WorldWide Drilling Resource

18 JULY 2021 WorldWide Drilling Resource ® How Dense Are We? by Ronald B. Peterson Drilling Products Specialist, Mountainland Supply Company In previous columns, we have danced around the drilling fluid property of weight or density. I would like to take this opportunity to put a wrap on the discussion of density. Density, or mud weight, is one of the key properties of a drilling fluid. It is a measure of how heavy the drilling fluid is. Quite frequently, I will have someone say their drilling fluid is very heavy. I will ask them how much it weighs, and they say, “I don’t know. What I meant is that it is very thick.” These are two totally different properties. A drilling fluid can be anything that flows, lacks rigidity, has a low resistance to force when applied to it, and typically will assume the shape of its container. Fluids can be gases or liquids; we will discuss this in more detail in a future column, or columns. For this discussion, we will stick to air- and water-based fluids. The lightest, or least dense, fluid typically used in drilling is air, with a density at sea level of approximately 0.0086 pounds per gallon, which is very nearly 0. Water has a density of 8.33 pounds per gallon. Drilling fluid weight or density is typically measured with a mud balance. A mud balance has four scales on it that are typ- ically used in regards to drilling fluid. Two of the scales are on each side of the balance beam. Those scales are: • Pounds per gallon - the most commonly used method of reporting drilling fluid weight. • Specific gravity - also called drilling fluid density. • Pounds per cubic foot - used in some areas and under some circum- stances to report drilling fluid weight; however, it is not very commonly used. • Pounds per square inch per 1000 feet of depth - an excellent way to determine the hydrostatic pressure the drilling fluid exerts at any point in the borehole. To measure the drilling fluid or mud weight: • Verify the mud scale is correct by weighing water in it and verifying a weight of water at 8.33 pounds per gallon. • Remove the cap and fill the cup on the end of the beam with drilling fluid. • Replace the lid, taking care to make sure some fluid comes out of the small weep hole in the center of the lid. • Wipe the entire balance off, making sure it is dry. • Place the scale in the fulcrum that accompanies the balance and move the slide as necessary on the arm to make sure the small bubble is in the center of the sight glass on the balance arm. • Read the desired property from the appropriate scale. Water-based drilling fluids can range from the weight of water at 8.33 pounds per gallon to a very dense drilling fluid at 21 pounds per gallon. Drilling fluid weight, along with funnel viscosity (a topic for another column), can be used as an indicator of the quality of the drilling fluid. You can have a very viscous drilling fluid with a low mud weight. It is possible to have a viscosity of 50-60 seconds per quart with a mud weight of only 8.5 pounds per gallon with good quality bentonite or bentonite and polymer- based fluid. On the other hand, you can have a relatively high mud weight with a low viscosity fluid which is usually a result of entrained or incorporated drilled solids with minimal to no high-quality bentonite. The low viscosity/high mud weight fluid will usually have high filtrate or fluid loss along with other adverse properties which can lead to borehole issues and drilling problems. In general, a lower drilling fluid weight will result in a faster penetration rate because the bit chips or cuttings can come off of the bottom of the hole faster, cleaning the bottom of the hole so the bit will have better contact with the bottom of the hole. Thus, it can more easily generate new cuttings. Larger cuttings are also more easily removed on the surface, which will help avoid drilling fluid weight increase. Drilling fluid weights increase as the hole is advanced because the drill cuttings are allowed to build up in the drilling fluid. Drill cuttings from the formations drilled are the biggest contaminant found in most drilling fluids. The fastest penetration rates are typically achieved with air or gaseous fluids. Sometimes drilling fluid weights will need to be increased to control pressures encountered in the formation. If the drilling fluid weight is high, make sure it is by design and not inadvertent. To control the drilling fluid properties, make sure you use the correct product for the drilling fluid property you are trying to achieve. Remember, this column is to be an interactive tool. If you have any questions on mud weight or density, or if you have another topic you would like addressed, please contact Michele (below) and she will get your question to me. Ron Ron Peterson may be contacted via e-mail to WTR