Subsurface Laboratory Methods 309 



tions when the fault lies in the disagreement between driller's and electric- 

 log depths. Cores are usually submitted to the core analyst with the 

 driller's depths designated. Electric-log depths may differ by several feet 

 from the driller's depths, and, unless the correction is specifically made, 

 the core-analysis report will be in error as to depths. The writer strongly 

 emphasizes the importance of correct depth designation. 



Careful note should be made as to the type of drilling fluid used. 

 Special muds such as low-water-loss, modified-starch drilling fluids are 

 often used in coring sands with low productive capacity. Total water 

 saturations in the same sand may vary with the type of water-base drilling 

 fluid used, and this factor nmst be taken into consideration in the data 

 interpretation. It is readily apparent that the practice of washing a well 

 with water after drilling or coring with low-water-loss mud defeats the 

 purpose for which these special muds were used. Wells of this type should 

 be washed and cleaned with crude oil. In other words, the completion 

 techniques employed by the operator may nullify accurate work on the 

 part of the core analyst. 



Failure properly to cement or squeeze gas zones above an oil sand 

 will result in high gas-oil-ratio production (fig. 127) . Sand bodies drilled 

 into bottom water must be squeezed or water will be produced (fig. 127) . 

 The use of excessively high pressures to break down the sand while 

 cementing may result in sand fractures which will cause channeling and 

 bad completions. The necessity of squeezing the same sand several times 

 in order to shut off gas or water may permanently damage the sand and 

 result in abandonment of the well. 



No definite set of rules or tables can be made for the interpretation 

 of core data. As a matter of fact, the practice of some analysts in making 

 interpretations as to probable production from the data without seeing 

 the cores often leads to bad predictions. 



The interpretation of data from flush, deep, high-pressure reservoirs 

 is entirely different from the interpretation of core data of samples cut 

 from shallow, low-pressure, depleted reservoirs. However, there are some 

 characteristics which enable the interpreter to predict correctly the type 

 of production to be expected and to locate the fluid contacts.^ 



Gas Sands 



The following physical characteristics are associated with permeable, 

 high-pressure gas sands: 



1. Low water saturation dependent on permeability, formation pres- 

 sure, and degree of contamination by drilling water. 



2. Residual-oil saturation absent or present in varying degree, de- 

 pending on the gravity of the oil and the height above the gas-oil contact. 

 Residual-oil color will vary from amber to yellow. 



3. If thoroughly flushed, the salinity of the core water will approach 



^ Caran, J. G., Core Analysis — An Aid To Profitable Completions : Mines Mag., vol. 37, no. 2, 

 Feb. 1947. 



