For HDD construction the primary impact on the environment revolves around the use of drilling fluids. Many regulatory problems result from a misunderstanding of the effects of drilling fluids. Informing the regulatory agencies about the purpose and composition of HDD drilling fluids is important for many successful HDD projects1.
For many HDD projects the drilling fluid system comprises a substantial portion of the HDD equipment and cost. The system is composed of both the equipment and the drilling fluids. The proper selection of the equipment and fluids can be a major factor in the project's success. Drilling-fluid systems may consist of mixing systems, holding tanks, and cleaning systems. Mixing systems are used to mix the drilling-fluid additives with water to make a mixture that is suitable for the specific HDD conditions. Generally the drill rig should have holding tanks with enough volume to provide at least 15 minutes of full pumping capacity. The longer the bore, the higher the fluid requirements and the larger the storage should be. The hopper allows for faster mixing and hydration of the additives. Separate tanks for
FIGURE 5-1 HDD Drilling Fluid System
mixing and storage are necessary to avoid delays in mixing water and hydrating additives. Cleaning systems are used to remove the cuttings and recycle the drilling fluids. In urban and environmentally sensitive areas a vacuum truck is required to contain the drilling fluids.
An effective drilling-fluid system saves on water, additives, and disposal costs. It also reduces costs on drills that use mud motors. When cleaning drilling fluid for reuse, a mud balance is required to measure the density of the fluid. If the fine solids in the fluid exceed the ability of the fluid to suspend cuttings in the bore hole, the fluid must either be disposed of and replaced with a new fluid or cleaned to reduce the weight. The drilling fluid weight should be kept below 9 pounds per gallon for small to medium-size systems.
If neutral pH water is not available at the HDD site, a water truck will be required to deliver water to the site. Soda ash can also be used to adjust the pH of the water before filling the mixing tanks. The water truck should not be used for any other fluid or additives. The presence of solids in the water may cause problems with the mud pump and can plug the downhole water source.
A pit pump is an essential item if a cleaner is used. To increase the pump life, the pump should be suspended above the bottom of the recovery pit to minimize the solids being pumped. A second smaller pit can be excavated beside the entry or exit pit to allow most of the solids returning from the bore hole to settle in the first pit, with the overflow going to the second pit.
There is much mystery surrounding the topic of drilling fluids. Drilling fluids perform the primary functions of enhancing the bit cutting, suspending the cuttings, removing the cuttings from the bore hole, stabilizing the hole, preventing loss of the drilling fluid, cooing the bit, reamer, and tracking sonde, reducing pipe to borehole friction, and providing lateral support for the pipe.
Drilling fluids also perform the secondary functions of preventing soil swelling, controlling the formation pressure, and minimizing wear and corrosion of the drill equipment. The key point to remember is that the essential function of all drilling fluids is to aid in the drilling process.
The main component of any drilling fluid is water. In some soils, water can be the only component. However, in many instances the water by itself does not do the required job. In order for water to perform the functions previously listed, it is often necessary to modify its properties by adding a viscosifier. The viscosifier almost exclusively used for HDD applications is naturally occurring clay in the form of bentonite. The water is treated with drilling-fluid additives such as bentonite, polymers, surfactants, wetting agents, or a combination. There is no one universal or ideal drilling fluid that meets all the possible requirements for HDD applications. The best drilling fluid for the application depends on many factors, such as soil type and conditions, composition of the water, site conditions, drilling application, and drill equipment. Deciding which additive to use involves matching the additive or additives to these factors. Tables 5-3 and 5-4 provide some application guidelines for drilling fluids.
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