We’ve been very diligent in the keeping the content of Inflection Points informative and non-commercial. However, in this issue we are going to brag a little; wouldn’t you if you think you broke a world record? More on that later…
Let’s do a quick review of horizontal/directional (remember the wells don’t have to be horizontal or straight) well installation methodology. There are two types of horizontal well configurations. The first is a double-ended well, installed similar to a utility bore. We design our bore/well path, drill down to depth, across the screen section, then back up to the ground surface on the other end of the well. The screen and casing are pulled back to the rig in tension. Using this method, extremely long wells can be completed. The longest horizontal environmental well is over 2,850′ long, the longest utility bore is over 12,000′!!! This type of well construction is often referred to as a “continuous” or “entry-exit” well.
While continuous wells are the norm, many sites don’t have enough room to exit the ground surface due to obstructions or real estate issue (the responsible party does not own the land at the exit point). In that case we install what the industry terms a “blind” well; a well that terminates under the ground surface and does not exit.
Blind wells can be somewhat tricky to complete. First we have to drill the borehole, then, while the drilling fluid maintains hole stability, the drill pipe and bit are pulled from the hole. With the borehole held open by the drilling fluid, we then push the well materials into the open borehole. Obviously this type of installation is more difficult than a continuous well for several reasons:
The borehole is held open by the drilling fluid, but in unconsolidated formations, the hole will collapse over time (gravity always wins). Gravels and sands tend to be more unstable than silts and clays.
We can’t push well materials as far as we can pull them. As we push the screen and casing, the string of pipe becomes somewhat sinusoidal in the hole and the push force is not transmitted to the end of the pipe. Think about trying to push a rope…
Most threaded connections for well screen and casing are designed for tensile forces while hanging vertically in a straight hole, not being pushed around a curve.
Having stated the above issues with blind wells, most contractors are proficient in their installation in cohesive soils around the following general parameters:
Total well length of less than 300’ – HDPE or PVC screen and casing
Total well length of 300’ – 500’ – HDPE screen and casing
Total well length of 500’ – 850’ – fiberglass, steel, or stainless steel screen and casing.
What about blind wells over 850’, can they be installed? Yes, but it gets even more complicated. First we have to drill the curve section of the borehole. Then we ream that section and set/cement surface casing. After the cement has cured we go back in the surface casing, drill the horizontal section, pull the drill pipe from the hole and install the well materials through the surface casing, into the open borehole. This is a time consuming and expensive process. The longest blind well with the above configuration is 1,444’ long. The depth of the screen section is 78’ bgs, with 553’ of 10” HDPE surface casing and 891’ of 4” stainless steel screen. For those of you keeping score – that well was completed in Louisiana, by Longbore,
Inc. (now defunct) in July of 2000.
The time and cost of the above well configuration is in most cases prohibitive to site owners and consultants. There had to be a better way…
Finally, it’s time to brag a little. DTD has found a better way. It took many man-hours of research and development to invent what we call the Knock Off blind well installation method. Here’s how it works:
First we use 5” inside diameter drill pipe to drill the borehole. The drill bit is the key to the Knock Off method. It is attached to the drill pipe in manner that allows us to literally remove it from the drill string down-hole. We have invented a de-latching assembly that can be activated from inside the drill pipe with the well screen and casing.
The following is the process to drill and install a well using the Knock Off technology:
Drill/navigate the well to total depth with 5” I.D. drill pipe and the Knock Off bite.
Leave the drill pipe in the hole.
Insert 4.5” O.D. PVC or HDPE well materials inside the drill pipe. The distal end of the well materials includes the de-latching tool.
Run the well materials to the end of the drill pipe.
Push the de-latching tool into the Knock Off bit.
The bit latches on to the well materials and detaches from the drill pipe.
Pull the steel drill pipe from the borehole, leaving the well screen and casing in the hole anchored in place by the sacrificial Knock Off bit.
The Knock Off method saves time, money and gives the client piece of mind in a variety of ways:
For wells over 850’ in length, PVC or HDPE can be utilized instead of steel, stainless steel or fiberglass.
The small difference in diameter between the 5” I.D. drill pipe and the 4.5” O.D well materials prevents them from becoming sinusoidal. This allows much more push pressure to advance the screen to the end of a long bore.
The smooth inside of the drill pipe has much less frictional effect than the subsurface formation, thus less push pressure is required for long well lengths.
Surface casing through the curve is not required.
The well materials are placed at the precise navigated location.
We do not have to be concerned with unconsolidated formations collapsing because the drill pipe is in the borehole during the well screen and casing installation.
Does the system work? You bet it does. In fact let’s review what we think are the longest blind wells ever completed:
DTD installed two air sparge wells under an active manufacturing facility. Both wells were constructed as single entry (blind) wells utilizing the Knock Off drilling/installation tooling/methodology. The pilot bore of the well was drilled, steered and located using a gyroscopic steering tool (GST). Once the pilot bore was advanced to the final drilled length (measured depth or MD), the entire drill string was pulled from the borehole. The Knock Off bit was attached to 5” inside diameter drill pipe and run into the open hole back to the final MD, clearing any obstructions. Well screen and casing (4”, schedule 80 PVC) were then inserted into the drill pipe and pushed with the rig to the end of the drill string. The proprietary Knock Off tool, attached to the distal end of
the well screen, was inserted into the bit, releasing the bit from the drill string. The 5” ID drill pipe was then pulled from the borehole, leaving the PVC well screen and casing in place. Well HSW-1 has 720’ of blank and 805’ of screen (installed at 130’ bgs) for a total well length of 1,525’. Well HSW-2 has 790’ of blank and 680’ of screen (installed at 130’ bgs) for a total well length of 1,470’. Bio-polymer drilling fluids were utilized for the drilling operation and well development consisted of flushing the well and jetting the screen section with fresh water and enzyme breaker.
There you have it. What we believe are the two longest blind horizontal environmental wells ever installed. Kudos to the DTD drill crew and field manager for this successful installation. If you’ve been involved with long blind well completions, let us know, we’d like to hear about your experience.
We wish everyone a safe holiday season and happy new year.