Horizontal Well Installations – Site Preparation and Preplanning are Paramount
We have discussed a wide range of topics in this newsletter over the last year or so. Last February the focus of Inflection Points was set back distance – the horizontal distance from the point where the bit enters the ground to the start of the screen section (usually, but not always the start of the horizontal section of the borehole).
We reviewed that a good rule of thumb for calculating set back is 5:1; for every 1′ of vertical depth figure about 5′ of horizontal displacement. Therefore, if we are 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’re 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 path that is 38′ long. UH OH – now what?
Which brings us to the topic of this newsletter, Site Preparation and Planning. 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 utilities “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 conflicts are not a deal breaker. Before we go any further, note 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 (see below).
If you don’t understand all the paint markings on the ground, you do not 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 finding the utilities can be a lengthy process. Potential standby time can be avoided by marking and finding the utilities before the rig 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 (see below). However the potential for injury or infrastructure damage far outweighs the cost of hand clearing.We have drilled closer than 2 feet to a utility (we do not like or recommend this practice), but only by exposing the line and visually observing the drill bit pass safely by the utility.
Finally, after the paper “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 and perform the proper planning and site preparation and have a safe job. You do not want to be the driller who did this…