We’ve all heard about “call before you dig” and most environmental drillers are well aware of potential utility strikes, but did you know April is National Safe Digging Month? With that in mind let’s discuss utility clearance for horizontal environmental wells.
We must remember, clearing utilities for a horizontal well is a completely different animal than traditional vertical drilling. We’re not clearing a “point” to a pre-specified depth; we need to clear the entire bore path. Since this is “National Safe Digging Month” let’s review a past “Inflection Points” newsletter (April 2015 to be specific – albeit slightly updated and modified).
Back in February of 2014 we discussed the rule of thumb for calculating set back is 5:1; for every one foot of vertical depth figure about 5’ of horizontal displacement. Therefore, if we’re discussing a well at 20’ depth, the set back is about 100’. Now let’s assume that the range of entry angles for most rigs is between 5° and 25° above horizontal.
Next, let’s also assume that MOST (not all) utilities we may encounter on a site are at depths of 8′ or less. It’s trigonometry time. How much horizontal displacement will we have from the entry point to reach 8′ of depth, if we have an entry angle of 12°? Thinking about a right triangle, we calculate using the tangent of 12° and come up with a horizontal displacement of about 37′. Now when we are planning a horizontal well project we need to consider much more than just math – we just learned that we need utility clearances down to 8′ over a bore segment that is 37′ long.
Which brings us to the topic of this newsletter, utility clearance. Once a site is slated for horizontal well installations, the first thing we do is start with a utility map. If you’ve done any amount of field work, you know that most “as builts” are nothing more than a fantasy at best. However, start here we must.
Are there any obvious utility conflicts with the bore path? Remember horizontal drilling is based on a steerable system so potential conflicts are not a deal breaker. Also, consider that utilities are not the only buried objects we need to avoid. Existing monitor/remediation wells, piezometers, abandoned bore holes, CPT/DPT holes and remediation system piping all need to be avoided. You don’t have to strike an existing well to trigger a mess; even drilling too close to the screen section may cause drilling fluid to rise to the surface.
Back to the topic at hand, utilities – after we review all paper data and before the bit enters the ground, the consultant, site owner and driller must comply with state and/or local regulations for underground clearance. In some states this is the “Call Before You Dig”, “One Call” or “Call 811” service. This mobilizes all utility owners on or near the site to mark their lines. However, some companies will not mark lines on private property; in that case a private locating company may need to be contacted. Once all the lines are marked it may get somewhat confusing.
If you don’t understand all the paint markings on the ground, you don’t trust the mark outs, or your bore path will cross close to a line – DO NOT DRILL! Get positive confirmation of the lines before you start. All utilities with potential conflicts should be exposed and the depth to the top and bottom of the utility confirmed by direct measurement. How do you get confirmation? Couple of ways. One – pull the manhole covers and use a tape measure. Two – expose the lines though hand excavation or air knife activities. Potholing of utilities should be done before the drilling team arrives on site. At many private and government facilities the utilities are often mismarked and even finding them can be a lengthy process. Potential standby time can be avoided by marking and finding the utilities before the rig drill and crew arrives. In many instances, the open holes/exposed utility can be covered with plywood or a plate, and fenced off until the drilling occurs. The drill team can backfill the potholes after the drilling is safely beyond the utility. Note that on some sites excavating lines can be a complex, time-consuming operation. However the potential for injury or infrastructure damage far outweighs the cost of hand clearing. Finally, we have found in many instances, large petroleum pipeline companies ask for visual verification of our bore path when crossing under their lines. The line is exposed and we watch the bit pass under the line at the depth mandated by the pipeline owner.
After the paper utilities “as built” is reviewed, the “One Call” ticket is in hand and all potential conflicts are exposed, drilling can begin. This process can be time consuming and expensive. However the alternative can be deadly and/or expensive. Take the time, perform the proper planning, site preparation and utility clearances and have a safe job. You don’t want to be the driller who did this…