Planning Horizontal Directional Drilling for Pipeline Construction

Complex subsurface stratigraphic conditions can give rise, in turn, to rapidly
changing variations in bedrock properties, potentially resulting in directional
control and steering problems. Hard (high compressive strength) and/or highly
abrasive bedrock will also affect schedule and costs and, in some instances, the
feasibility of the HDD project could be put in question. Mitigative approaches
may include avoiding high compressive strength bedrock units in the design of the
drill path and/or minimizing the length of drill path that encounters these
formations. Application of air hammering or air drilling techniques could also be
considered if drilling through these materials is unavoidable.
All coal seams encountered during the geotechnical investigation should be
identified on the bore hole logs. In many cases, they are extensively
jointed/fractured and, as such, can be a source of loss of circulation and/or mud
control problems. During directional drilling, coal particles can also clog pumps
and create problems with cutting returns.
Depending on their extent, voids related to bedrock solution processes can also be
a source of loss of circulation and mud control problems. Karst openings in
limestone and dolomite formations are most commonly encountered but solution
cavities can also occur in gypsum and salt- and potash-bearing bedrock
formations.
6.1.4 Hydrogeology
The main hydrogeological issue relates to the presence of artesian conditions.
These are typically encountered where impermeable clay or shale bedrock layers
overlie permeable water-bearing sands-gravels or sandstone bedrock at depth,
forming a confined aquifer. When intersected by the pilot hole, such aquifers may
be large-volume sources of groundwater under pressure. As such, mud quality and
fluid management problems may result. Mitigative approaches include casing or
cementing off the confined aquifer zone.
Cross-contamination of aquifers may also be a concern, particularly when the
directional drill path is very deep.
6.2 Geotechnical Investigation
The geotechnical investigation is a critical part of the information gathering and
risk assessment phases of planning an HDD. The scope of work should include:
review of background information, a field reconnaissance, completion of a
program of field drilling and sampling, geophysical surveys if appropriate,
laboratory testing, and office analysis. It should be recognized that subsurface
conditions are generally not homogeneous and, for this reason, may be difficult to
fully investigate.
Results of the investigation should be presented in a geotechnical report
(Section 6.3). The report can be referenced in final design and should be suitable
September 2004 Planning Horizontal Directional Drilling for Pipeline Construction Page 6-3