Chapter 3: Drilling Fluids and Back Reaming

The overwhelming majority of the bores that fail do so because of the improper use of drilling fluids and poor back reaming techniques. Soil conditions can vary from one extreme to the other, even in the same bore. Although drilling fluids and backreaming are two entirely different topics, it is important to understand that they are integral to each other and that the end result is contingent upon the proper application of these two components. This chapter will address drilling fluids in general, backreaming, and special drilling-fluid considerations for remediation.

The major base component of any drilling fluid is water, and in some rare instances, water alone might work. However, in most cases, straight water is not satisfactory. There are numerous products on the market that can be added to the water to enhance the drilling fluids' performance. Which additives to use will largely be dictated by the ground conditions encountered. A good drilling fluid should provide cooling for the drill head and the sonde, lubrication of the drill string and the product line being pulled back, the ability to suspend the cuttings, and the ability to flow cuttings out of the hole. Another key requirement is the ability to retain the drilling fluid in the hole without having it dissipate into the surrounding formation. Before any further discussion can take place concerning drilling fluid, a determination of soil types must be made.

For the sake of discussion, soil can basically be classified into two general categories: coarse and fine. Coarse soils consist of sands and gravels. Fine soils are comprised of clays and shales. The drilling fluid requirements for these classifications vary greatly. Coarse soils are noncompactable and allow water to flow freely into the formation. Fine soils will usually prevent water from flowing into the formation, but there is a strong tendency for them to become sticky and to swell when mixed with water. Of course, it is possible to have a combination of the two general classifications.

Drilling-Fluid Additive

Bentonite is an absorbent aluminum silicate clay that was formed from volcanic ash more than 60 million years ago. It was first discovered around 1890 near Ft. Benton, WY, hence the name. It is widely used for many commercial applications ranging from the suspension of more nitrogen in liquid agricultural fertilizer to its use in the oil-drilling industry as a fluid loss preventative. Although bentonite is produced in many areas around the world, Wyoming bentonite is regarded as being the highest quality. Its many uses make bentonite fairly easy to obtain around the world.

When bentonite is added to water, it breaks up into microscopic, disk-shaped particles called platelets. When used while drilling, the platelets have a shingling effect and form a filter cake on the sidewalls of the hole. It is this filter cake that prevents the fluid from escaping out into the formation of the bore. If it were possible to break bentonite into its most minuet particle size, there would be enough platelets in one cubic inch of premium grade sodium bentonite to cover 66 football fields. Of course, this is not possible in the field. However, the ability to mix bentonite with water can be greatly improved by using clean water with a pH of 8.5 to 9.5. Soda ash can be added to water that has a lower pH to bring the pH up to acceptable levels.

Bentonite also helps to suspend cuttings in the hole. This carrying capacity is called gel strength and should not be confused with viscosity. Many novice drillers think that thicker-or higher-viscosity drilling fluids improve the fluids' ability to carry cuttings. This is not true. It is the gel strength of the drilling fluid that is important. Up to this point, the HDD industry has paid little attention to the viscosity of drilling fluids.


Polymer is the name given to describe any of numerous organic and synthetic compounds of usually high molecular weight consisting of millions of linked structural units, each being a relatively light and simple molecule. The simple molecules that may become structural units are called monomers. A structural unit is a group having two or more bonding sites.

Polymers are classified as branch or linear, which is determined by the number of bonding sites on the individual monomers. The monomers in a linear polymer have only two bonding sites and resemble a long, continuous chain. A branched polymer is comprised of monomers that have three or more bonding sites. Under a microscope, branched polymers resemble a tree branch with little branches jutting out in all directions from the main branch. Polymers are widely used in plastics, rubbers, finishes, processed foods, and synthetic fibers.

An example of an organic polymer that everyone is familiar with would be household baking flour. When added to milk and hot grease, the individual monomers of the flour bond together to form structural units, which make the milk and grease thicken into gravy.

Polymers are mainly used in drilling fluids for two specific reasons. First, polymers are attracted to clay, and when introduced into the soil, they will wrap themselves around the individual clay particles. This slows down the clay particle's ability to absorb fluid, which in turn reduces the clay's ability to swell and become sticky. The second main function of the polymer is to provide lubrication in the bore hole, which reduces friction on the drill stem and on the product line being installed.


Surfactant is commonly used as an additive in drilling fluid. It leaves a thin film on the drill stem, which helps prevent sticky clays from attaching to the drill stem. The most commonly used surfactant in HDD is a biodegradable, nonfoaming detergent.

