Drilling Fluid:  bentonite There are some contractors that have mini rigs and boring small diameter, shallow holes using only water. How well does water meet the functional requirements. water will clear cuttings from the hole when in turbulent flow. However, water that is not in turbulent flow is a poor hole cleaning medium. The hole cleaning ability of water is entirely dependent on the pumping capacity of the rig. Water will lubricate and cool the bit. In fact water has a better coefficient of friction than bentonite fluids at high pressure. Water will not stabilize the hole. Water in turbulent flow will erode weak formations leading to hole enlargement. Large amounts of water can be lost to highly permeable formations such as gravel. In clays that hydrate and swell, water is your worst enemy. Since water is a Newtonian fluid it does not have any yield point or thixotropic characteristics. This means that as soon as the pump is shut off all the cuttings will fall to the bottom of the hole. Any colloidal clay particles will stay in suspension but the majority of the cuttings will settle out on the low side of the hole. Water is fairly efficient at transmitting hydraulic energy to the bit. Efficiency is lost as the water becomes loaded with solids. Water is not a good choice for a drilling fluid. BENTONITE The vast majority of drilling contractors will utilize a bentonite slurry when doing an HDD bore. The bentonite that is used will normally be high yield or a comber nation mix that  has additives such as polymer, soda ash, water loss control agents and thinners added to the bentonite. Bentonite slurries in general are good hole cleaners as long as a couple of rules are followed. Utilize an elevated viscosity from the start and use the highest possible annular velocity as it provides the impact force necessary for good cuttings transport. This is only applicable to the main hole since cutting transport is severely limited when reaming. During reaming suspension is the most critical function rather than cutting transport. however the reader should  be aware that boycott settling will still occur in the 30-60 degree section of the hole and that cutting beds will form on the low side of the hole especially during extended periods of sliding. Bentonite slurries were thought to provide lubrication since they feel slippery. However, they are good lubricants only under low pressure conditions. They are not good lubricants when exposed to high pressure conditions. The coefficient of friction in the bentonite slurry is higher at 720 psi than water. However bentonite slurries are good hole stabilizers in permeable formations such as sand and gravel. The slurry will deposit a filter cake on the permeable formation allowing the hydrostatic pressure to push against it. The filter cake should be thin and tough. How well bentonite slurries do in stabilizing clay sections is dependent on the water loss. The lower the water loss the better it will stabilize the clay. Since clay is not permeable no filter cake will be deposited. Suspending cuttings when circulation is stopped is another critical function. Settling of cuttings in bentonite slurries are controlled by the size, shape and density of the cutting, as well as, the physical properties of the fluid. The bigger and heavier the cutting the faster it will settle. It is impossible to know the exact physical properties of the fluid without testing being done. A simple way to check settling is to collect a sample of mud at the return pit in a container. Let the container set for 5-10 minutes and then carefully pour out the mud and see how many cuttings are on the bottom of the container. If there are more than you want then increase the viscosity of the fluid with bentonite and/or polymer. The suspension ability of bentonite slurries are much greater than water but not as great as some other fluid types. When it comes to transmitting hydraulic energy to the bit the bentonite slurries are not good. Bentonite slurries contain 4 - 6 % solids. The solids create friction as they rub against the pipe and each other. So the pumping friction loss is increased. That means that less pressure reaches the bit and less work is done. If you had to give an honest evaluation of bentonite slurries then the best rating would be average. Of the five critical functions they are deficient in at least two to three areas. This is the reason that most drilling fluid suppliers will recommend adding some supplemental products in certain drilling situations. The products most often recommend are a bio-polymer to increase the gel strengths and improve suspension of solids, PAC polymers to decrease the filtrate water loss, PHP A polymers to inhibit clay hydration and swelling and drilling detergent to water wet the tools and reduce torque. These products will improve hole conditions and reduce certain problems. However, drilling detergent although widely used is not an efficient torque reducer and has a serious drawback. If you refer back to the table that compare various lubricants you will see that the detergents did not provide much reduction in the coefficient of friction.