Subsurface Logging Methods 



363 



TABLE 21 



Detailed Recovery Estimate, University Well No. 2, Andrews County, Texas 



Generally, it is found that this volumetric estimation is somewhat above 

 an estimate made from pressure decline or production decline curves, 

 and allowances should be made. In spite of its imperfections, this method 

 of valuation is the only one available in many fields in which cuttings 

 samples have been kept but no other type of reservoir information is 

 available. 



A somewhat similar method may be used for sandstone reservoirs. 

 However, since sandstone reservoirs are much more uniform in their 

 porosity and permeability, it is commonly possible to assign an acre-foot 

 recovery for an entire field and to obtain the ultimate recovery by multi- 

 plication of the net pay — sand thickness in any well by the acre-foot re- 

 covery, rendering detailed pay analysis unnecessary. 



Logs of cuttings samples are also very useful in studying the char- 

 acteristics of limestone reservoirs. They give some idea of volume and 

 relative permeability of lenticular pay zones. They are a useful supple- 

 ment to any coring program undertaken as part of a reservoir study. It 

 is often possible to select gas-oil contacts from cutting logs so as to select 

 points for plug-back or for packer settings in remedial work. 



Summary 



Samples of well cuttings can be taken from both cable-tool and rotary 

 holes in such a manner that the nature of the formations penetrated can 

 be determined accurately. These samples can be described by a geologist 

 and the description plotted so as to give a graphic section of the formations 

 encountered in the well. These sections can be correlated to give a picture 



