Drilling Fluid Weight

It is important to know how to measure the density of drilling fluid.

By Ron Peterson

I would like to discuss the primary drilling fluid properties in depth in this and future columns in Water Well Journal. We will start with mud weight or drilling fluid density. Density or mud weight is one of the key properties of a drilling fluid. It is a measure of how heavy or dense the drilling fluid is.

The testing equipment used in testing drilling fluids is all designed to meet specific American Petroleum Institute (API) specifications and standards. The proper equipment is available through your drilling fluid vendor.

There are multiple options for guidance in proper drilling fluid testing protocol. You can get training through your drilling fluid vendor and various drilling fluid manufacturers. You can even find educational online videos in many cases too.

Know When to Check

Checking the drilling fluid properties on a regular basis will help you know when you have encountered a potential problem and may need to take corrective action.

Knowing when to take action—and what action to take—can save you rig time, the expense of potential hole problems, and help minimize well development issues. Avoiding problems in the first place is almost always easier and less expensive than taking corrective or remedial measures to correct them.

When on a jobsite and discussing problems, I frequently will ask how heavy the mud is. The contractor will often say their drilling fluid is very heavy. I will then ask them how much it weighs, and the reply I hear back is, “I don’t know but 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. But for this discussion, we will stick to air- and water-based fluids.

The lightest or least dense fluid typically used in drilling is air, which has a density at sea level of about 0.0086 pounds per gallon which is nearly 0. Water has a density of 8.33 pounds per gallon.

Measuring Fluid Weight

Drilling fluid weight or density is typically measured with a mud balance (Figure 1). A mud balance has four scales on it that are typically used regarding 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 circumstances 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 that 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 fluid weights can range from the weight of water at 8.33 pounds per gallon to a dense drilling fluid at 21 pounds per gallon.

The Quality of the Fluid

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 viscous drilling fluid with a low mud weight. It is possible to have a viscosity of 50 to 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 also have high filtrate or fluid loss along with other adverse properties that 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 the bottom of the hole faster, cleaning the bottom of the hole so that 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.

Higher drilling fluid or mud weight increases the hydrostatic pressure on the bottom of the borehole. This creates a phenomenon known as “chip hold down” which makes it difficult to remove the cuttings from the bottom of the borehole and get them moving up and out of the borehole.

The fastest penetration rates are typically achieved with the lighter fluids. The lightest fluids are 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.

Increasing drilling fluid weight must be done carefully. I have been on boreholes where 0.02 pounds difference in drilling fluid weight will be the difference between controlling the downhole pressure and losing returns. The current issue  we have with overpumped, depleted aquifers makes this even more critical.

Excessive mud weights may also cause problems with well development and well production.

______________________________________________

It comes down to knowing your drilling fluids. To control the drilling fluid properties, be sure you use the correct product for the drilling fluid property that you are trying to achieve.

If anyone has a question or subject that they would like to see addressed, please contact me either through NGWA or directly at petersonrb@msn.com.

---

Ronald B. Peterson has been involved with the drilling industry for more than 40 years. He previously worked for Baroid Industrial Drilling Products and is now with Mountainland Supply Co., a supply company in Orem, Utah. He served as The Groundwater Foundation’s McEllhiney Lecturer in 2015 and was given NGWA’s most prestigious award, the 2013 Ross L. Oliver Award. He can be reached at petersonrb@msn.com.