Choosing the Right Drilling and Well Development Method

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The success of the well project can depend on selections made by the groundwater professional.

By Marvin F. Glotfelty, RG

There are a lot of drilling methods that can be used for installation of a water well, and each of those drilling techniques are appropriate for some, but not all, hydrogeologic conditions and well designs.

Similarly, there are numerous tools and techniques that can be applied for well development, and just as with drilling methods, some methods will be successful, and others will be ineffective, to develop the well under certain conditions.

Most groundwater professionals will select the drilling method and development method for a particular well based on their prior experience. However, as Will Rogers said, &#;Good judgement comes from experience, and a lot of that comes from bad judgement.&#;

We all learn from our past successes and failures, but as we install wells at various locations with differing hydrogeologic conditions and well performance expectations, our previous experience may no longer be applicable.

Consideration of the selection process is worthwhile because the success or failure of a well installation project will turn on the groundwater professional&#;s initial selection of the drilling method and well development technique that will be compatible with the specific location and purpose of that well.

Well Drilling Methods

Many drilling methods are available, and in some cases, any one of them is suitable for installation of a particular well. However, just about every well drilling technique has its limitations, and those confines should be considered at the outset of any well drilling project.

Mud Rotary

The direct mud rotary drilling method is one of the most common methods and is suitable for a broad range of well designs and hydrogeologic conditions. There is essentially no practical limit on the depth capabilities of direct mud rotary boreholes. Direct mud rotary drilling rigs can be equipped with a variety of drill bits (tricone button bits, tricone mill tooth bits, drag bits) as appropriate for hard or soft formations.

As shown in Figure 1, drilling fluid is circulated through the mud pits (or above-ground tanks) and the borehole to remove cuttings and stabilize the hole. The drilling fluid is pulled from the pits by a mud pump and delivered up the standpipe to the kelly hose, which conveys the drilling fluid down through the center of the drill pipe to the bit. The drilling fluid then carries the cuttings back to the land surface as it flows up the annulus outside the drill pipe.

Since this drilling method involves keeping the borehole full to the land surface with drilling fluid, the hole can be maintained in a stabilized condition even when unconsolidated or friable formations are encountered. The properties of the drilling fluid must be consistent with the formation being drilled to achieve borehole stability, but the chemical and physical properties of the drilling mud can be monitored and adjusted as needed.

Some hydrogeologists claim drilled cuttings from mud rotary boreholes are difficult to log due to the impact of the drilling mud on the cuttings. However, that argument is unsubstantiated because properly mixed drilling fluid has only about 1% to 3% solids, and the drilling fluid film can be rinsed off the cuttings. Thus, a hydrogeologist can evaluate both the washed and unwashed samples of drilled cuttings
from a mud rotary borehole to prepare a good and representative lithologic log.

The limits to the direct mud rotary drilling method arise when the drilling fluid is not properly maintained, or when the hydrogeologic conditions of the borehole involve severe lost circulation conditions.

Improperly maintained drilling fluid may lack the critical characteristics (adequate viscosity, low filtrate, low fluid weight and solids content) to stabilize the borehole and drilling process. Lost circulation conditions can occur when porous formations such as coarse-grained gravel or fractured rock aquifers are penetrated.

During lost circulation conditions, the drilling fluid will seep out of the borehole into the adjacent formation voids, which destabilizes the borehole and makes well development more challenging. Lost circulation material can be added to the drilling fluid to address this problem, but that approach will have uncertain results and generally lead to increased efforts required for well development. Therefore, in areas where severe lost circulation conditions are expected, different drilling methods should be considered.

Air Rotary

The direct air rotary drilling method involves a similar rig setup as mud rotary drilling, except that the circulated fluid is compressed air instead of drilling mud. The carrying capacity (viscosity) of the compressed air is improved by adding water mist, surfactant (soap), and polymer.

Direct air rotary drilling rigs can be equipped with any of the drill bit types that are used for mud rotary drilling and can also be equipped with downhole hammers and hammer bits. This versatility makes air rotary drilling appropriate for both hard and soft formations.

In general, penetration rates are higher for air rotary boreholes compared to mud rotary boreholes, and because there is minimal wall cake accumulation during air rotary drilling, well development requirements will be greatly streamlined.

As shown in Figure 2, the compressed air is circulated up the standpipe to the kelly hose, which conveys the compressed air down the center of the drill pipe to the bit. The compressed air carries the cuttings upward as it flows up the annulus to the blooey line at the land surface. Mud pits are shown in the figure, but those are not always used with this type of drilling.

Note that in the figure the fluid level in the borehole does not extend all the way to the land surface. While compressed air is being circulated, the entire borehole is under positive pressure and will be stabilized. However, when the drilling crew &#;makes a connection&#; to add another joint of drill pipe, the compressor must be shut off, so the fluid level will fall to the water table and the upper portion of the borehole will be exposed to atmospheric pressure.

