Bridging the Gap Between Horizontal and Vertical
As a young vertical driller, I believed that vertical drilling and horizontal drilling were radically different. I had a grasp of basic drilling fundamentals; I could never drill a horizontal hole. My internship with Baroid IDP made me realize that vertical and horizontal drilling, like most facets of drilling, share the same fundamentals. Both types of drilling share the same goal: to create a temporary pathway to extract resources or install product. Once a driller realizes that Mother Nature and physics are always working against the borehole, he can be successful. Although these fundamentals start the same, completing the hole is when the major differences occur. For a simple comparison in this article, I will compare a typical 400-foot vertical borehole and a 400-foot horizontal borehole with no major drilling issues.
The start of any borehole requires proper planning, preparation and patience. Regardless of vertical or horizontal, the driller must understand the proper steps it will take to complete the borehole. The potential for inadvertent returns or fracking out at the surface in most situations has to do with the formation density. Top soil has a soft density with much porosity, allowing fluid to push through it fairly quickly. On the other end of the spectrum, rock can be hard to break and frack outs can occur while attempting initial penetration.
A starter hole, or “pilot hole,” will help prevent fracking out at the surface. Think back to woodshop class in grade school. If you were like me, you skipped Mr. O’s instruction to pilot drill all the fasten points on your cedar birdhouse. Sure enough, when you started to assemble the birdhouse you split the planks. I attempted to use putty and glue to repair it, but the stability was not the same as it would have been if I had not split the joints. That bird house only lasted one Michigan winter before it fell apart. Splitting the fracture gradient of the ground is the same. Yes, we can use loss circulation material and install surface casing, but the damage is done. The fracture wants to become the path of least resistance, slowly opening into a larger fracture area.
Often I hear, “A pilot hole or test hole is not important because we plan to open the hole up to a larger diameter.” That statement is a huge mistake. In reality, it is the most significant hole drilled because any damage done on the pilot hole will continue to destabilize the hole. Remember that the initial penetration of the bit is the most invasive and disruptive process of a borehole.
Now let’s imagine that the bit is 100 feet away from the drill. When drilling first begins, fractures in vertical drilling and horizontal are caused by similar issues, but as the bit travels farther away from the drill, the downhole conditions change and the issues that cause the fractures change. In vertical drilling, once the bit penetrates beyond the top 30 feet, fractures to surface substantially reduce because the compaction of the ground helps prevent major fractures back to the surface. The bit and tools stay centered in the hole and, as long as the mud pump continues to provide an excellent fluid return, issues are minimal. However, in horizontal drilling, the farther the bit travels from the drill, the higher the possibility of frack outs. Drilling of the pilot hole on a horizontal directional drill (HDD) can rarely rely on 30 feet of overburden, like a vertical drill. The path is always up, and when you do not have 100 feet of overburden, the HDD fluid returns are always trying to take the path of least resistance, regardless if that is 12 feet up or 100 feet back to the drill. Regardless of vertical or horizontal, the key to preventing frack outs is to maintain flow.
Maintaining flow has three parts: volume, velocity and gravity. All have a significant impact on the success of both vertical and horizontal boreholes. Vertical drills have it much easier starting out. A vertical drill rod and bit are centered in the borehole and, between rod connections, any cutting that did not make it to surface falls to the bottom of the hole. On an HDD drill, the rods are located in the lower third of the hole, and when a connection is made cuttings can quickly fall and start to cover the drill rod. Vertical drills rely on the velocity of the mud pump to carry cuttings to the surface, understanding that some cuttings will fall to the bottom when the pump is turned off. An HDD drill relies on drill fluid volume and suspension of the cuttings to clean out the borehole. An HDD driller will watch his return fluid and wait until cuttings stop coming out. Often the operator will use a stopwatch to time how long the pumps need to run to clean the hole completely. A 12-inch borehole in HDD will require three times the fluid to transport the same amount of cuttings than a vertical borehole. Consider that water well return fluid is 8.8 pounds per gallon, whereas a typical HDD return fluid is 10 pounds per gallon. In horizontal drilling, heavy soils such as large gravel and clay require two to three times the amount of drilling fluid to push the solids-laden fluid out of a hole. HDD cuttings have inches to fall before a rod can be stuck, whereas vertical cuttings will fall to the bottom of the hole. Flow must be maintained to remove cuttings out of any borehole.
The major difference between horizontal and vertical occurs at the end of the hole. HDD challenges happen during the pilot shot and middle of the hole. At 400 feet, the HDD drill is pushing through into an exit pit. The fluid and cuttings suddenly have a shorter path of resistance, substantially reducing the hydrostatic pressure. At 400 feet, a vertical hole has encountered many new issues. First, the drilling fluid properties are starting to break down with incorporation of ultra-fine drill solids. In HDD the drilling fluid is discarded or recycled, thus preventing the loading of ultra-fine solids. Horizontal drilling fluid properties are better maintained because of the constant introduction of new drilling fluid.
My buddies at Baroid IDP will tell you that a project’s success depends on creating a stable borehole. Borehole stability can only be achieved by maintaining good drilling fluid properties. These properties will incorporate low solids content and high filtration control, and inhibit reactive soils. Contact your local mud engineer for help on the right drilling fluid properties for vertical or horizontal holes.
Vertical and horizontal drilling have more in common than not. The goal is to prevent physics and Mother Nature from destabilizing the borehole. The goals will be achieved when a driller follows these steps. First, plan to drill a pilot hole. Next, create a drilling fluid that will stop fluid invasion into the formation, properly suspend drill cuttings and prevent reactive soils from reacting downhole. Finally, the job can only be successful if the mud pump is properly designed for the job. Maintaining flow means that cuttings are carried out of the borehole, creating the temporary pathway required to finish the job.