Miscellaneous Subsurface Methods 719 



for treating drilling fluids. The complex phosphates are good for lowering 

 viscosity and have a beneficial effect on filter loss. The tannins are very 

 effective for lowering water loss and frequently have a beneficial effect on 

 viscosity. Under certain conditions it is possible to use only complex 

 phosphates or tannin compounds for maintaining satisfactory properties. 

 In other cases complex phosphates and tannin compounds may be used 

 together. When first applied, these chemicals are very effective in nearly 

 every case, and. only small quantities are required. With the repeated 

 treatment that is required from time to time during the drilling of the 

 well, the further addition of chemicals does not always have the expected 

 effect. This is one of the problems in the successful chemical treatment of 

 drilling fluids. 



Only complex phosphates, such as tetrasodium pyrophosphate, sodium 

 acid pyrophosphate, sodium tetraphosphate, and sodium hexametaphos- 

 phate, are useful for treating drilling fluids. The orthophosphates are not 

 used because they are ineffective as thinners. 



Tannic acid or tannin is a complex organic compound occurring in 

 certain trees and plants. Pure tannic acids can be extracted from these 

 and other sources, but because of the expense involved, the less costly 

 crude extracts are used for treating drilling fluids. The tannin extracts 

 such as quebracho extract are commonly used primarily to reduce vis- 

 cosity and filter loss and to condition the filter cake. The proper use of 

 tannin extracts will almost always yield a fluid which loses very little 

 water to underground formations and will form a thin, tough cake that 

 protects the walls of the open hole. 



One of the most common troubles in drilling fluids is cement con- 

 tamination. Usually no "symptoms" are required to locate this trouble, 

 because it may be expected whenever operations require that cement be 

 drilled, and particularly when the cement has not set thoroughly. Calcium 

 hydroxide is present in cement slurries and is mostly responsible for the 

 observed changes in the fluid properties. The calcium ion replaces the 

 sodium on the clay particles and tends to convert them to calcium clays. 

 The calcium clays are not so highly ionized as the sodium clays. Conse- 

 quently, the degree of hydration and dispersion of the clay colloids may 

 be reduced. This fact results in an increase in filter loss and a flocculation 

 of the clay colloids. Flocculation results in high viscosities and gel 

 strengths. 



Commercial materials have been devised to recondition cement-cut 

 drilling fluids. These materials remove the calcium ions as an insoluble 

 precipitate, reduce the pH of the fluid to the approximate value before 

 contamination, and redisperse the clay aggregates. Frequently, as a result 

 of the treatment of the fluid for cement contamination, the fluid displays 

 better performance characteristics than it did originally. This is because 

 some of the commercial products include substances which counteract 

 the calcium ion. 



