It should always incorporate the proper drilling method.
By Marvin F. Glotfelty, RG
Well designers need to consider numerous variables and select the most appropriate attributes for incorporation into their technical specifications for water well construction.
The design elements include types of well construction materials (including applicable ASTM standards), and also the construction methods to be implemented during well installation. The list of potential well design attributes is extensive, and the selection of just about each and every design feature is critical since each design element will ultimately impact the well’s performance, stability, and longevity.
Critical well design features include:
- The well casing material type, diameter, and wall thickness
- The well screen type, slot size, open area, and depth setting
- The filter pack source, grain size, and gradation
- The cement or bentonite annular seal materials and depth intervals
- The well construction and development methods, and protocol requirements
- Logistical considerations for the well site, such as sound abatement and traffic control
- Standard requirements for contractor licensure, insurance or bonding, reporting, etc.
With the broad array of well design alternatives to be selected by the well designer, there are cases where the well designer may address many elements of the well design but overlook an even more important fundamental consideration: the drilling method to be used.
Well designers are sometimes distracted from calling for the appropriate drilling method, either because they’ve become obsessed with multiple other well design variables, or because they simply lack an understanding of the advantages and disadvantages of various drilling techniques.
I was recently reminded of the importance of specifying the correct well drilling method when a driller I know from an adjacent state told me the story of a well he’d recently drilled in which the mud rotary drilling method was specified even though the well was located in an area where extensive lost circulation conditions would be expected.
The mud rotary drilling method is an excellent drilling technique for most situations, but it can be problematic when the formation consists of extremely porous sand and gravel, or fractured rock that will allow the drilling fluid to be lost out of the borehole by rapid seepage (called lost circulation).
The driller requested permission to use an alternative drilling method that would be more consistent with the local hydrogeology, but that request was rejected by the well designer. The driller was able to get the well installed, but only by adding significant amounts of lost circulation material (LCM) additives to his drilling fluid.
LCM is designed to clog pore spaces in the formation so that the drilling fluid can be circulated back up to the land surface for the removal of cuttings, as is required to advance the hole. The LCM enabled the driller to stabilize the borehole and drill it to the total depth. However, the LCM could not be completely removed, so the water production from the completed well was disappointing.
Rather than taking responsibility for his own specification that required use of the mud rotary drilling method, the well designer blamed the driller for the result, and told the landowner that the low water production from the well was due to faulty work on the part of the driller.
Stories such as this are unfortunate and can be avoided if the well designer is familiar with the compatibility between various drilling methods and their respective hydrogeologic environments.
Each and every drilling method has advantages and disadvantages, and there is no “one-size-fits-all” approach that will accommodate a single drilling technique for all wells. Just as with tools in a toolbox, we shouldn’t use a screwdriver to hammer nails, and we shouldn’t try to tighten nuts with a claw hammer.
Following are some brief descriptions of the advantages and limitations for several common drilling methods.
Direct Mud Rotary Drilling Method
The direct mud rotary drilling method involves stabilizing the borehole and circulating cuttings out of the hole by pumping drilling fluid down through the drill pipe and back up the annulus, as shown in Figure 1.
Direct mud rotary drilling is one of the most commonly used drilling methods because it enables the driller to address downhole problems such as swelling clays, unstable formations, or fluid loss by adjusting the chemical and physical properties of the drilling fluid.
If the drilling fluid is maintained properly, a thin and (relatively) hard wall cake will be applied to the face of the borehole, which will stabilize the hole during drilling. After the well installation is complete, that same wall cake can be broken down and removed during well development. Boreholes have been drilled to thousands of feet in depth all around the world by direct mud rotary drilling, but this method also has some limitations.
One disadvantage is the inability to identify the water table or quantities of groundwater production during drilling (since the borehole is maintained full to the land surface with drilling mud). Also, drilling fluid costs can be excessively high for direct mud rotary-drilled wells if the borehole volume is extremely large.
