There has been limited prior work performed in assessing the annular space region created during horizontal directional drilling (HDD) installations. In one project, researchers performed testing on three 530-foot HDD installations of 12-inch diameter steel pipe near Vicksburg, Miss., to monitor changes in pore pressures before, during and after installation to assess impact on stability. Dye tracers were added to the drilling fluid to enable visual determination of suspected fluid migration. Postconstruction investigation of the annular space performed immediately after installation revealed a relatively limited migration path of drilling fluid, suggesting no evidence of significant voids in the annular space region.
Another research program was undertaken in which two 8-inch-diameter high-density polyethylene pipes – one 180 feet long, the other 290 feet long – were installed 6.5 feet below the ground surface. Excavations were performed approximately two years after installation to gain a visual perspective of the annular space. The research concluded that there was no evidence of significant voids present in the annular space region.
Fluid CompositionDrilling fluid is composed of a carrier fluid (water) and drilling fluid additives (bentonite and/or polymers). Bentonite is a naturally occurring clay mineral (montmorillinite) that forms a mud when mixed with water. When bentonite is mined, the clay platelets, which have been subjected to high confining stresses, are closely compressed and have very little water between them. An aggregate is a unit of stacked clay platelets. When water enters between some of the clay platelets, it immediately causes them to disperse, separating the clay platelets. The dispersion of the bentonite is aided by shearing through good-quality mixing equipment.
Drilling fluids are characterized by seven key properties:
- gel strength
- fluid loss and fluid density
- filtration control and filter cake
- sand content
To create the optimal drilling fluid, each of these factors must be considered. It should be noted that the native soil exerts the greatest influence on the selection of a drilling fluid mixture. Therefore, it is imperative that the native soil is properly identified and characterized to facilitate selection and formulation of the proper drilling fluid.
The principal functions of drilling fluids used in horizontal directional drilling:
- Transporting drill cuttings to the surface by suspending and carrying them in the fluid stream flowing in the annulus between the drilled bore wall and the drill pipe and product pipe.
- Cleaning the buildup of soil on drill bits or reamer cutters by directing fluid streams at the cutters.
- Cooling the downhole tools and equipment.
- Lubricating to reduce the friction between the drill pipe and product pipe and the bore wall.
- Stabilizing the bore, especially in loose or soft soils, by building a low-permeability filter cake, and exerting a positive net hydrostatic pressure against the bore wall. The filter cake, along with positive hydrostatic pressure, reduces collapse of the bore and prevents formation fluids (i.e., ground water) from flowing into the bore, or drilling fluids from exiting the bore into the formation (i.e., loss of circulation).
- Providing hydraulic power to downhole tools such as mud motors.
When the drilling fluid is pumped into the borehole, the fluid, just like water, attempts to flow through the soil. However, the bentonite platelets will start to plaster or shingle off the wall of the borehole and form a filter cake that seals off the flow of fluid from the bore into the native soil. The ideal filter cake is smooth, forms quickly during construction of the borehole, reduces migration of drilling fluid into the formation, and reduces intrusion of both ground water and soil into the bore. Optimum filter cake thickness should range between 1⁄32 inch and 3⁄32 inch.
The water that does manage to filter through the cake is referred to as filtrate. Filter cake quality can be improved to reduce the amount of filtrate entering the surrounding soil. This can be accomplished by one of two methods – adding more bentonite or using certain polymers in conjunction with bentonite to tighten the filter cake. It is more effective to use a bentonite/polymer mix because it is less viscous, more pumpable, and flowability at the annular space will be maximized due to the shear-reducing properties of the fluid (i.e., more slurry will flow).
In addition to providing a filter cake layer, the drilling fluid must provide suspension characteristics or gel strength. The drilling fluid must be able to support, suspend and carry the cuttings. If the fluid cannot suspend the drilled material, that material will quickly settle out of suspension and pack around the drill pipe or around the product line being pulled. Even if the fluid has a high viscosity, it may have a very low carrying capacity. Proper control of gel strengths is an important factor in avoiding excessive downhole pressures. Gel strength usually is checked with a clean fluid. When solids are added to this fluid, the drilling fluid properties change drastically. The yield value of a drilling fluid (not the gel strength) is the measurement of the drilling fluid’s internal resistance to flow, and, thus, the carrying capacity of the fluid when it is moving. Gel strength and yield value are more important parameters in horizontal drilling than viscosity. Plain water has low viscosity and no gel strength or yield, and polymers, by themselves, have high viscosity but low gel strength and yield. Therefore, bentonite is required to provide the necessary carrying capacity for cuttings from coarse soils.
At times, additives such as detergents are added to the drilling fluids to counteract some of the formation characteristics, such as swelling and stickiness commonly found in expansive clays. Other additives are used to adjust the pH of the fresh water constituent of the drilling fluid.
For HDD, the proper drilling fluid mixture is heavily dependent upon the soil encountered. It must be formulated for the anticipated geological conditions. For simplicity, soil conditions may be defined as either a coarse soil (i.e., sand and gravel) or a fine soil (i.e., clay, silt and shale). When drilling through sand and gravel, a drilling fluid needs to serve two important functions – stabilization of the borehole and suspension and transportation of cuttings. When drilling through clay, the same functions need to be performed; however, an additional requirement of the fluid is to retard swelling, and reduce sticking of the soil to the downhole tooling and product line being installed. Geological conditions may vary between fine and coarse soils; consequently, different combinations of drilling fluid additives will be needed to perform the required function under actual conditions. In general, for coarse soils, bentonite should be used, while for fine soils, polymers (possibly added to a bentonite base) are recommended. When drilling through sands and gravel, drilling fluids may migrate from the bore into the native soil formation. Bentonite and lost circulation materials reduce fluid losses into the formation.
Undoubtedly, the native soil will dictate – first and foremost – what type of drilling fluid is required. Once the native soil is properly identified, the suitable drilling fluid can be chosen – and quite possibly modified – for optimal performance.