34 SEPTEMBER 2021 WorldWide Drilling Resource® Marsh Funnel Viscosity by Ronald B. Peterson Drilling Products Specialist, Mountainland Supply Company Marsh funnel viscosity, commonly called vis or viscosity, is one of the most frequently used drilling fluid terms. The viscosity of a fluid is a measure of its resistance to deformation at a given rate. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water. Viscosity can be conceptualized as quantifying the internal frictional force that arises between adjacent layers of fluid that are in relative motion. For instance, when a viscous fluid is forced through a tube, it flows more quickly near the tube's axis than near its walls. In such a case, experiments show that some stress (such as a pressure difference between the two ends of the tube) is needed to sustain the flow through the tube. This is because a force is required to overcome the friction between the layers of the fluid which are in relative motion. So for a tube with a constant rate of flow, the strength of the compensating force is proportional to the fluid's viscosity. A fluid that has no resistance to shear stress is known as an ideal or inviscid fluid. Zero viscosity is observed only at very low temperatures in superfluids. Otherwise, the second law of thermodynamics requires all fluids to have positive viscosity; such fluids are technically said to be viscous or viscid. A fluid with a high viscosity, such as pitch, may appear to be a solid. The viscosity of a drilling fluid is affected by the chemistry of the water; the additive used to make the fluid; the chemistry of the formations drilled; and to some degree, the temperature of the fluid. The water used as the base liquid for the fluid needs to be as free as possible of any chemicals and or total dissolved solids, it needs to be close to a neutral pH, and it needs to have a minimal concentration of polar or charged material. To obtain maximum performance from most drilling fluid additives, a pH between 8.5 and 9.5 is ideal. Most drilling fluid additives produce the best results in that range. Hardness from calcium, magnesium, and manganese (total hardness) needs to be below 200 parts per million (ppm). If we can avoid any magnesium or manganese, it is preferable. The average hardness of water in general is somewhere above 200 ppm. There are, as usual, exceptions . . . The use of soda ash at approximately ½-pound per 100 gallons to one pound per 100 gallons is usually sufficient to adjust both the pH and hardness to the desired range. The use of pH strips and total hardness test strips can be used to verify both the need for treatment and the success of the treatment. The most effective and cost-efficient method of building viscosity is usually a good quality, high-yield bentonite. There are other qualities of a bentonite which need to be considered in building your drilling fluid. Some polymers provide viscosity, but few of the other characteristics that are beneficial in a drilling fluid in many circumstances. We will discuss the individual additives and the functions and properties in a later column. Viscosity is basically resistance to flow; and the higher the viscosity, the more pump pressure it requires to keep the fluid moving. It is best to have the viscosity as thin as possible yet still provide the fluid characteristics needed to successfully drill the well. You need to know what the desired viscosity is, based on the formations you will drill. Make sure it is where you want it when you start, and measure it frequently to make sure it is where you need it to be. Marsh funnel viscosity is often referred to as apparent viscosity or relative viscosity. It simply gives you a good idea of and changes in the fluid, which can indicate changes in formations and the need to make other adjustments in your fluid properties. Viscosity of the drilling fluid is measured using a Marsh funnel, a viscosity cup, and a timing device. The marsh funnel has a specifically sized orifice in the bottom of it. Make sure the orifice is undamaged and clean. There is a screen halfway across the top of the Marsh funnel, and under the screen is a line. Hold your finger under the tip of the Marsh funnel and pour the fluid through the screen to ensure no particles large enough to plug the orifice get into the funnel. Fill it to the line - this is 1500 mL (milliliters) or 1½ quarts. The vis cup has a line near the top. This line is 1000 mL or 1 quart. When you have filled the Marsh funnel to the line, quickly hold it over the top of the vis cup and allow enough fluid to run into the cup to fill it to the line. Time how long it takes for one quart to run into the cup. This is reported in seconds per quart. The reason you need to move quickly is because drilling fluids typically have another rheological property called gels. This is the ability of the fluid to get thicker when it is sitting idle. If the fluid has a chance to develop these gels, then the viscosity reading will be higher than that of the fluid when it is in motion and you will get an inaccurate viscosity reading. Marsh funnel viscosity is the most elementary of the fluid properties that are called Rheology and Hydraulics. In a later column, I will go into these properties so we can have a feel for the overall impact of them on drilling operations. Remember, I would like this to be our column. I need your input to make it so; otherwise, it becomes my column. If you have any questions about this topic or if you have another topic you would like me to address, please contact Michele (below) and she will let me know. If I don’t know the answer, I will find someone who does and is willing to share. We work in an amazing industry and have a tremendous resource base to pull form. Ron Ron Peterson may be contacted via e-mail to michele@worldwidedrillingresource.com WTR Atlantis Vault • Self-Contained • Simple installation • Trouble-free operation For more information call: (270) 786-3010 or visit us online: www.geothermalsupply.com
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