8 NOVEMBER 2021 WorldWide Drilling Resource® Borehole Cleaning Part 2 of 2 by Ronald B. Peterson Drilling Products Specialist, Mountainland Supply Company If a drilling fluids professional uses a Fann model 280 field viscometer to measure some of the properties of the drilling fluid, he can use the numbers he generates to determine other drilling fluid properties such as Plastic Viscosity (PV), Yield Point (YP), and Gel Strengths. These are all significant properties in drilling: 1 Plastic Viscosity (PV) - Generally, the plastic viscosity is related to the size, shape, and number of particles in a moving fluid. It is an indication of the contribution to the fluids viscosity under dynamic flow conditions, of the entrained particles in the drilling fluid. PV is calculated using shear stresses measured at 600 and 300 with a Fann model 280 field viscometer. 1 Yield Point (YP) - The force or interaction exerted between the individual particles. Yield point is exhibited as the force required to initiate flow. YP is calculated using shear stresses measured at 600 and 300 with a Fann model 280 field viscometer. 1 Gel Strengths - The time-dependent measurements of a fluid’s shear stress under static conditions. Gel strengths are commonly measured after 10-second, 10-minute, and 30-minute intervals, but they can be measured for any desirable length of time. Plastic viscosity can be an indication of the density of the fluid. A higher number of solids entrained in the fluid will usually result in a higher drilling fluids weight. Yield point is an indicator of the suspension capability and the carrying capacity of the fluid. In general, the higher the yield point, the greater the capacity of the fluid to carry and suspend cuttings. A high yield point reduces the ability of the cuttings to settle out on the surface. In general, you want the yield point to be as low as possible and still maintain adequate suspension qualities in the borehole, with minimal suspension qualities in the pits. If you need to run a weighted fluid to control an artesian flow, then you will need a yield point of 12 to keep the weighting material in suspension. Gel strengths are the ability of the fluid to become thicker under static conditions, or when the fluid is not moving. Gel strengths help keep the cuttings in suspension. Generally, you want the gel strengths to be fairly low but progressive. One of the things that needs to be considered is it requires a force called shear stress to break the gels or gel strengths. This puts additional force on the borehole and can result in loss of returns. The best way to break the gels is to start rotating the drill pipe before you start the mud pump. This will minimize the impact on the borehole in starting circulation and reduce the chance WTR Peterson continued on page 30
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