Horizontal remediation installations, regardless of the drilling technology used or the type of remediation performed, are of two basic types—blind holes (single-entry completion) and surface-to-surface (double-entry completion). All vertical wells are blind holes.
Double-entry completions are the most frequently installed.
In these, a pilot hole is drilled from an entry, or launch, pit. The pilot hole is steered to the desired target depth, traverses the contaminated zone horizontally, then rises to an exit pit. Here, the pilot bit is replaced with appropriately sized reamers to enlarge the hole to the correct size to accept the casing.
As the reamer is pulled back towards the drill rig, additional drill rods are sometimes attached at the trailing end of the drill string so that the next size reamer may be attached without leaving the borehole open and subject to unrecoverable collapse.
The final step is to pull the well casing into place behind a final reamer pass.
Double-entry completions are preferred for remediation installations, because they are relatively easy to install and the drilling process is better controlled than single-entry completions.
Single-entry completions are most frequently used where a contaminated zone must be accessed beneath a large building or other surface obstruction.
In a single-entry completion, the surface is penetrated only once and all drilling, casing installation, and remediation system activities take place at that one end. The pilot hole is drilled, then the drill rods are withdrawn to attach a succession of reamers to enlarge the borehole. However, because reaming is accomplished in a forward direction, it requires skillful manipulation of the drill rig to ensure that the reamer doesn’t wander out of the pilot boring and off course. Finally, all rods are withdrawn and the casing is pushed into place.