Thus, in locations with unstable or unconsolidated formations, the viability of the air rotary drilling method may be limited because the upper borehole could slough in against the drill string while a connection is being made.

Another limitation of air rotary drilling is in areas where the aquifer produces groundwater at a high rate. Situations may occur where groundwater flows into the borehole faster than the compressed air can remove it. Such conditions will cause the drill bit to be in contact with a thin layer of water instead of in contact with the formation at the base of the borehole. Thus, the penetration rate will be impeded. This situation is sometimes referred to as &#;waterlogging&#; or &#;being flooded out.&#;

Some drilling contractors convert the same drilling rig between air rotary and mud rotary by simply switching out the circulation system, since the overall drilling rig configuration is unchanged. This versatility enables the driller to adjust the drilling method in response to whatever hydrogeologic conditions are encountered.

A challenge with both direct air rotary and direct mud rotary drilling methods arises when large-diameter boreholes are being drilled. This challenge results from the need to achieve adequate uphole velocity to bring the cuttings to the land surface. The circulation velocity can be much lower for mud rotary drilling compared to air rotary drilling, but both those direct drilling methods rely on creating enough fluid (mud or air) velocity in the annulus to transport the cuttings.

When a large borehole is drilled, the cross-sectional area of the annulus becomes quite large. For example, if a 10-inch-diameter borehole is drilled with a 3½-inch-diameter drill string, then the annular cross-sectional area between the drill pipe and the borehole wall is about 1.9 square feet. If that same drill string was used for a 20-inch-diameter borehole, the annular cross-sectional area increases to almost 8.5 square feet (an area almost 4.5 times larger).

This means that applying direct circulation to remove cuttings from a large borehole would necessitate a high fluid flow rate to achieve the required uphole velocities to clear the cuttings from the hole. In addition, for the case of mud rotary drilling, the larger volume of drilling fluid required to fill the large-diameter borehole would be expensive.

Flooded Reverse Circulation Rotary

For the reasons just mentioned, water supply wells with large-diameter boreholes are commonly drilled using the flooded reverse circulation rotary drilling method. This drilling technique is like mud rotary drilling in many respects, except that fluid is circulated through the borehole in the reverse direction, as it flows down the annulus and up the inside of the drill pipe (about 6-inch-diameter for this drilling method) to the land surface.

The reverse flow direction of the fluid is facilitated with compressed air that is either introduced via dual-walled drill pipe or with an interior airline, as shown in Figure 3. This drilling technique is probably the most common method for installation of large-diameter municipal, industrial, or agricultural wells in many areas. Since the fluid flow path in reverse circulation is coming up the inside of the drill pipe, the flow of drilling fluid through the relatively small cross-sectional area inside the drill pipe (about one-half square foot) will be adequate to clear the drilled cuttings from the borehole, even without high circulation flow rates.

At some locations, unstable formations introduce severe borehole stability challenges that are difficult to resolve while using any of the aforementioned drilling methods. Some of the most common and problematic unstable formation conditions are gravels and cobbles, and flowing sands. Schematic images of these two hydrogeologic conditions are shown in Figure 4.

Cobble-dominated strata, shown in the left image of the figure, commonly occur in stream sediment environments, and those loose and uncemented strata are susceptible to lost circulation conditions and sloughing of sediment into the borehole.

Flowing sands occur when a layer of well-sorted, poorly cemented sand is encountered, which is exposed to a hydraulic pressure gradient (either because it is part of a confined aquifer or has been penetrated by an underpressurized borehole). Flowing sand can rapidly move into the borehole, and fill both the annulus and interior of the drill string quickly.

At those locations (as well as at less challenging sites), a casing advance drilling method may be the best choice. The two most common casing advance drilling methods for water wells are the cable tool drilling method (Figure 5) and the dual rotary drilling method (Figure 6).

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Cable Tool

The cable tool drilling method is an old and time-tested technique for well installation either in an open hole or with casing being simultaneously driven into the borehole as it is drilled. Cable tool drilling involves advancing the borehole with a drill bit that is suspended from a cable that is reciprocated up and down due to the action of a walking beam, which is stationary at one end (see red arrows in Figure 5).

After the drill bit has pulverized the formation into cuttings, the bit is pulled from the borehole and the cuttings are removed by bailing. In unstable boreholes, the well casing is driven as the borehole is advanced. The base of the casing is outfitted with a drive shoe to prevent it from becoming battered or deformed.

Dual Rotary

Another casing advance drilling system is the dual rotary drilling method. With this drilling technique, drill pipe is rotated with a tophead hydraulic drive (shown as A in Figure 6) and the casing can also be independently rotated by a second hydraulic drive (shown as B in Figure 6).

The circulating fluid brings the drilled cuttings up to the land surface where they are discharged into a mud tank or cyclone solids separator. The base of the casing is outfitted with a drilling shoe with tungsten carbide buttons, so it can be advanced through the formation. The interior drill string can be advanced down below the bottom of the casing, or maintained above it, as needed to address formation conditions (Figure 4).