Larger-diameter boreholes have the challenge that a significant volume of drilling fluid is required to maintain the borehole full to the land surface. Moreover, some of the drilling fluid additives that can be introduced to solve borehole problems, such as LCM, may constitute well performance problems later on. As illustrated in Figure 1, if an unstable or porous formation is encountered in a direct mud rotary borehole, the drilling fluid treatment that will be required to maintain circulation during the drilling process may impede groundwater production later on.
Direct Air Rotary Drilling Method
The direct air rotary drilling method involves stabilizing the borehole and circulating cuttings out of the ground by use of compressed air that is circulated down through the drill pipe and back up the annulus, as shown in Figure 2.
The carrying capacity of the compressed air to remove drilled cuttings is typically improved by adding water (mist drilling), detergent (foam drilling), or both detergent and polymer (stiff foam drilling).
In the case of foam or stiff foam drilling, direct air rotary drilling can be used to advance the borehole through porous formations that would be problematic for mud rotary drilling methods because the foam acts as a low-weight and high-viscosity fluid that is capable of removing drilled cuttings without applying hydraulic pressure to the borehole that can cause lost circulation conditions in porous formation intervals (Figure 2, left image).
The direct air rotary drilling method also accommodates rapid penetration rates and provides minimal wall cake buildup, so the well development will generally be quick and effective. Another advantage of the direct air rotary drilling method is the ability to identify the water table and estimate groundwater production rates during drilling.
Whether mist, foam, or stiff foam drilling is used, the direct air rotary drilling method will eventually require addition of more drill pipe as the borehole is advanced, and unlike the mud rotary system, the borehole is not filled to the land surface with fluid. Rather, the compressed air pressure is released from the drill string and borehole each time the drill crew makes a connection.
When the air compressor is turned off to enable the drill crew to make a connection, and sometimes even while the compressed air is still being circulated, there can be a collapse of loose or unstable formation material into the borehole (Figure 2, right image), which can result in stuck drill pipe.
Flooded Reverse Rotary Drilling Method
The flooded reverse rotary drilling method is a good choice for deep, large-diameter well installation projects such as municipal, industrial, or agricultural wells. This method involves stabilizing the borehole by maintaining it full of fluid (which in some cases may be essentially clear water). Upward circulation of the fluid through the drill pipe is achieved by airlifting, and the fluid then flows back down the annulus via gravity (Figure 3).
The drill bit orifice size and drill pipe diameter must be large enough to accommodate the flow of fluid as it carries the drilled cuttings up through the interior of the drill string. That is why larger drill string dimensions are commonly used for flooded reverse drilling projects (17½-inch drill bits and 5½-inch drill pipes are typical).
The principal advantage of this drilling method is that the cuttings can be circulated up out of the borehole directly through the drill pipe even though the annulus is very large. The large annulus would otherwise require very high flow velocities or very high viscosity of the fluid to directly circulate the cuttings to the land surface.
The hole is stabilized by being flooded to the land surface, but a disadvantage of this drilling method is that large quantities of construction water may be needed to maintain the hydraulic head in the borehole. To meet the requirement of adding water to the borehole faster than it can seep out into the adjacent formation, some drilling sites may require a construction water source of several hundred gallons per minute.
In some situations, the driller may need to “mud up” the fluid to address formation problems such as swelling clays or lost circulation conditions. As with direct mud rotary drilling, this drilling method also has the disadvantage that the driller is unable to identify the water table or quantities of groundwater production during drilling.
Casing Advance Drilling Methods
Problems with lost circulation conditions and unstable boreholes can almost always be addressed with drilling techniques that involve simultaneously advancing the well casing as the borehole is drilled. Two of the most common casing advance drilling techniques are the dual rotary drilling method (Figure 4, left image) and the cable tool drilling method (Figure 4, right image). These drilling techniques are actually quite different from one another but have similar considerations with regard to their casing advance attributes.
