Thinking of taking on a job that requires drilling through rock? Or maybe you’re on a job that has suddenly run into rock… Rock-specific HDD tooling solutions are about more than just buying the right bit. Rock drilling requires thinking big-picture about the full scope of needs that will enable you to tear through these challenging ground conditions.
Here’s what you need to know about the true cost of drilling rock.
A basic HDD set-up just won’t cut it
Trying to use a standard duckbill on the end of your drill string while upping the power of your rig will just result in frustration—and possibly broken HDD equipment. The only way to get through rock successfully is to invest in buying or renting HDD tools for rock drilling.
Mud motor (+ drill fluid)
A mud motor is designed to drill through rock by chipping away at its surface, while a two-degree bend in its shaft enables steering. A mud motor is specialty equipment with a price that reflects it. This type of high-impact grinding requires significantly more drill fluid to power the mud motor and slurry out cuttings. For comparison, conventional drilling (using a duckbill blade) may consume about 5-15 gallons per minute—but even the smallest mud motor will require at least 40 gallons per minute. Together, the cost can add up.
(Mud motor: $1,000 - $3,000+ per week to rent, $14,000 - $65,000+ to buy; drill fluid: explained at end of article)
When using a mud motor, you’ll also need a few critical pieces of additional HDD equipment before you even displace the first foot of earth.
Because of all the drill fluid required to move through your mud motor, you’ll need a specialized high-flow housing. A high-flow housing provides better protection for your transmitter and other electronics in your drill string, but more importantly, it allows larger volume of liquid to flow through without restriction.
(Cost to buy: $3,000-$5,000)
Drill fluid reclaimer
Since your rock operation will require so much drill fluid, it doesn’t make economic sense to use the fluid just once. A drill fluid reclaimer siphons off the cuttings, leaving you with clean drill fluid to re-use.
(Cost to buy: $40,000-$300,000+)
Tri-cone or PDC bit
Tri-cone bits or PDC bits are specialty HDD tooling solutions that are built to stand up to ultra-hard surfaces. Without a bit like this, you’ll spend hours grinding away with little progress to show for it. The good news is: these bits will get the job done efficiently. The bad news is: they’re a bigger investment than a standard duckbill.
(Cost to buy: $4,000-$7,000 new; $1,500 - $4,000 used)
So far, we’ve only covered the equipment necessary for your pilot shot. Anything bigger than a 4”-6” hole will require a hole opener designed to back ream through solid rock. The cost of a hole opener relates to its size—a good ballpark is about $1,000 per inch. So, drilling an 8” final hole using a hole opener would set you back about $8,000.
(Cost to buy: ~$1,000/inch)
More bentonite & additives
Drill fluid volume aside, you’ll need more additives when you’re in rock. Drill fluid must have a higher viscosity to hold the weight of the cuttings and move them out of the hole. If not, rock chips build up at the bottom of the hole and can cause you to get stuck and lose your mud motor down hole. Upping your additives protects your equipment. See the chart below for a comparison of necessary drill fluid vs. a standard HDD job.
Slower production and other hidden costs
In addition to the investment in equipment and accessories, rock drilling is, by its nature, slower going than dirt drilling. It’s a double whammy. Plus, you’ll also incur added personnel costs since the jobs take longer. Here’s a quick run-down of the areas where you can expect greater investment than you might be used to:
- Additional 1-2 people to mix fluid and keep the reclaimer clean
- More time spent sucking and dumping cuttings into pits
- Renting or buying a bigger vacuum to manage the extra flow
- More operating hours on your equipment
- Additional equipment maintenance
- Increased labor costs (due to a longer job)
- Consultant fees to help inexperienced crew plan the bore path and use equipment properly
Comparison: Dirt vs. Rock Job
Curious about how rock drilling stacks up against a standard job? Here’s a side-by-side comparison of
of rate of production, tooling required, and fluid costs while drilling a pilot shot in dirt vs. rock. Here are the specs for this sample pilot bore:
- 100k drill rig (JT10, D100x140, etc.)
- 6 – 7” bit
- 600’ shot
- Standard walk-over locator
- Reclaimer cost not factored
We’ve made some assumptions (as noted) but this will give you a good comparison of what it costs to drill in dirt compared to rock.
Fluid required is in gallons
Bentonite required is in lbs.
Go into rock jobs with your eyes open. Though job budgets are bigger, so are associated costs. Slower production and greater investment in HDD tooling and accessories might make the job less profitable than you originally anticipated. Even on non-rock jobs, it’s always a good idea to insert a clause into your bid or contract that enables you to re-scope the project if you run into rock.
As with any HDD job, unknowns will arise. But if you’re geared up right and know what you’re getting into, you’ll be able to successfully complete that rock project on-time and within budget.
Want to know more about the right tooling for drilling rock—or any ground? Our HDD experts are ready to help.