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Accounting for
Elevation During Horizontal Well Design and Installation
Since a vertical well or boring is a
single point relative to the surrounding topography, it
generally only has one important reference elevation:
“ground surface”. An HDD boring, on the other hand,
could transect a “ground surface” with widely varying elevation
relief.
This adds several different complicating factors that
need to be considered when planning an HDD project. In
this month’s Inflection Points we’ll explore some of
the different implications of changing elevation on HDD
bore path planning, particularly in the context of
designing horizontal wells.
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Design the Bore
Path in Terms of Relative Elevations
When designing environmental remediation wells, it’s common
to refer to the target depth in terms of “below ground
surface” or BGS. It’s a natural habit most folks developed
from vertical well design. But HDD bore profiles are designed
relative to “entry elevation” or “rig elevation”. This is the
point at which the bit enters the ground. Understanding this
distinction can prevent well screens from being designed or
placed to target the wrong depth. Don’t laugh; it happens.
For example, consider the following hypothetical. Based on
monitoring well and sampling data, we may know that our
highest contaminant concentrations are at a depth of 35ft
BGS. So, we plan to target this zone with our air sparge
screen. Of course, to achieve this depth, we’ll need a
horizontal setback of ~5:1, which means we’ll need to set up
our rig ~170ft away.
But is the ground surface elevation the same at the rig up
area as in the screen area? If it isn’t, the bore profile
needs to be designed with elevation relief taken into
consideration.
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Fig 1 –
Sometimes the ground surface elevation is not the same in your
target area as it is in the rig set up area
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The savvy project planner can use a change in
surface elevation to her advantage. If the ground surface at
the rig up area is below the elevation at the target area,
this effectively reduces the required setback necessary to
achieve the desired depth. Of course, if the rig is at a
higher elevation then the reverse is true.
Another way to ensure the screen is located at
the proper elevation is to use mean sea level (msl)
elevations for all the significant data points; well entry,
screen start and screen end.
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Make Sure Your Steering Technology is Going to
Work Across the Entire Bore Path
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There are several different locating technologies
available, and each one has its own advantages and
disadvantages. For the vast majority of HDD projects, the
walkover system works very well, and it certainly represents
the most economical choice.
One limitation of the walkover locating system
though is depth. The walkover relays bit location and
orientation information to the operator by way of a radio
signal and an electromagnetic field (EMF). Both signals
can attenuate as the distance between the receiver and the
bit housing (or “sonde”) increases. First the EMF signal
becomes unusable, and not long after the radio signal is
lost, the walkover system ceases to be an effective locating
tool.
For the more robust walkover systems the depth range is
limited to ~70-80ft. Elevation changes along the bore profile
(like drilling underneath a big hill for example) may cause
the bit to be “out of range” for some portion of the bore
path.
If it’s just a brief period that the bit is too deep, the
driller may be able to “go blind” for a few rods without
deviating too far from the planned running line. Please note
that “blind” is sort of a misnomer, the driller and locator
can drill for a short time at the last known pitch of the
drill bit. We know where we are and where we’re
going. However, if a significant portion of the bore
path is out of range, or if extremely precise knowledge of
the bit location is crucial to the project’s success, then an
alternate locating technology is probably more appropriate.
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Fig 2 –
Walkover locating tools have a maximum effective depth range,
which needs to be considered relative to ground surface
elevations along the bore path.
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It’s Better to Drill Up than Down
Another factor to consider when a bore path
crosses a dramatic change in elevation is the rig layout. A
substantial change in elevation across a bore path will exert
a significant amount of head pressure on the drilling fluid
in the borehole.
Increased head pressure with very little
overburden between the borehole and ground surface can
drastically increase the likelihood of a frack out (or
“inadvertent fluid return”).
Although it is a bit counterintuitive to drill
“up”, the head pressure issue tends to cause HDD contractors
to prefer to rig up on the lower elevation side of a borehole
if given the choice.
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Fig 3 –
Logistically it’s much easier to set up the rig at the
lower elevation side of the bore path.
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Not only does this configuration reduce the head
pressure at the distal end of the bore, limiting frack out
issues, but it also simplifies mud management logistics in
double ended bores. The mud will flow towards the lower
elevation side of the bore path. It is more convenient if
this is the “rig side” since the pumping equipment,
roll-offs, etc. are already set up there.
While duplicate pumps and containments systems
can be set up on the exit side, it’s not only more costly,
the change in head can make it very difficult to manage the
mud when it comes rushing out.
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You Probably Should Plan to Survey
There are a lot of unique aspects to HDD that
can make the projects more complicated than a vertical
drilling project. The fact the ground surface elevation can
change across the length of the bore profile is one of the
more prominent ones.
Being aware of elevation relief is an important
aspect of planning an HDD project, so much so that it’s
probably worth planning an elevation survey as part of your
project costs.
DTD will provide a final as-built of all the
horizontal wells that we install; however, our reference
point is always going to be relative to “rig elevation”. For
a complete picture of the exact well location in the
sub-surface, you’ll want a firm understanding of how the
ground surface elevation changes along the way.
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