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Basic Drilling Fluids
Let’s try to take some of the mystery out of drilling fluids. To start, let’s agree that the main component of any drilling fluid is water. In some soils, water can be the only component. However, we’ve seen that in most cases water by itself is not as user friendly as we need it to be. In these cases, we need to treat the water with drilling fluid additives. These additives will usually be bentonite, polymers or a combination of both. How do we decide which additive to use? We simply have to match the additive or additives to the soil type.
For our purposes, we can say that we only encounter two basic soil types: coarse and fine. For coarse soils (sands and gravels), we are dependant on the use of bentonite or bentonite/polymer mixes. For fine soils (clays and shales), we can use polymer or a bentonite/polymer mix. Today, let’s talk about coarse soils and the need for bentonite.
In drilling sand and gravel, we need a fluid that will do two things. First, we need a fluid that will stay in the hole. We know that water by itself doesn’t do that because water flows through sand. This is where bentonite comes in. Bentonite, when thoroughly mixed in water, breaks down into small particles called platelets. These flat plate-like particles are small, very small. In fact, if you take one cubic inch of high quality sodium bentonite and mix until it’s broken down to its smallest dimension (the platelet), you have enough surface area to cover 66 football fields. (When you realize this, you can see why quality mixing equipment is so necessary.) When we pump this bentonite fluid into the hole under pressure, the fluid, just like water, wants to flow through the sand or gravel. However, in this case the bentonite platelets in our fluid start to plaster or shingle off the wall of the borehole and form a filter cake that cuts off the flow of fluid into the surrounding sand or gravel. The water phase of the fluid that does filter through this filter cake is referred to as filtrate. We can improve the filter cake quality and reduce the amount of filtrate going into the surrounding soil by one of two methods. We can add more bentonite (more available platelets) or we can use certain polymers in conjunction with the bentonite to tighten the filter cake. In most cases, it’s more desirable to use a bentonite/polymer system because the end result is a more pumpable fluid and more flowable slurry. It’s important to remember, however, that in this case we are using a certain polymer to enhance the performance of the bentonite. This is not a polymer system. Polymer by itself does not have the necessary beneficial solids to form a filter cake.
The second thing we need our drilling fluid to do in sand or gravel is to provide suspension characteristics or gel strengths. When we think of a bit or a reamer, we think of cutting or drilling tools. However, these tools serve a very important secondary function. They are also responsible for mixing the soils they are cutting into a flowable slurry with the fluid. The fluid we are using has to be able to support, suspend and carry these drilled spoils (cuttings). If the fluid doesn’t have the ability to suspend the drilled material, that material is going to quickly pack off around our drill rods or, more dangerously, around the product line (or lines) we’re pulling. Bentonite provides the carrying capacity (gel strength) we need to support this material. You have probably heard the term viscosity used to describe the thickness of a drilling fluid. The viscosity is the number of seconds that it takes for one quart of fluid to flow through a Marsh viscosity funnel. The problem is that viscosity only tells us the thickness of a fluid. We can have a thick fluid (high viscosity) that has low carrying capacity (gel strength). This is where gel strength becomes much more important than viscosity. Water by itself has low viscosity and no gel strength. Polymers by themselves can give us rather high viscosity but low gel strengths. Bentonite, on the other hand, can give us both viscosity and gel strength.
How much material does it take to give us these desirable properties? Usually at least 30-35 pounds of high quality bentonite per 100 gallons is required before any margin of safety is achieved in sand or gravel. Sometimes as much as 50 pounds per 100 gallons will be needed. Sands and gravels can be tough. We have to have enough margin of safety to be successful. Remember- don’t go into a sword fight with a pocketknife.
Now that we are keeping our fluid in the hole (filter cake and filtrate) and have good carrying capacity (gel strength) we can come to the subject of flow. When we have our slurried spoils flowing out of the hole either from the exit or entry side, we know that we have an open borepath. We know that if we have an open borepath we don’t get stuck. Some drillers say that they don’t like to have flow because they wind up with a mess. The problem here is that they also get stuck a good part of the time. On the other hand, other drillers state that they live by having constant flow or run the risk of dying because of the lack of it. Having good slurried flow on both the bore and the backream is highly recommended.