The PV is also a function of the viscosity of the fluid phase of the mud (as temperature rises, the viscosity of water decreases, and the PV will decrease). As the mud density is increased, by the addition of barite or hematite (more solids), the PV will automatically increase. In the mud system, we have solids that are an integral part of the mud (bentonite, starch, CMC, etc.) and solids that are undesirable (sand, limestone, dolomite, etc.). Workbook correct mud density and carrying capacity are maintained without exerting unnecessary pressures on the annulus. We can see that control of the solids will give us control over our PV! This leads to "Why are we controlling the solids?" Since the viscosity of the mud is one of the principal factors contributing to the carrying capacity of the mud, the suspension of weighting materials, and pressure surges applied to the formation through frictional pressures in the annulus, it is obvious that increased solids will increase these annular pressures (and may increase the mud density), so a balance must be found in which the This is a measure of that part of resistance to flow caused by mechanical friction between solids in the mud, solids and liquids and the shearing layers of the mud itself. The " apparent viscosity" is given by the 600 reading divided by 2. The 600 dial reading minus the 300 dial reading gives the slope of the shear-stress/shear-rate curve. To obtain a value for this constant rate of increase, readings are taken with a viscometer at 511 sec- sec-1 (300 and 600 rpm).
With a Bingham plastic fluid, a finite force is required to initiate a constant rate of increase of shear-stress with shear-rate. The viscometer is designed to give the viscosity of a Newtonian fluid when used at 300 rpm.įor Non-Newtonian fluids, the ratio of shear-stress to shear-rate is not constant and varies for each shear rate. The Fann viscometer reading is therefore multiplied by 1.067 to obtain shear stress in lb/100ft2 or multiplied by 478.8, and divided by the shear rate in second-1 to get Dynes/cm2.ĥ11 x dial reading / shear rate (sec-1) since 511 sec-1 = 300 rpm or (300 x dial reading) / Fann shear rpm This produces viscosity as Dynes (sec/cm2) or poise. Viscosity (poise) = (F/A) / (V/H) where: F = Force (dynes) A = Area (cm2)
The units of viscosity are poise or centipoise (1/100 poise) and is derived as follows: To convert the dial reading to shear stress, the dial reading is multiplied by 1.067 to give a reading in lb/100ft2. (ft/sec)/ft) and is usually written as seconds-1. Shear rate is the rate of change as the fluid layers move past one another per unit distance, and is measured in reciprocal seconds (i.e.
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