Tips for Calculating Setback in Horizontal Drilling

In a past column, we provided a general overview of horizontal wells and their uses for environmental remediation applications. Now it’s time to get a little more detailed. One of the first questions usually asked during the initial phase of a horizontal well project is, “What’s the distance from where the bit enters the ground to the start of the screen section?” Or, “How deep do I have to drill vertically before I can turn to go horizontal?” This starts to raise flags with the horizontal drilling contractor because what the consultant really means is either “My site is tiny and I’m not sure if the rig will fit,” or “I didn’t know that you start drilling at an angle.” What we’re talking about in horizontal directional drilling vernacular is the setback distance.

As a rule of thumb, the minimum setback distance can be estimated using a 5:1 ratio. For every 1 foot of depth, figure about 5 feet of horizontal displacement. So a 20-foot-deep well would require approximately 100 feet of setback. Review the figures in the table on this page and you’ll see what we’re talking about. A word of caution needs to go here: Always remember that the drill rig still needs to fit on the site and some rigs are really long — over 55 feet. To make matters worse, some of the longer rigs are trailer-mounted, which means additional room is required to manipulate the equipment into the proper position. What’s more, depending on the angle at which the drill rods enter the ground, as much as 15 feet of drill pipe may be exposed between the front of the rig and the actual entry point.