of crystallization. 



Cores cut with oil-base mud from low-pressure reservoirs normally 

 show total water saturations that may be assumed to be the connate or 

 interstitial water saturation of the formation. Deep, high-pressure reser- 

 voirs cored with oil-base muds show total water saturations that may not 

 be the true interstitial-water content because of the high temperatures en- 

 countered. 



The total core water is usually presented in percentage of the pore 

 space. 



Connate Water 



Connate water is present in varying degree in all water-wet sand or 

 lime formations. It has been termed both "interstitial" and "connate" 

 water. In the writer's opinion, the term "connate" water is the more 

 desirable and will be used throughout this discussion. 



Connate water is defined as the water in the formation at the time 

 the formation is cored. It is immaterial in core-analysis work whether 

 it is the same water that saturated the sand when it was first deposited 

 or water that migrated into the sand during later geologic time. The 

 connate water may be made up of "free" water, and the water may be held 

 in the interstices of the sand by capillarity. The free water may be pro- 

 duced, but the capillary water is not producible. Most oil or gas sands 

 are water-wet and contain some connate water, although water-free fluids 

 may be produced. 



A number of methods are available for measuring or calculating the 

 connate-water saturations of sand formations. 



1. Capillary pressure versus water saturation by gas-pressure and 

 centrifugal methods. ^^ 



2. The use of tracers in the drilling fluid.^^ 



3. The calculation of saturation based on electric-log resistivities.^® 



=>< McCullough J. J., Albough, F. W., and Jones, P. H., Determination of the Interstitial-Water Con- 

 tent of Oil and Gas Sands by Laboratory Tests of Core Samples: Am. Petroleum Inst. Drilling and 

 Production Practice, 1936. / o r c j » 



i^ Pyle, H. C, and Jones, P. H., Quantitative Determination of Connate-Water of Oil Sands: Am. 

 Petroleum Inst. Drilling and Production Practice, 1936. 



58 Jones, J. P., Water Saturation vs. Resistivity: Petroleum Production, vol. 1, pp. 54-56, 1916. 



