AUG2020
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.
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.
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.
Make Sure Your Steering Technology is Going to Work Across the Entire Bore Path
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.
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.
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 containment 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.
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 subsurface, you’ll want a firm understanding of how the ground surface elevation changes along the way.