Over the past few years there have been numerous articles on bore planning. Most of these articles focused on a major problem in the horizontal directional drilling (HDD) industry: OUT RUNNING OUR DRILLING FLUIDS. Let's look again at the formulas that help avoid this problem and address other problems not associated with the fluids, which are causing most of the contractors' unwanted problems.
To understand drilling fluid requirements, we need to establish parameters of the bore: length of bore, reamer type and size, and soil conditions. Always keep in mind there is no universal soil type; therefore, there is no one universal drilling fluid, reamer, or set time to do a successful bore. Look at every new bore as if it was your first, regardless of your experience or how successful you have been in the area in the past.
To calculate your fluid requirements, you need to determine amount of soil to be removed to have an adequate passageway for the product pipe being pulled in. To do this, we use the following formula.
- Diameter of reamer squared divided by 24.5 = volume (gallons/linear foot)
Example: 12" reamer
12 x 12 = 144 / 24.5 = 5.88 gal. drilling fluid / linear ft. soil should be removed.
To remove the soil and have flowable slurry in sands or dirt-like soil, you need to apply a minimum of two times the fluid-to-soil ratio to every foot of soil to be removed. This allows for fluid loss and absorption of fluids into soil being removed. In clays and harder, more reactive soils, you should apply three to five times the fluid-to-soil ratio to keep the drilled slurry flowing. If the soil is not removed, the product pipe has little or no chance of being pulled through.
Using this practice on a 500 ft. backream in a sandy or dirt-like soil, we can plan to use about 5880 gallons of drilling fluid (2 x 5.88 gal./ft. = 11.76 x 500 ft. = 5880 gal.). If we were in a clay or more reactive soil requiring use of polymers and additives, we would need to use three to five times more fluid to keep the soils mixed in the slurry. That would require us to use 8800 to 14,700 gallons of drilling fluid. These reactive soils can greatly increase the time and expense required to complete the bore.
Time required to do the bore is based on hardness of soil conditions, size of rig used, and how long it takes for the reamer and fluid to mix the soil into the slurry. In most cases, the power of today's machines makes it very easy to pull back too fast, not allowing enough fluid to be pumped down hole. This ultimately causes stuck drill pipe, stuck product pipe, and the ever-dreaded frac-out or speed bump. Pull-back time should be based on output of your pump: the faster you can pump drilling fluid in the hole, the faster your pull-back time.
For example, using a 12" reamer on the 500 ft. bore, we calculated we will need 5880 gallons of fluid (for sands or dirt). If we divide that volume by output of our pump, which in this case is 40 GPM, it will take three to six hours to do the pull-back. This does not take into consideration down time, which leads me into what I perceive as one of our biggest problems today: NOT ALLOWING ENOUGH TIME TO DO THE JOB RIGHT.
Time. We never allow enough to do the job right the first time, but always seem to spend a lot of it redoing the project, which costs us money. If time is money, why are we not using it effectively?
Most projects have time restrictions, which usually allow us to work between 7a.m. and 5p.m., or about 10 hours a day. Of these 10 hours, only about seven are productive hours, as in most jobs we need the first hour to get ready and the last hour to clean up/reset barricades, and we shut down for an hour lunch. How you use these seven hours of production is critical to a successful bore. Using the example bore mentioned-500 ft. 12" without a pre-ream-we estimated three to six hours to do the boring. That is machine time only, not including breaking rods, changing racks, mixing fluids, getting water, vac and hauling time, visiting with inspectors, and talking with your girlfriends on the cell phone. The biggest problem with time management on the job is vac and hauling time. Very few contractors allow for time it takes to dispose of spoils or provide enough vacs or large enough vacs to do the job efficiently. This problem, along with inadequate mixing systems and water sources, is adding six to 10 hours to the job and, in more cases than we want to admit, causing us to lose the bore. Because of added time for vac and hauling, we are not allowing needed drilling time to remove and mix up the soils.
Looking at vac time alone, say we have a 750 gal. vac unit. Using the example bore mentioned above, we need to remove between 8800 and 14,700 gallons of material. (Drilling fluid required + soil removed from bore path). This will require filling and dumping our vac 12 to 22 times. As I have seen on most jobs, it can take 30 to 60 minutes to dump a load. This can add six to 22 hours of unproductive time to the job. We would need to seriously look into having a second or larger vac available. A larger vac alone will not save money (due to down-time) but will decrease number of trips to the dumpsite. Mud mixing and having a source of water available also calculates into lost production when not properly planned.
Also, make sure you have the product pipe on site and ready to pull when needed. Check your inserts and pulling cables for wear. Don't stop pulling back product pipe once you start, and avoid unnecessary cell phone calls when work needs to be done. I have seen crews lose one to two hours of production time a day because the operator or key person was on the phone, unnecessarily in a lot of cases. Remember-time is money; use it as effectively and efficiently as possible.