Drilling Fluids: Back to the Basics

With advances in HDD technology and equipment it is easy to overlook the basics. This article will explain the fundamentals of testing and concepts used in HDD drilling fluids. All brands of bentonite can be evaluated using the same concepts and tests. Barrel yield, viscosity, gel strength and fluid loss are terms we hear in the industry daily. Understanding and applying these concepts allow you evaluate your drilling fluids.

Often, you hear drilling fluid suppliers talking about the barrel yield of their material. A ton of bentonite is hydrated with de-ionized water to a Marsh funnel viscosity of 36 seconds per quart. The resulting fluid is measured in oil field barrels (42 gals). A 240-barrel yield product would produce approximately 10,000 gals of mud while a 150-barrel yield mud would produce 6,300 gals. The higher the barrel yield of the mud, the less bentonite you will have to add to produce a given viscosity. Remember, this is a laboratory test using DI water. The use of water of poor quality will have a significant impact on the amount of material used in the field.

Viscosity is another term used in our industry daily. It is simply resistance to flow and is measured using a Marsh cup and funnel. Techniques such as visual inspection and feeling mud do not produce accurate results. Water is about 27 seconds per quart. Adjust your viscosity to the soil conditions. Generally speaking, a drilling fluid in clay should be in the low 40s while fluids used in sand are much higher, often in the mid 60s. Using a Marsh cup and funnel is simple. Place a finger over the bottom of the funnel and fill it up to the bottom of the screen. Place the cup under the funnel and remove your finger from the bottom of the funnel. Measure the amount of time it takes for the fluid to touch the one quart line on the cup.

Gel strength is a concept often misunderstood. It is simply the ability of a drilling fluid to suspend cuttings while the fluid is at rest. Gels are measured with a viscometer at 10 seconds and 10 minutes. A gel that increases substantially over time is termed progressive. When we turn our mud pump on, the first thing we must do is break the gels to get the mud circulating. A highly progressive gel in an unconsolidated formation can cause a frac-out.

Fluid loss is one of the least understood but most important concepts in drilling. It is the loss of the water phase of the drilling fluid into the surrounding soil, measured with a filter press. A fluid with a high fluid loss will form a thick, mushy filter cake. A very thick filter cake will decrease the amount of annular space available for the drilling fluid to flow. This pressurizes the hole and can cause a frac-out. High fluid loss also water wets the area around the bore hole. In sand or cobble conditions this can cause your hole to collapse. A fluid with a low fluid loss will have a thin, tough filter cake. This maximizes the space available for the fluid to flow and limits the loss of water into the formation.

A test for water hardness or calcium content will reveal if you need to add soda ash to your make-up water. Soda ash will increase the pH while rendering the calcium insoluble. Simple pH test strips are dipped and color-matched to determine the pH of the water. Add the bentonite to the water via a high shear mixer and check with a Marsh cup and funnel. Polymers are added last. Once the polymer is fully mixed, check the finished viscosity and make sure it is appropriate to the soil conditions. Each family group of polymers has different characteristics. Determine the characteristics of the polymers you use and make sure they are appropriate for the current project. Selecting appropriate polymers for each HDD job will help control fluid costs.

Rick Zavitz is a drilling fluid specialist with Mears Group Inc., a Quanta Services Co.

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