Yield point drilling fluids

Of even more economic importance is the rheological impact of the addition of KC1 to conventional water-base drilling fluids. KC1 causes undesirable increases in both yield point and gel strength that can only be eliminated by chemical disperants or by dilution with fresh water. Dilution in turn requires more KC1 for clay inhibition, and the cycle continues with mud costs escalating exponentially. 

The results of these tests and the fluid formulations axe shown in Table IV. The adverse effect on fluid rheology of adding potassium, KC1, Test B, and TKPP, Test C, is evident. Both salts cause increases in yield point, YP, and initial gel strength. TKPP produced only about half as much increase as did KC1 and can be added in much higher concentrations before the rheological properties of the drilling mud become unacceptable. 

Apparent vis.=18.5 cp, Plastic vis.=13.0 cp, Yield point=ll lb/100 ft.2 These values are characteristic of a superior, low-solids, drilling fluid that promotes a high drilling rate and good solids removal. In addition, maximum inhibition of hydrating clays is provided. 

The results of the latest research into helical flow of viscoplastic fluids (media characterized by ultimate stress or yield point ) have been systematized and reported most comprehensively in a recent preprint by Z. P. Schulman, V. N. Zad-vornyh, A. I. Litvinov 15). The authors have obtained a closed system of equations independent of a specific type of rheological model of the viscoplastic medium. The equations are represented in a criterion form and permit the calculation of the required characteristics of the helical flow of a specific fluid. For example, calculations have been performed with respect to generalized Schulman s model16) which represents adequately the behavior of various paint compoditions, drilling fluids, pulps, food masses, cement and clay suspensions and a number of other non-Newtonian media characterized by both pseudoplastic and dilatant properties. 

In addition to the plastic viscosity and yield point, the gel strength of the drilling fluid is measured after it has been at rest for a given period of time. The usual procedure (64) is to shear the sample and then allow the sample to be unsheared for 10 s or 10 min. After the prescribed... 

Figure 7. Determination of yield point and plastic viscosity of drilling fluid using the Bingham plastic fluid model.

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