348 Subsurface Geologic Methods 



large enough for visual examination. However, since methods have been 

 recently developed for determining porosity and permeability from extra- 

 large cuttings obtained by reverse circulation, this method undoubtedly 

 still will be used to some extent in pay sections. 



Loss OF Cuttings 



Loss of cuttings samples is due to two chief causes: lost circulation, 

 and blow-outs. Both of these are primarily mud problems. Lost circula- 

 tion is caused either by too high water loss in the mud or by excessively 

 heavy mud which overcomes the formation pressure of a porous or frac- 

 tured bed so that the mud enters the pores or fractures. The cuttings are 

 then carried into the porous beds and may not ever be recovered unless the 

 well is completed in this zone and the cuttings come out later with the 

 oil. Blow-outs are caused by unexpectedly high formation pressures or 

 by carelessness in handling the mud. The cuttings are blown out of the 

 hole and not recovered. 



Elutriation 



Elutriation or separation of the coarse from the finer part of the 

 sample by the upward movement of the circulating fluid is due to the 

 use of mud with low viscosity and gel strength. In good mud the cuttings 

 are held in suspension and there is little change in the relative position. 



Methods of Obtaining Correct Depths 



Obtaining proper depth measurement for rotary cuttings is another 

 major problem. The depth of the well must be checked often either by 

 steel measuring line or by measuring the drill pipe under tension. Atten- 

 tion must be given constantly to see that the crew catches the samples at 

 ^ the proper intervals and that they do not anticipate the sample by filling 



several sacks at one time. 



A great help in assuring correct sample depth is to have the driller 

 keep a record of the drilling time on a form, such as table 18. Each 

 interval, 1, 5, or 10 feet, depending on the importance of the section, is 

 shown on this sheet with the time drilling began, the time it ended, the 

 time taken out for mechanical work, and the net time drilling. These 

 intervals are usually marked on the kelly with grease or chalk and 

 checked by the pipe measurement each time the kelly is drilled down 

 to the derrick floor. In addition to assuring the correct depth of the 

 samples, the drilling time gives a very valuable clue to the nature of 

 formations penetrated as the time of drilling has a very close correlation 

 with the lithologic character of the formation. This drilling time also 

 may be taken with a mechanical device. 



In pay sections where the exact measurements are very important 

 or in deep holes where tlie cuttings take a long time to come to the sur- 

 face, samples should be labelled with the depth at which they are actually 



