Feb2020
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.
Lately, DTD has seen an increase in requests for horizontal well maintenance or rehabilitation. These requests include wells installed by DTD years ago and wells installed by other contractors, both active and defunct. What is our approach for improving the performance of an existing well? Let’s take a look.
As with vertical wells, a horizontal well may not perform for a variety of reasons. The site may have highly variable geology, leading to poor placement of the well. There may be a mismatch between the well design and the formation, or the well may have been installed improperly, among other reasons. Often, however, the well may just require a little tender loving care to return it to its former service level.
One of the first tasks we suggest is to perform a video survey of the well. Sometimes a visual inspection can shed 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.
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.