water, and the gas-oil contact is interpreted as lying between the depths represent- 

 ed by samples 4 and 5. Sample 16 shows a decrease in oil saturation, with an 

 increase in water; and the transition zone, or water table, is predicted at the 

 depths represented by samples 15 and 16, with water definitely and unquestion- 

 ably present 3 feet lower. 



Preliminary data are usually available to the operator before the well is 

 ready for perforation, and these data will enable the operator to perforate below 

 the gas-oil contact (if present) and above the oil-water contact. The perme- 

 ability profile from the preliminary report also assists the operator in placing 

 the shots to avoid highly permeable streaks that might permit entrance of water 

 to the bore hole and tight, unsaturated rock that would not add to production. 



Reservoir Information 



The saturations so useful in well-completion information are seldom used 

 in a reservoir study. The reservoir engineer is primarily interested in the 

 storage capacity of the rock, the permeability distribution in the reservoir, and 

 the percentage of water under reservoir conditions. Porosity and permeability 

 are reported in the routine analysis, but connate water must be estimated or 

 determined in some other manner. 



The same plugs on which porosity and permeability measurements were 

 made can be used for connate-water study provided that the routine core- 

 analysis methods were of the type that do not contaminate or destroy the 

 specimen. The thoroughly cleaned and dried plugs are saturated with formation 

 water or a brine equivalent and subjected to a displacing fluid, either oil or air, 

 which displaces part of the water. Pressure differential is obtained by placing 

 one face of the saturated specimen in contact with a water-permeable plate or 

 membrane and applying pressure to the displacing fluid. Connate water satura- 

 tion is reached when a large increase in displacement pressure results in a neg- 

 ligible decrease in water saturation. 



The Messer evaporation technique (Messer, 1950; Dodd, 1951) is just 

 as reliable and much faster than the plate method above described. The cleaned 

 and dried specimen is saturated with a volatile solvent (usually toluene), 

 weighed, and suspended from a recording balance similar to the Gramatic. A 

 slow current of dry, warm air is passed across the specimen, and the weight is 

 recorded at equal time intervals. Connate water saturation is assumed to be 

 reached when the rate of evaporation, measured by the loss in weight, becomes 

 constant. 



The use of oil or oil-base mud as a coring medium gives a reliable measure- 

 ment of connate water. Connate water, being that fluid left coating the inner 

 surface area of the pore space not displaced by the invading oil, is assumed 

 to be immobile and will not be displaced by filtrate invasion or fluid expansion ; 



246 



