Various technologies can be used for installing horizontal wells for subsurface pollution control. Such wells are typically installed in unconsolidated soils 10-200 feet deep. Selection of drilling technique depends on surface access, well placement and completion requirements, and subsurface hydrogeology.
Jetting and moling are the most common techniques used to create horizontal borings in shallow, unconsolidated soils. Both are mechanically simple and require minimal labor, using small tubulars that can be handled by one person. Small trucks can carry the hardware for both techniques.
Moling employs a rotating bit, compressing soil into the hole wall. This requires a soft, compressible soil. Directional control is limited and relies chiefly on trial-and-error targeting methods. Holes are only a few inches in diameter, limiting completion options. No drilling fluid is used, and no cuttings are generated. Clayey soils may become "damaged" (i.e., their permeability may be reduced) because the cuttings are not removed from moling boreholes.
Jetting employs an off-axis, high-pressure water jet to fluidize the formation. Downhole transmitters and a walk-over receiver are used for surveying the well location and determining jet orientation. The operator can steer the system accurately in unconsolidated, homogeneous soils. Jetting requires that there be no interference sources between the surface receiver and downhole transmitter.The transmitter must not be located deeper than 25 ft below the surface. Drilling fluid lubricates the drill string. Cuttings mixed with drilling fluid enter the soil formation along the wellbore, which may cause formation damage as with moling.
Rotary drilling employs a stabilized assembly allowing control of well path inclination but no horizontal directional capability. The drilling rig must be large enough to provide rotating, push, and pull forces. The technique is used for oil and gas recovery and for installing utility lines under rivers. Low drilling fluid flow rates are used in utility line installation, where the fluid serves only to carry cuttings through the hole wall to keep it open and to lubricate the soil. High fluid flow rates are used in hydrocarbon recovery to remove cuttings from the wellbore and thereby preserve formation permeability. A large radius of curvature is required in rotary drilling to prevent failure of tubulars. The larger the tubular diameters, the larger the curve radius required to prevent failure of tubulars. Surveying tools are similar to those used for magnetic and gravitational orientation surveys. These tools work at any depth, but only in the absence of magnetic interference.
Positive displacement steerable motor technology offers vertical and horizontal directional control. The radius of curvature is small compared to that of rotary drilling, because the drill-string rotation is minimal. Penetration rates are high because of rapid bit rotation. Drilling fluid forms an impermeable layer on the borehole wall, preventing fluid and cuttings from entering the formation, as with rotary drilling. The impermeable layer also prevents hole collapse by maintaining positive fluid pressure within the wellbore. The impermeable layer must be removed following installation of the screened casing, to expose undamaged formation. Drilling fluid must remove cuttings from the wellbore. Viscosity and turbulence of the drilling fluid allow it to clear the wellbore. By comparison, vertical wellbores rely solely on viscosity. Turbulence prevents cuttings from accumulating along the bottom of the wellbore. Since turbulence increases with decreasing viscosity, ideal drilling fluid balances both factors. Thixotropic gels are well suited as drilling fluids. Turbulence is also maintained with the help of centralizes, which provide a uniform annulus around the tubulars .
Percussion and vibration drilling techniques use air as a drilling fluid. These techniques are particularly suitable for drilling through heterogeneous soils containing boulders or coarse gravel or through highly fractured rock formations. The wellbore remains dry, so fluid loss is avoided. However, percussion and vibration drilling techniques currently lack well-developed steerability and directional control.
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