Miscellaneous Subsurface Methods 713 



sions of the intergranular pores and those of the solution channels is 

 reflected very well by the abrupt changes of slopes in the capillary- 

 pressure versus water-saturation curves of figure 385. Nevertheless, the 

 connate-water saturation of all the samples is about the same (17, 16, 

 and 17 percent respectively), the ability to retain water being controlled 

 chiefly by the dimensions of the intergranular pores and the mineral make- 

 up of the rock, both of which are essentially constant in these samples. 



The net result of all these factors is that connate-water saturation may 

 be expected to differ from place to place in sedimentary rocks but that 

 capillary pressure at the same horizontal datum plane in a rock that is 

 permeable vertically as well as horizontally is single valued. The porosity 

 and permeability of rocks are intimately related to capillary pressure. 

 Although the relationship is intricate, the problem of the relationship, 

 leaving aside the effect of the properties of fluids, is one of stereogeometry, 

 which in sedimentary rocks or clastic sediments is fundamentally one of 

 geology. The concepts of capillary phenomena aid the geologist who is 

 interested in the problem of origin, accumulation, and discovery of oil 

 and gas. These concepts also aid the petroleum engineer who is con- 

 fronted with production problems, the solution for which must be based 

 upon the behavior of fluids in porous media. Quantitative data are made 

 available from capillary-pressure tests of core samples, and these data 

 can be used advantageously by both geologist and engineer. Capillary 

 data, combined with porosity, permeability, and other physical properties, 

 can be applied practically to increase the efiiciency of the finding and the 

 production of oil and gas, which is the ultimate objective of the production 

 branch of the petroleum industry. 



DRILLING FLUID CHEMISTRY 

 H. F. SUTTER 



The properties of the drilling fluid used in rotary drilling of oil wells 

 should be such as to promote safe and speedy drilling and completion 

 of the well with maximum productive capacity. This fluid under the pro- 

 pulsion of powerful pumps is forced down through the drill pipe and out 

 into the well through holes in the bit. Jetted against the bottom of the 

 well with high velocity, the circulating fluid is deflected upward and 

 flows back to the surface between the drill pipe and the walls of the well, 

 carrying in the ascending stream the cuttings formed by the drill. At the 

 surface, the drill cuttings are segregated by screening or gravity settling, 

 and the fluid is discharged into a mud pit, later to be again circulated 

 through the well. 



The drilling fluid performs a variety of different functions. Prompt 

 and continuous removal of the material loosened by the drill prevents 

 its accumulation in the well with the possibility of freezing the drill pipe. 

 The fluid must possess thixotropic properties so that, in the event of 



