Let’s face it, there are hundreds, if not thousands of small contaminated locations across the country that are being neglected because access is “impossible”. Most of these sites are located in highly urbanized areas where contamination is under occupied commercial/residential buildings and surface infrastructure such as streets and parks.

DTD and other horizontal drilling contractors have small-footprint horizontal drilling systems that are ideal for sites like active or former UST releases and dry cleaners. How small? Some of our equipment can be placed in an area as small as a few parking spots, 30’ x 50’. This includes the drill rig, drilling fluid mixing/pumping system, waste containment and materials storage.

Another unfortunate perception is that horizontal remediation wells need to exit the ground and the well materials must be pulled back to the drill rig; requiring site access and work far from the rig up area. Again this is not reality. Blind wells in many geologic settings are routinely completed up to 300′ in length using PVC screen and casing. By utilizing HDPE well materials, blind well installations up to 500′ are possible. Please note that geology plays a role in blind well installations, and many times is the limiting factor when it comes to the ultimate length of the completion.

Perceived cost is another issue that limits the use of horizontal drilling and well installation at a small site. In almost all cases horizontal wells, used for remediation activities are less expensive than vertical wells. You are thinking how can that be? First, in many instances blind PVC wells in unconsolidated sand, silty sand and clay can be installed at about $100/ft. That is still expensive when compared to $25/ft. for vertical wells. Unfortunately some consultants stop their cost analysis at this point. But the cost per foot of the well installation is a very small part of the total remediation cost. Other items that must be included in the cost analysis include:

• Disruption of business, if vertical wells must be located in customer access areas.
• How many vertical wells are required compared to the horizontal wells?
  • Multiple vaults
  • Multiple pumps/injection strings
  • Multiple feet of piping for vertical well tie-in to the system
  • Trenching
  • Concrete/asphalt repair
• If you need to put vertical wells in the street think about these costs:
  • Permits for the initial well installation and any ongoing access requirements.
  • Street repairs after drilling.
  • Potential utility conflicts
  • Traffic rated vaults
  • Safety of field crews working in traffic
• Ongoing operation and maintenance of a multiple vertical well system versus one horizontal well.
• Can the horizontal wells access the contamination under a building? If not, how is the remediation attacking the plume? Are you only tackling the edges of the plume with vertical wells?
  And finally, how long will the remediation activities take?
• And with today’s in-situ chem-ox remediation techniques, contact with the contaminant is the name of the game. Remember plumes are typically long and “skinny”; one horizontal well screen can place treatment remedy in touch with tens or hundreds of feet of plume.

When all of the above issues are factored into total project cost, horizontal wells offer a very cost effective solution. But you can’t just look at the cost per foot of the well. If you want to compare cost per foot, look at the cost per foot of SCREEN in the actual contaminant plume.

Enough discussion – let’s look at an actual project. The site is located in a small town west of the Rocky Mountains. Diesel fuel has contaminated both soil and ground water in the downtown area. The majority of the contamination is located under a busy street and parking lot as an elongated plume. Surface access was limited due to multiple property owners and the street. The consultant proposed two horizontal wells, at a depth of 26′. One well would be utilized for the injection of an amendment and the other well would be pumped to pull the amendment across the plume.

Double-ended (continuous) wells could not be installed at the location – there was insufficient room at the distal end of the screen to exit the ground. Therefore single ended (blind) wells were utilized. The consultant and DTD worked through several iterations of possible well designs based on plume location, screen depth, possible entry locations, potential utility conflicts and surface access. The final design consisted of two blind wells, constructed of 3″ diameter high density polyethylene (HDPE) screen and riser. The screen length in each well is about 260′, installed at an approximate depth of 26′ bgs.

The project started on day one with delivery of the drilling and support equipment. DTD crew members arrived on-site day two and proceeded with equipment rig up, utility confirmation by pot-holing and final well design review. The rig up area for the small rig required an area of about 50′ x 50′ (see below). Drilling commenced in the afternoon and 60′ of borehole was completed.

Typical small rig set-up area

Drilling continued on day two and the first part of day three. Several times the drilling fluids returns (we were drilling with a bio-degradable polymer) slowed. This normally indicates that the borehole is not fully clean/open, therefore the drilling assembly was pulled from the bore to insure the hole was clean and the risk of inadvertent returns was minimized. Late on the third day the bit reached the end of the predetermined path and was removed from the hole. The screen and casing was then pushed into the open hole.

Looking from the end of the well back toward the rig

The second well was drilled and installed in a
similar fashion and by day six the installation, well development, vault
installation and clean-up activities were complete.

Plan view of blind well installations (proposed – actual installed location slightly modified due to field conditions)

In summary – two blind horizontal remediation wells were completed at the site. Each well is approximately 380′ long, consisting of 260′ of 3″ diameter HDPE screen and 120′ of riser. The installation of both wells, including well development, site cleanup and vault installation took six working days – without any disruption to the normal downtown activities. Just think about that for a second; 520′ of screen installed in the plume, two well heads located in a parking lot in the same vault, no well vaults in the street/street repairs, no trenching and no need to close the street for future access to the remediation wells – all completed in six days.