Choosing The Right Additive

Which additive to use is determined by the soil conditions that will be encountered during the bore. The rule of thumb is to use bentonite in a coarse soil condition and polymer in a fine soil condition. Of course, soil conditions can vary considerably in the same bore. Therefore, it is often necessary to use a bentonite and polymer combination in the drilling fluid. The use of a surfactant is also recommended when sticky clay conditions might be encountered. It is advisable to contact the drilling-fluid manufacturers for specific recommendations and applications.

Drilling Fluid Considerations For Remediation Application

It is important to remember that in a remediation application, the purpose of installing the well is to allow the contamination to enter the well screen for removal. If a drilling fluid is used that prevents loss of fluid into the formation, it is reasonable to assume that it will also prevent contaminates from entering the well screen. Therefore, the proper drilling fluid should be selected when installing a remediation well. Chapter 8 of this handbook addresses well development and drilling-fluid considerations for remediation applications in greater detail.

Back Reaming

One function of a back reamer is to enlarge the bore hole to a size large enough to allow for the installation of therequired product. However, the mixing ability of the back reamer is equally important. During the back-reaming process, the main objective is to mix the cuttings from the back reamer with the drilling fluid to create a slurry that can be displaced to the side of, or discharged out of, the bore hole to allow room for the product.

The back-reaming process is critical to the successful completion of the bore. Not only is it necessary to use the proper drilling fluid, it is also important to use an adequate amount of drilling fluid. To create a flowable slurry requires, at a minimum, a 50/50 ratio of drilling fluid to solids. It is not uncommon for the "drilling fluid to solids" ratio to increase in favor of the drilling fluids. As discussed in Chapter 2, it is possible to calculate the minimum drilling-fluid requirements.

It is important that you not rush the back-reaming process. The back reamer needs time to cut the formation and to mix the cuttings into a slurry. The capacity of the mud pump on the machine also needs to be taken into consideration during the back-reaming process. For example, a 16-inch-diameter hole requires a minimum of 10.45 gallons of drilling fluid per foot of length to obtain a 50/50 ratio of drilling fluid to solids. If the mud pump has a capacity of 38 gpm, then the pullback speed of the back reamer should not exceed 1 foot per 16.5 seconds.

Diameter of back reamer squared

16 x 16 = 256

Divided by 24.5 = gallons per foot

256 / 24.5 = 10.45

Pump volume per minute divided by required gallons per foot = number of feet per minute

38 / 10.45 = 3.63

60 seconds / feet per minute = seconds per foot

60 / 3.63 = 16.5

In this example, if the pull back speed is faster than 16.5 seconds per foot, an undesirable condition of dry and poorly mixed cuttings is created down hole. This is referred to as "outrunning the mud." This example also assumes no fluid loss into the formation and no flow exiting the bore hole. In reality, mud flow exiting the bore hole is highly desirable and the best indicator that an adequate amount of drilling fluid is being used.


Hydra-lock is an undesirable condition created during pullback, when not enough mud is pumped into the hole or a poor mud mixture is used. Hydra-lock occurs when the drilling fluid that is being pumped through the back reamer is totally contained in the hole, rather than flowing out of the entry and/or the exit hole. Without an escape route, the fluid being pumped into the hole becomes pressurized, acting like a hydraulic cylinder. The pressure prevents the pipe from moving until the fluid finds an escape route. When hydra-lock occurs, there will be no flow, pullback pressure will increase to the drill's maximum limit, and rotational pressure will be very low. A hydra-lock condition can be reversed by waiting for the pressure to subside or by digging up the back reamer to relieve the pressure.

To better understand hydra-lock, there are five illustrations at the end of this chapter dealing with various scenarios involving hydra-lock.


Prereaming is when a back reamer is pulled through the hole one or more times before the product is pulled into the hole. Instead of pulling product behind the back reamer, drill pipe is attached to it. When the back reamer gets back to the rack, the back reamer is removed and the drill pipe is coupled back to the machine. The back reamer is attached to the drill stem at the exit pit and is pulled back through the bore with the product attached. Sometimes it is necessary to preream several times before the product is installed. Prereaming is never a bad idea, but it is often not necessary. The difficulty of the bore will largely dictate whether prereaming is required. Prereaming should be considered any time that the product being installed has a low tensile strength (i.e.: slotted well screen or PVC). Prereaming is often necessary when installing large diameter product because of the total amount of cuttings that must be displaced in the bore. In this case, it is often advisable to start with a smaller back reamer and work up in size on subsequent trips through the hole.

Back Reamer Selection

There are a number of various styles of back reamers on the market today, not to mention all of the different styles of homemade back reamers that are in use. The type of back reamer required will be determined by the soil conditions encountered during the pilot bore. Following are descriptions of some of the more popular styles.


