Figure 8-37. Effect of KCI and polymer solutions on linear swelling. Same shale as in Figure 8-36. (From Steiger.50 Copyright 1982 by SPE-AIME.)

The performance of KCI muds is greatly increased by the addition of certain long chain anionic polymers that coat shale surfaces, and thus protect the walls of the hole from disintegration (see Figure 8-37). The most likely explanation of the coating action is that the negative sites on the polymer chain are attracted to the positive sites on the clay crystal edges. Tests in the model borehole by Clark48 showed that partially hydrolyzed polyacrylamide-polyacrylate copolymer (PHPA) is the best polymer for maintaining hole stability in an illitic shale (Atoka). 10.5 lb/bbl (3%) KCI was added to prevent swelling (see Table 8-4). Tests by Bol62 in a similar machine showed that PHPA was the best polymer to prevent erosion of a montmorillonitic shale (Pierre), but 10% KCI was necessary to prevent swelling. Strain gauge tests have shown that PHPA does not inhibit swelling50—that is the function of the KCI. As already mentioned, the optimum concentration of KCI depends on the CEC of the shale. Hole stability can often be maintained with KCI alone, but considerably higher concentrations are required if the PHPA is omitted.

The degree of hydrolysis of the PHPA is important (see Table 8-4), presumably because it spaces the negative sites along the chain so that they match the positive sites on the clay platelet edges.

Another important benefit of formulating KCI muds with PHPA is that it coats the drill cuttings (usually referred to as encapsulation) thereby inhibiting their dispersion and incorporation into the mud. Table 8-5 shows the effect of various additives on the rate of dispersion of Pierre shale cuttings, expressed as a per-

Table 8-4

Polymers Evaluated for Shale Protection*1

* From Clark et al.48 Copyright 1976 by SPE-AIME.

tTest conditions. Alolcu shale, 3500 psi stress. S00 toot minuie l)ou rate.

"Designates no sample failure at test termination. cRun on different Aioka shale. 2500 psi stress

Table fc-5

Water Activities of Various Saturated Salt Solutions*

Desiccator number

Salt employed

0 0

Post a comment