Dual rotary drilling rigs have two hydraulic rotary drive heads, so that the upper drive head can be connected to the interior drill string (similar to a top-head drive on a conventional rotary drilling rig), and the lower rotary head can be connected to the casing string.
The upper and lower rotary heads operate independently, so the interior drill string can be extended down below the bottom of the casing (as shown in Figure 4), or it can be kept up above the base of the casing. The dual rotary casing string is capped with a casing shoe that is equipped with tungsten carbide inserts (Figure 4, left image). The casing shoe is welded to the base of the casing, which allows the casing string to be advanced downward as it is rotated.
Drilled cuttings can be brought to the land surface via the interior drill string, using either direct mud rotary, direct air rotary, or flooded reverse rotary circulation.
The cable tool drilling method involves advancing the borehole by breaking up the formation with a heavy drill bit that is suspended from a cable that is intermittently raised and dropped on the base of the borehole.
The reciprocal motion of the drill bit results from the movement of the cable as it passes through a sheave at one end of the walking beam, which pivots at one end, and is moved up and downward at the other end by its connection to a pitman arm attached to the rig’s crankshaft. After the cable tool drill bit has broken up the formation into cuttings, they are removed by bailing.
As the borehole is advanced, it can be stabilized by driving the casing downward. In cable tool wells, the casing is capped with a drive shoe (Figure 4, right image) that has been welded to the base of the casing.
There are a variety of advantages and disadvantages with these two casing advance drilling methods. Although the dual rotary and cable tool drilling methods are actually extremely different, one consistent challenge with both of them is the need to complete the well installation without excessive sand production while the well is being pumped.
At some locations, dual rotary and cable tool wells can be completed by simply perforating the well casing with a Mills knife or other cutting tool. However, the relatively large perforation cuts will not impede native sand from flowing into the well as groundwater is being pumped, so filter pack sand will need to be installed to prevent sand invasion from occurring during operation of the well.
Sand invasion can be extremely damaging to pump equipment and water distribution infrastructure, so in cases where sand invasion is a concern, the driller must use a “pull-back completion” to install the well. A pull-back completion involves first installing a smaller-diameter well screen down to the appropriate depth interval of the well, and then jacking or rotating the advanced casing back upward as the filter pack sand is simultaneously installed.
This is an intricate and tricky operation, which should be attempted only by highly skilled and experienced drillers because the filter pack sand can easily become locked between the interior well screen and the exterior casing. If sand locking occurs, the outer casing and the inner screen will effectively become connected, so as the outer casing is pulled, the inner screen will come with it and will be removed from its appropriate depth in the well.
The pull-back approach to well completion can be simplified in some cases by use of pre-pack type well screens that have been manufactured with two bonded well screens that encase a thin layer of filter pack material. Pre-packed well screen is more expensive, but it provides assurance that the intended well construction will be successful.
The general well drilling methods summarized in this column represent only a portion of the variations and methodologies that are available for well installation. It is important that each well designer make themself aware of the various well construction methods and the positive and negative attributes of each drilling technique.
Well designers routinely adjust the well’s depth, diameter, casing/screen material, annular seal material, filter pack sand, and even its location to suit the situation-specific purpose and hydrogeologic characteristics of the well. With all those adjustments, let’s not forget to also include the fundamental consideration of calling for the proper well drilling method.
Since we are interacting with Mother Nature, we can really use all the help we can get, so let’s design every well with the best possible chance for success.
Marvin F. Glotfelty, RG, is the principal hydrogeologist for Clear Creek Associates, a Geo-Logic Associates Co. He is a licensed well driller and registered professional geologist in Arizona, where he has practiced water resources consulting for more than 35 years. He is author of The Art of Water Wells (NGWA Press, 2019) and was The Groundwater Foundation’s 2012 McEllhiney Lecturer. Glotfelty can be reached at firstname.lastname@example.org.