Drilling Fluids Selection, Performance, and Quality Control
Authors: Kelly John 
Published: December 2007

Summary
The composition and properties of selected drilling fluids and the influence of the quality and kinds of fluid materials on their properties are discussed. The fluids considered are currently in use. With many sources of supply for mud-treating chemicals, it is important for operators to maintain some system of quality control to ensure receipt of products that will perform as expected.

Introduction
Because drilling conditions vary widely, the drilling fluids referred to in this paper will be those most applicable to the most difficult conditions. More important, it is hoped that the comments made concerning the mud treatment and behavior will be of special value. Considerable reliance has been placed on experience gained through consulting with operations personnel and through research on drilling fluids for 27 years. In many instances, the drilling fluid problems encountered in the field are a result of poor solids-control measures or the use of inferior products. Hence, these two subjects are also considered. Drilling problems often occur while drilling the surface hole; therefore, this paper begins by treating that portion of the well as it relates to drilling fluids.

Surface Hole Muds
Relatively little attention is usually given to the mud used to drill to the surface casing point in most wells. Its importance may be illustrated by the finding that in one drilling area, the surface-hole mud cost on one well was more than $20,000, whereas in a similar well the cost was, about $3,000. The difference was a factor of almost seven and resulted solely from the cost of mud treatment--one drilling procedure used expensive chemicals, and the other used simple inexpensive ingredients. The results in hole condition and ease of setting surface casing were very similar.
The principal factors in formulating the surface mud are cuttings-carrying capacity and, in some instances, filter-loss value.
For most cases, the surface mud can be composed simply of fresh water, bentonite, and slaked lime. The bentonite is hydrated first, and then slaked lime is added to increase the true yield-point value and hence the cuttings-carrying capacity at low shear rates. The amounts of bentonite and lime required will depend on the desired yield point of the mud. For example, 15 to 25 lbm/bbl (42.8 to 71.3 kg/m3) bentonite and 0.5 to 1 lbm/bbl (1.4 to 2.9 kg/m3) slaked lime may be used in many wells. If large gravel is being drilled, higher true yield points may be required and hence more bentonite and lime.
If some filtration control is needed, it may be achieved preferably by adding a nonionic filter-loss additive such preferably by adding a nonionic filter-loss additive such as starch. The reason for this treatment is best illustrated by the plots in Fig. 1. Treatments of the bentonite/lime mud by carboxymethyl cellulose (CMC) will eliminate the true yield-point value in most cases and, as shown in the figure, will significantly reduce the cuttings-carrying capacity at the low shear rates present in the annulus flow. Under these conditions, CMC acts as a thinner or deflocculant; it is an anionic material that adsorbs on the clays and acts like any other thinner.
JPT
P. 889