Rehabilitation of
Horizontal Remediation Wells

Although horizontal
directional drilling for environmental remediation has been
around since 1987, most of the horizontal remediation wells
installed have been constructed in the last 20 years or so.
Like almost any man-made structure, wells are dynamic in
operation and may change over time. Many, if not most,
horizontal wells are successful in their goal and quickly
contribute to the site cleanup. Still others initially
perform as designed but suffer performance losses over
time. As horizontal well professionals, we receive calls to
investigate wells like these and suggest methods to restore
or improve their performance.

FEB2019

One of the
first tasks we suggest is to perform a video survey of the
well. Sometimes a visual inspection can shed a light on
what is causing the trouble. Our equipment inventory
includes high resolution borehole cameras to inspect every
inch of the affected well. An initial video inspection will
establish a baseline for future rehabilitation efforts. We
can observe for potential damage to the screen or casing,
see where sand or silt is entering the well, and check for
other performance-robbing factors, such as chemical or
biological encrustation or fouling.  Additionally, a
video can identify any obstructions that may be in the
well.  It’s not a good thing to run into a horizontal
well with a jetting tool and become entangled in something
that should not be in the well.  You’d be surprised at
the things you find in a well – hand tools, gloves, pump
cable/pumps, rope, and heck once we even saw some kind of
lizard.

For an
example check out this horizontal well video:

If there is a
submersible pump in the well; the pump, drop pipe and cable
must be removed first. This generally stirs up the water
and may cause cloudy conditions that may persist for hours
or even days. Low visibility water will hinder the video
inspection, so, plan to pull the pump a couple of days in
advance of the camera team arrival.

After we review the video, there are several
options to continue with the maintenance or rehabilitation.
One of the most common is to jet the well with a
high-pressure jetting nozzle. With a combination
vacuum/pump truck it is possible to jet out sand and silt
or bio-fouling through hundreds of feet of screen. 

For longer
screens, or where there is considerable coarser grained
sediment it may be necessary to run tubing, or even drill
pipe down into the well and pump out of one end while
jetting from the other. Obviously, this is only possible in
a double-ended well configuration. Particularly in a well
with limited open area (such as an air sparge well) that
was drilled with bentonite, additional development by
jetting can improve performance.

If review of the video confirms bio fouling or mineral
build-up, the best approach is to collect samples of the
offending material for lab analysis. After the nature of
the build-up is determined there are a variety of chemical
treatments that may be applied to dissolve or remove the
fouling. Injection of acids, bases, sterilizing agents, or
even less common treatments such as liquid nitrogen may be
used to improve the well performance.

After jetting or other treatment, it is generally useful to
perform a second video survey of the well. This will verify
the effects of the treatment attempt.

What if the initial video survey shows damage to the well,
sustained either during or subsequent to the installation?
This may show up as a kink, break, or even dislocation in
the pipe. Someone may have trenched into a riser, or even
driven a direct-push well through a horizontal well. You
might see broken out slots in the screened section, which
can allow sand to enter the well unimpeded. Even in this
severe situation, it may be possible to salvage the well
rather than abandon and re-drill it. If the well screen is
large enough in diameter, it may be possible to insert a
smaller-diameter slotted liner either through the entire
screen section or just to bridge the damaged section. DTD
has used this method to repair a well in the past.

Another task
that we have found ourselves performing with some frequency
is locating the position of an existing well. Even older
wells generally were completed with some sort of as-built
that provides the profile of the installed well screen and
risers. However, unless the consultant was diligent about
surveying in the location markings left as the bore was
drilled (assuming walkover locating), these markings
(either pin flags or paint marks) are soon obliterated –
leaving no permanent record of the well location. In addition,
site owners have been known to lose or misplace as-built
diagrams. This can result in situations where facility
expansion at the site, or even the addition of a few
monitoring wells, risk damage or destruction of the
horizontal well.

DTD has
recently completed a couple of projects where we had to
locate existing legacy horizontal wells. At one Department
of Defense facility, the limited site access (no walkover
allowed) required us to use a data-logging gyroscopic
steering tool through the well. The tool, using an inertial
guidance system, logged the position of the well in three
dimensions from a known control point with sub-foot
precision. The resulting data was used by architects and
engineers to avoid placing support columns over the well
during a building expansion project.

In another
project, we pushed a standard walkover locating tool
through the well, while locating normally with our receiver
and re-marking the bore path. The survey confirmed some
vertical discrepancies in the bore path (installed by
others) and will help the owner and DTD in deciding how to
resolve well performance issues.

One final note of caution. A well has to be of
sufficient diameter for the insertion of a camera, jetting
tool, tubing, drill pipe and/or a locating tool. We have
recently seen an increase in small diameter HDPE tubing
used as horizontal wells, primarily to reduce up-front
costs. Consultants, site owners and regulators need to be
aware that a 1” diameter well is difficult to properly
develop post installation and virtually impossible to rehabilitate.

If you have an underperforming well, or as a new consultant
on site are just trying to figure out what sort of legacy
HDD wells you have inherited, there may be more options
than you think. Contact your horizontal well specialist to
see if there are alternatives that you may have overlooked.