As a rule of thumb, the minimum set-back distance can be calculated based on a 5:1 ratio. For every 1′ of depth, figure 5′ horizontal displacement. So a 20′ deep well would require approximately 100′ of set-back. Review the figures below and you will see what we are 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 drill rigs are really long – over 55′. To make matters worse, some of the longer rigs are trailer-mounted which means additional room is required to manipulate the rig 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 rod may be exposed between the front of the rig and the actual entry point.
The set-back calculation seems pretty simple, just use the 5:1 ratio. Well maybe not. Let’s look at our diagram again, you’re good at geometry right? You think to yourself what if I change the entry angle or the build radius, won’t that have an effect on the set-back distance? You would be correct. But before you get too carried away we need to think about the most important item that determines the set-back distance – the drill pipe.
We use steel drill pipe in our operations, and steel is flexible. But at some bending radius the pipe goes from elastic deformation (it will spring back to its original shape) to plastic deformation (permanent deformation/change in shape). If you keep bending the pipe through the plastic deformation phase it will ultimately buckle and then break. So now it’s time for another rule of thumb: for every inch of outside diameter (O.D.) of steel drill pipe, estimate it can be bent in a 100′ radius. So if we are drilling with 2 3/8″ diameter drill pipe we can safely put it in a 250′ radius, 5″ drill pipe a 500′ radius and so on.
Why would we use one drill pipe size over another? Could be several reasons. For example, the larger the borehole/well diameter the larger the drill pipe required. Also the more challenging the geology — think gravels or bedrock — the larger the drill pipe and finally, bigger rigs require bigger drill pipe.
Before we go any further, a word of caution. Designing the bore plan at the tightest drill pipe bending radius is risky. If for some reason we can’t steer to the profile we have no “wiggle room” to go tighter (we call that “busting a radius”). The result will either be a deeper screen start depth or a lengthened set-back distance and shorter screen section. Bottom line, we always want to plan the bore path with a bending radius greater than the drill pipe minimum, this will give us some “wiggle room” to meet the bore plan if we encounter issues during drilling.
Now that we know the drill pipe has a minimum bending radius, let’s go back and play with the geometry. Assume that we have a drill rig that can enter the ground (entry angle) between 10° and 22° above horizontal, drill pipe that cannot be placed into a tighter radius than 250′ and a screen start depth of 20′. Look at the table below and see how the set-back distance changes as we vary the entry angle.