Featured in: Trenchless Technology
By Jeff Davis, Ditch Witch Product Manager, HDD
Tough terrains and hard-to-navigate underground construction jobsites are increasing the demand for horizontal directional drilling (HDD) installations. When a project takes place along a highway or in a protected environment, boring under the ground is often the only solution.
While HDD units offer a profitable solution for these jobsites, crews do run into one major issue: inadvertent returns. When drilling fluid isn’t contained by the bore and seeps into the environment, the jobsite may shut down and crews are responsible for ensuring containment and cleanup.
Jobsite costs expand as containment efforts ensue. A common method for containment is excavating a sump pit at the fluid-release point and filling it with absorbent materials. This task then requires additional equipment and results in excess waste through all the lost fluid.
Although operators are constantly on the lookout for seepage, they often aren’t aware that these issues begin with pressure. Namely, having too much of it underground.
Complications of Excessive Downhole Pressure
Environmental conditions like soil type and aboveground structures put pressure on every bore. While these conditions create minimal stress, each bore can still handle some additional pressure before opening or cracking and releasing drilling fluid into the ground.
Restriction to the circulation of drilling fluid is the main cause of additional pressure downhole. Efficient boring involves sending drilling fluid down from the surface equipment to the bit and then back up to the surface, where it’s collected in a pit for recycling or removal. On its return trip, drilling fluid follows the path of least resistance. When the bore is stable, that path is the space between the drill pipe and the sides of the bore.
As more fluid is sent downhole, however, higher fluid pressure is created which can impact stability. When this additional pressure exceeds what the bore can handle, it hurts the profitability of an HDD job. With excess pressure, fluid can begin to seep out of the bore and into the environment.
Operators must be aware of the risks related to downhole pressure, especially when using downhole tooling that requires higher mud flows. While debris should always be flushed out of a bore, only a small portion of the drilling fluid is used for that purpose when using a mud motor. The fluid pumped downhole is primarily powering the drill bit.
Inadvertent Returns from Drilling Fluid Power
Mud motors are a common HDD tool found on many directional drills, and they can have a major impact on pressure. By transferring the fluid that’s pumped downhole into mechanical power, these motors build up horsepower to help drive and steer the drill bit. Efficient bit operation requires a steady and constant supply of mud. This high fluid volume increases downhole pressure and the likelihood of inadvertent returns as the fluid makes its way back to the return pit.
Mud motors can often send fluid downhole that is five times more than what is actually needed on the job. A bore that requires 20 gpm of mud flow to efficiently flush the bore of cuttings, for example, ends up using an additional 180 gpm of fluid to run the mud motor. And as the drill units become larger or the ground harder, mud motors can send as much as ten times more mud downhole than what is needed to clean the borehole.
The additional drilling fluid results from inefficiencies of creating mechanical power.
To accumulate enough energy for the drill bit, mud often flows over a 15- to 30-foot power section. On average, only 50 percent of the power generated makes it downhole.
Fluid pressure loss can also be an issue when using mud motors, further decreasing their efficiency. As a drill bit gets farther away from its entrance path, more mud pressure is required downhole to keep a steady and consistent stream of fluid returning to the rig at all times.
While mud motors can be effective in mixed hard soils, such as rock between 5,000 to 20,000 psi, the risk of inadvertent returns with this technology is higher in HDD applications. Today, however, technology has advanced and new developments have led to a decrease in the amount of fluid sent downhole for better jobsite efficiencies.
All Terrain Technology Tackles Fluid Efficiencies
A switch from powering the bit with drilling fluid to All Terrain (AT) technology helps operators minimize how much fluid is pumped downhole. And AT systems provide a way to use the HDD unit’s power more efficiently for driving the drill bit.
This is possible because of the dual-pipe technology offered by AT systems. With an inner drive shaft that reaches back to the HDD unit, these motors physically control exactly how much horsepower is sent to the bit.
By reducing operator reliance on drilling fluid, AT technology is a more efficient power option. In comparison to the 50 percent of power sent downhole with mud motors, 95 percent or more of the HDD unit’s inner drive power is successfully transferred downhole using AT systems. For instance, when a job calls for 22 hp, these systems provide 24 hp of AT power and don’t create any additional downhole pressure since the AT system is not dependent on fluid flow or pressure.
The efficiency improvement plays a large role in reducing the risk of opening the bore in adverse ground conditions. When a hard rock is hit underground, mud motors need to build up additional power to drill through the obstacle, increasing the risk of inadvertent returns. AT systems however, can drill through the hard rock by delivering more power to the bit without any change in pressure underground.
This direct connection to an HDD unit also reduces the overall size of an AT system. Instead of a long power section, AT systems are between 3 and 5 feet long. And by placing the electronic locating package in the middle of that system, the shorter length puts the package 1 to 2 feet behind the bit. Locators are then able to more accurately track the location of the drill bit’s cutting face, instead of predicting where the bit could be when the electronics package is embedded behind a mud motor’s 15-foot power section.
All Terrain Technology Reducing Jobsite Footprint
HDD technology is often used on jobsites where environmental disruptions need to be kept at a minimum. Drilling fluid seeping into the ground is the most problematic example since it can temporarily shut down a project.
A small setup space is also a common requirement for minimizing environmental disruptions.
When using a mud motor, a number of problems can result from the size of this space because of the support equipment that helps it remain productive.
For instance, the continuous flow of drilling fluid requires a large supply of water. Yet additional water tanks can exceed what is permitted in the setup area. HDD crews are then forced to establish tanks elsewhere and use large pumps to supply fluid.
Large pumps are only one example of the additional equipment required on jobsites due to the use of mud motors. Jobsite costs continue to grow when using equipment to reclaim, clean and reuse drilling fluid. By using less drilling fluid, AT motors comfortably fit within small setup spaces where mud motors do not, and require less support equipment. The only additional equipment required with the use of AT motors are fluid management systems that can contain enough water to keep the drill in operation for a full day.
Less additional equipment and drilling fluid keep HDD jobsites productive and increases profitability. AT motors with dual-pipe technology let operators control how much fluid is used on varying jobsites. And by keeping drilling fluid to a minimum, HDD crews can decrease the risk of inadvertent returns and stay efficient and profitable on any jobsite.