The spiral reamer is long and has a very gradual taper to the full diameter of the reamer. This long, tapered design gives it the ability to move obstacles such as rocks, gravel and tree roots off to the side, allowing the product being installed to pass by.

Although there are probably more spiral reamers in use than any other style, they do have their disadvantages. The long, tapered design exposes a lot of surface area of the reamer to the surface of the formation. This increases the horsepower required to rotate them in a sticky or abrasive formation. Since the spiral reamer does compact to some extent, mud flow past the reamer is somewhat restricted. In a very dry clay or a very compacted sand condition they have a tendency to wood screw or feed themselves into the formation too fast making it difficult to back ream with them in these conditions.

Wing Cutter

In contrast to the spiral reamer, the wing cutter has a very steep angle. The length of the cutting edge is very short. There is less cutter surface exposed to the face of the formation, which reduces drag and also reduces the energy required to turn them. The open design allows the drilling fluid and cuttings to pass easily through the reamer. Since wing cutters do not compact, they are a good choice when drilling at shallow depths under driveways, sidewalks and streets where cracking concrete or blacktop is a concern.

Wing cutters come in various styles. They may be two bars, three bars or four bars, and can be built very heavy and rugged, or they may be fragile and light. All are designed to cut the formation and mix the cuttings with the drilling fluid to create a slurry that the product can be pulled through. A light wing cutter will use less rotational power and will actually do a better job of mixing than a heavy wing cutter. However, they are fragile and more susceptible to abrasive wear. Heavy wing cutters, on the other hand, do not mix the cuttings quite as well and will not allow the slurry to pass through as well, but they will last longer in adverse conditions.

Helical Reamer

A helical reamer is best described as a large egg beater. It provides excellent mixing and allows the slurry to flowfreely through it. However, it is somewhat fragile and does not hold up well under rocky or abrasive conditions.


Fluted Reamer

The fluted reamer was specifically designed for use in sand and hard-packed sand conditions with rock. Although it is tapered and will compact rocks into the side wall of the bore, it does not wood screw itself into the formation like the spiral reamer does. The flutes allow for the slurry to pass by the reamer. Cutter teeth can be welded on to provide better cutting and mixing action. The fluted reamer is rugged and has proved to work well in a number of various conditions. However, this reamer has a tendency to ball up in a very sticky clay condition. 

Illustration #1


This illustration is an example of everything operating without hydra-lock. A good hole has been created by having a good mud mix which will not only prevent the hole from collapsing but will also suspend and carry the cuttings out of the hole. A good sign that everything is fine is having good fluid flow out of the back of the hole and possibly flow at the rack.

Illustration #2 

This is a good example of hydra-lock. The fluid travels along the outside of the pipe, but it is blocked from traveling out of the hole. This creates pressure on the outside of the pipe, including the front of the pipe. As more fluid is pumped, this pressure increases, which keeps the pipe from moving. Good signs that this has happened are:

1. Pullback pressure increased considerably.

2. Rotation pressure decreases.

3. Flow at either end of the bore has stopped.

The best suggestions to prevent hydra-lock are:

1. Use the proper type and quantity of drilling fluid for the soil conditions encountered.

2. Don't outrun the mud. This happens when the pullback speed is faster than the pump can fill the hole with fluid. This results in the pipe going into the ground dry, which could lead to a possible cave in of the hole. Even if hydra-lock is not a problem, the pipe may still become stuck from lack of lubrication or cave in of the hole.

3. Use a large enough back reamer, but not too large. As a rule, use a reamer 1.5 times the size of the pipe. On pipe that is 10" or larger, selecting a reamer 1.3 times the size of the pipe is usually sufficient.

 Illustration #3

In this situation a hydra-lock has occurred and the fluid pressure in the hole has built up to the point that it cracked the ground. Fluid now travels to the ground surface. This is called a frac-out. While this improves the hydra-lock situation, it may not solve the problem. Remember, the cause of the hydra-lock was probably a lack of flow at the beginning of the bore causing the pipe to go into the ground dry and possibly closing up the hole around the pipe. Pullback may still be extremely difficult.


Illustration #4

During a frac-out, the fluid always travels along the path of least resistance. In this illustration the fence post provided the easiest path for the fluid to go. While this probably won't create a problem, try to imagine reaming under a driveway with a sand base. The sand will most likely be the path of least resistance. The frac-out will occur under the driveway, resulting in cracked concrete.

Illustration #5 

If the dreaded hydra-lock has already occurred, a burp hole should be dug to relieve the pressure of the fluid. This will allow you to continue backreaming An added benefit of the burp hole is that you have created a "controlled" frac-out. This will avoid cracking concrete or flooding a sensitive area, such as someone's yard.