Subsurface Logging Methods 347 



stable. Thus, the samples contained large amounts of extraneous material. 

 The practice of considering any new material appearing in the samples as 

 composing the entire content of the formation drilled then became general 

 and any other material was believed to be cavings. At present, however, 

 the treatment of rotary muds has advanced so that usually a properly 

 mudded rotary hole caves very little and the percentage content of the 

 sample may be taken as representative of the types of rock in the interval 

 covered. This enables the geologist to follow very closely the lateral grada- 

 tions in the section. 



It should be emphasized that the geologist must work with the drilling 

 superintendent to insure the maintenance of proper mud in the hole not 

 only for the sake of drilling progress but also for the securing of repre- 

 sentative cuttings samples. The mud must have a gel strength and viscosity 

 sufficient to bring the cuttings to the surface without recirculation and 

 regrinding. It must be also capable of cushioning the impact of the drill 

 pipe against the walls so as to prevent powdering of the samples. Once 

 on the surface, the mud should be run through pits sufficiently large to 

 insure settling of all the cuttings so that they will not recirculate. A shale 

 shaker will insure complete separation of the larger cuttings and the mud. 



Powdering of Cuttings 



The powdering of the sample to a size too small to be examined 

 effectively under the binocular microscope is largely due to poor mud 

 which allows regrinding of the sample by the bit and powdering by 

 whipping of the drill pipe against the walls of the hole. When extraordi- 

 narily large cuttings are needed for analysis of porosity and permeability 

 or for other purposes, it is helpful to circulate in reverse of the usual 

 manner, that is, to pump the mud down between the casing and drill pipe 

 and up through the drill pipe. This process results in higher mud veloci- 

 ties returning through the drill pipe which brings larger cuttings from the 

 bottom as soon as they are chipped off by the teeth of the rock bit. Some 

 of these cuttings are 4 to f inch across. 



Reverse circulation is especially useful when drilling into low-pres- 

 sure formations, by using oil as drilling fluid, as it prevents any of the fine 

 oil-borne cuttings from plugging the pores of the formation. In regular 

 circulation with oil, the low viscosity of the oil will not carry out the 

 cuttings, and they are commonly reground to a fine putty-like mass which 

 is useless for examination and has a plugging effect on low-pressure pays. 

 Of course, reverse circulation is not necessary in high-pressure gas or 

 oil pays since the high pressures tend to increase the velocity of the cir- 

 culating fluids and carry the cuttings out of the hole. The advantages of 

 using a water-free drilling fluid in low-pressure oil pays are combined 

 with the advantages of a high-viscosity mud in carrying out the cuttings 

 and cleaning the formation in oil-base muds recently developed. With 

 these muds it is not necessary to use reverse circulation to obtain cuttings 



