Thinking of taking on a job that requires drilling rock? Or, do you need a solution for a job that's unexpectedly hit rock? If so, you'll specifically need hard rock directional drilling solutions — but finding the right HDD rock bit for your job is about more than just buying the right bit. Rock drilling requires big-picture thinking about the full scope of needs that will enable you to tear through these challenging ground conditions; that includes having a realistic idea of what you'll need to spend on equipment. Here's what you need to know to assess true rock drilling cost.
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 only result in frustration — and possibly broken HDD equipment. The only way to get through rock successfully is to invest in buying or renting hard rock directional drilling equipment.
Mud Motor (+ Drill Fluid)
A mud motor tackles rock drilling by chipping away at the rocks' surface while a two-degree bend in its shaft enables steering. A mud motor is specialty equipment — and its price reflects that. 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 (Rent): $1,000-$3,000 per week
- Mud Motor (Buy): $14,000-$65,000
- Drill Fluid: (See end of article)
Rock Drilling Accessories Matter
When using a mud motor, you'll also need a few critical pieces of additional hard rock directional drilling 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 transmitter housing. A high-flow housing provides better protection for your transmitter and other electronics in your drill string but, more importantly, it allows a larger volume of liquid to flow through without restriction.
- Cost to Buy: $3,000-$,5000
Drill fluid reclaimer
Since your rock drilling 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 reuse.
- Cost to Buy: $40,000-$300,000+
Tri-cone or PDC bit
Tricone 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 drilling rock (or ineffectively grinding away at it) with little progress to show for it. The good news is that Tricone and PDC bits will get the job done efficiently. The bad news is that they're a bigger investment than a standard duckbill.
- Cost to Buy (Used): $1,500-$4,000
- Cost to Buy (New): $4,000-$7,000
So far, we've only covered the rock drilling 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 per inch
More bentonite & additives
Drill fluid volume aside, you'll need more additives when you're drilling 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 hard rock directional drilling equipment and accessories, rock drilling is, by nature, slower going than dirt drilling: it's a double whammy. Plus, you'll 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 Drilling Jobs
Curious about how rock drilling stacks up against a standard job? Here's a side-by-side comparison of rate of production, tooling required and fluid costs while drilling a pilot shot in dirt vs. rock. Check out 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 rock drilling cost compared to the cost of drilling in dirt.
*Fluid required is in gallons
*Bentonite required is in pounds
Go into rock drilling jobs with your eyes open. Though job budgets are bigger, so is overall associated rock drilling cost. 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 rescope the project if you run into rock.
Head Into Your Next Rock Drilling Job Prepared with Melfred Borzall
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 drilling 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.