Over the past few years, ground source heating/cooling using vertical loop installations has taken over our industry, and has provided a great new opportunity for many of us in the water well and drilling industry. With new opportunities also comes a new learning curve to accomplish the job effectively and efficiently. One of the major problems I encounter on a regular basis is installing loops after the borehole is drilled. These boreholes range in depth from as shallow as 100 feet to 600 feet in depth, with the average being about 300 feet.
Borehole sizes normally range from between 4 inches to 61⁄2 inches, depending
on the size of loop being installed, and in some cases, the size of the drill
pipe. Smaller seems to be better, as it reduces drilling and grouting costs.
But with a smaller-diameter borehole also comes potential problems with getting
the loop to the bottom of the hole. A smaller borehole does not allow much
extra space for swelling clays or from unconsolidated materials such as rocks
or cobbles from sloughing into the borehole. This will hinder getting the loop
placed to depth, and possibly cause the need for the borehole to be re-drilled,
which can be costly. But the number one problem I see with new contractors
getting into loop installations is their not knowing how to overcome pipe
Drilling normally is done with either air or conventional mud drilling, depending
on drilling conditions and available equipment. Some sonic drill rigs are being
utilized. With loop boreholes drilled using air either with a tri-cone bit or
air hammer, in most cases, the boreholes are dry or have little or no water,
which allows for easy installation of the loop and tremie pipe. Both need to be
placed at or near the bottom of borehole to properly seal the annular space.
However, mud drilling poses some potential problems, which need to be
understood before placing a loop in the borehole. The number one concern should
be buoyancy. Poly-pipe loops, even pre-filled with water, have buoyancy, and
weighted drilling fluids drastically increase the buoyancy of the loop.
Figure 1 (on p. 36) shows what weight is needed to be added to the loop filled
with water to offset the buoyancy from heavy drilling fluid. These increased
weights normally are found because of the lack of solids control found with
contractors doing loop installations. You might not have the space for
utilizing a mud-cleaning system or the resources to purchase one, but there
should be no reason that you could not be using a de-sanding cone while
drilling. The de-sanding cone alone can reduce your added mud weights from sand
and silt by as much as 90 percent to 95 percent. When looking at the chart, if
you can reduce your mud weights from 10 (lb./gal.) to 9 (lb./gal.) on a 1-inch
loop, you can cut your weight required by 21.1 pounds per 100 feet of loop.
When looking at a 300-foot loop, we drop the added weight required to get the
loop to the bottom from 112.8 pounds to 49.5 pounds. Some contractors are using
sinker bars attached to the bottom of the loop to assist getting the loop and
tremie pipe in the hole, but a lot of rigs I have seen do not even have a sand
line or other method to use a sinker bar. This leaves them using weighted loops
or brute force to get the loop installed.
Not understanding buoyancy has had a number of contractors pulling the loops
back out of the borehole and re-drilling, thinking they had a problem downhole
only to find a good hole and the same problem getting the loop down.
Smaller-diameter holes also can add to problems getting the loops installed as
mentioned with swelling clay and materials sloughing in, but friction from the
pipe and sides of borehole need to be considered.
So, if you are drilling with mud, do what you can to reduce the mud weight, or
be ready to weight up the loops before installing.
Drilling Fluids: Buoyancy Related to Vertical Loop Installations
January 3, 2011