306 Subsurface Geologic Methods 



Experience has shown that the total water saturations measured for 

 limestone cores from oil fields in Caldwell, Guadalupe, and Milam Coun- 

 ties in south Texas may be considered as the connate-water saturations. 

 Cores cut with diamond-core bits using water-base mud normally show 

 total water saturations between 15 and 25 percent of the pore space when 

 the cores are oil-productive. 



The practice of using dextrose in cable-tool coring of partially de- 

 pleted oil sands in some Pennsylvania fields has proved practical in differ- 

 entiating between drilling, connate, and flood or extraneous water.^"^ 



A recent development of the capillary-pressure studies shows possi- 

 bilities of using the high-pressure mercury pump in measuring that por- 

 tion of sand-core samples occupied by capillary water.^ 



The use of capillary-pressure measurements for the determination of 

 cgpillary water is commercially practical. Of particular value are the 

 restored-state techniques in that the core samples need not be fresh. It is 

 highly desirable to know the height above the water table when selecting 

 the portion of the pressure curve applicable to the particular zone being 

 tested. The portion of the curve which represents the irreducible-minimum 

 water saturation may not apply to all reservoirs and is dependent on the 

 closure. In practice, a core more than fifty feet above the water table in 

 the reservoir will probably contain its minimum water saturation.^ 



A series of comparison tests in which the connate-water content of 

 a large number of cores cut with oil-base mud was compared with results 

 obtained by the following methods showed reasonably close agreement; 

 these included the use of the capillary-pressure method, calculations from 

 electric-log resistivities, calculations based on salinities, and distillation 

 measurements.^ 



The salinity of the residual water in cores cut with water-base mud 

 has been used in an attempt to estimate the connate-water saturation. 

 This method has been found to be unreliable for most cores taken from 

 flush, high-pressure reservoirs because of the contamination by the water 

 in the drilling fluid. Zones of very low permeability may show salinities 

 within the range of the formation water, but connate-water content calcu- 

 lated from these salinities may not apply to more permeable flushed zones 

 because of the relationship between permeability and connate water. Com- 

 parison tests using electric-log, oil-base, and tracer data have established 

 a logarithmic relationship between permeability, total water, and connate 

 water.^ Unfortunately, no one relationship will apply to all sand bodies. 



*'"' Clark, A. P., A Method for Determining Connate and Drilling Water Saturation for Cable Tool 

 Cores: Producers Monthly, July 1947. 



' Purcell, W. R., Capillary Pressures — Their Measurement Using Mercury and the Calculation of 

 Permeability Therefrom: Am. Irst. Min. Met. Eng., Fall Meeting, Dallas, Texas, Oct. 1948. 



^ Bruce, W. A., and Welge, R. J., The Restored State Method for Determination of Oil in Place and 

 Connate Water: World Oil, Aug. 1947. 



' Thornton, 0. F^, and Marshall, D. L., Estimating Interstitial Water by the Capillary Pressure 

 Method: Am. Inst. Min. Met. Eng. Tech. Pub. 2126, Jan. 1947. 



* Earlougher, R. C, Core-Analysis Problems in the Mid-Continent Area: Am. Petroleum Inst. Drill- 

 ing and Production Practice, 1940. 



