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.

As you see in the above table, by changing the entry angle we could potentially decrease the set-back distance from 135′ to 98′. Note that the 18° and 22° entry angles work well with our 5:1 ratio we discussed earlier. However that rule of thumb does not hold true with the shallower entry angles.

Now let’s make things a little more complicated. A recent client was asking for total well lengths for a horizontal water supply project with the following specifications:

• Continuous well configuration (double ended well)
• 75′ deep horizontal screen section
• 500′ long screen section
• 6″ diameter stainless steel screen
• 8″ schedule 40 stainless steel riser at each end

First we use the 5:1 rule of thumb right? That would give us set back distance of 375′, a screen section of 500′ with a total well length of 875′. Well not so fast, let’s think about this for a minute. First, notice that the well is a continuous well, it has both and entry and an exit. Then we have 8″ diameter steel pipe in the riser sections of the well. As discussed above, the tightest we can bend 8″ steel pipe is an 800′ radius. However, 8″ schedule 40 stainless steel pipe is very expensive and we want to be doubly careful not to deform, buckle or break that material during the installation process. Therefore a 900′ build radius was planned for the well. Not only do we need to worry about the radii on the entry and exit sides, but the pipe will be pulled back to the drill rig,
and we want the transition from the ground surface (where the well material is laid out at the exit side) to the borehole to be as smooth as possible. So the entry angle was designed to be very shallow, 10° at both the entry and exit points. This geometry, 10° entry and exit angle, coupled with a 900′ build radius gives us a set-back distance of 504′. Now the total well length is 1,508′.

Finally, if you are looking at a site and determine that you don’t have the proper set-back distance, don’t panic. If you find yourself in this situation, call someone who is familiar with horizontal well/screen design; drillers are a clever bunch and we may be able to squeeze the well in anyway using some other options.

Hopefully this has helped you visualize the geometry of a horizontal well and not made the set-back distance calculation “clear as mud”. Just remember, the 5:1 ratio should be used as a rule of thumb to determine the minimum distance from the drill pipe entry point to the start of the screen section. To get a more accurate determination, contact a reputable horizontal drilling contractor with well installation experience: they can quickly review your project parameters to determine the well geometry.