Casing advance drilling methods allow for boreholes to be advanced to significant depths without significantly disrupting the adjacent formation and aquifer. The Achilles&#; heel of casing advance drilling systems is that the casing perforations cannot be easily emplaced.

The casing of a well drilled by the cable tool or dual rotary method can be perforated in place with a Mills knife or other downhole perforation tool. However, such perforation cuts are generally too large to prevent sand invasion, and since there would be no filter pack envelope, the well would likely produce sand while being pumped.

This problem can be mitigated by use of a pullback completion, wherein the blank casing is pulled upward after a smaller-diameter well screen has been lowered to the selected screen depth. Simultaneous with pulling the outer casing, the filter pack sand is installed between the borehole and the inner well screen. This is achievable but is a slow and tedious process that includes significant risk that the inner and outer casing strings could become sand locked.

Well Development Methods

There are numerous physical and chemical techniques available for well development, but none of them are suitable for each and every well. Consideration of several factors is warranted for selection of an appropriate well development technique. These factors include the site-specific hydrogeology, events and problems that may have occurred during drilling and well installation, and the specific well design details.

The hydrogeologic factors that influence well development techniques include the nature of the aquifer, such as whether it is composed of basin-fill alluvium, glacial till, fractured rock, etc. Drilling or well construction events that impact well development may include instances of lost circulation, fishing operations, or equipment breakdown. These events prolong the well construction and may result in intervals in the well that are more extensively disturbed and require more attention during development. The primary elements of the well design that must be considered during development operations are the well screen type, the filter pack materials, and the annular thickness.

For any well development program, the intent is to convey energy that will be passed through the well screen and filter pack envelope to break down and remove the wall cake on the face of the borehole. Removal of the wall cake is best achieved with high-velocity energy that is directed in both an outward and inward direction, relative to the well screen.

The bidirectional (inward and outward) energy will effectively break down the wall cake and prevent sand bridging. Effective well development is best achieved by development tools that can apply depth-specific energy. Although pump-and- surge operations are helpful for an overall well development program, that development method applies unfocused energy throughout the well screen, so the energy will be dissipated when the pumping/surging energy encounters the first productive zone of the well. Therefore, focused well development methods such as jetting or swabbing are generally much more effective for the bulk of the well development.

Horizontal Jetting

High-velocity horizontal jetting tools (Figure 7) achieve well development when water (with or without chemical additives) is pumped through the drill pipe or tubing string down to the jetting tool. The jetting development tool is outfitted with a series of tungsten carbide nozzles that direct high-velocity streams horizontally through the well screen slots. The high-velocity fluid streams are intended to roll and reposition the filter pack and deliver hydraulic energy to the borehole face at the far side of the filter pack envelope.

The horizontal jetting method delivers a strong outward force, and the jet nozzles are slowly rotated to provide good coverage of the screen&#;s interior. It is a good idea to also facilitate a strong inward hydraulic force that will complement the outward force delivered by the horizontal jets. That inward force could be accommodated via pumping, airlifting, or swabbing.

Swab and Airlift

Double swab-and-airlift development tools (Figure 8) achieve well development by being reciprocated upward and downward within the screened interval, which forces water inward and outward on each side of the development tool. This bidirectional hydraulic motion above and below the swab tool provides good inward and outward energy, which is delivered to the borehole face at the far side of the filter pack envelope. Any remnants of drilling fluid or fine sediments that are pulled into the well are removed by airlifting.

Clearly, the orientation of well screen slots (wire-wrap, louver, bridge slot, etc.) provide an important consideration in the type of development tool that is selected. Considerations for selection of the specific development tool should also include the other factors listed earlier (local hydrogeology, problems encountered during well installation, and well design). Additionally, the chemical additives and duration of well development will vary from well to well and should be adjusted in accordance with the site-specific conditions being addressed.

More on Drilling Methods from Glotfelty

Click here to watch Glotfelty covering differing drilling
methods and specifying the drilling method to meet the well&#;s design in past videos.

methods and specifying the drilling method to meet the well&#;s design in past videos.

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, ) and was the Groundwater Foundation&#;s McEllhiney Lecturer. Glotfelty can be reached at .

Are you thinking about installing a well on your property? More than 10 million American homes rely on well water, a healthier and cheaper alternative to the city water supply.

If you&#;re thinking of drilling a well, you should know that there are three commonly used methods of digging shallow wells and deep wells. The type of well that you&#;ll need depends upon your property, and you&#;ll want to consult a professional installation company.

We&#;ll walk you through the most popular types of well drilling methods, and give you some information about local well drilling professionals. Let&#;s get into it!

1. Rotary Drilling

The depth of your residential well will be anywhere from about 15 feet to more than 1,000. Rotary drilling uses a bit&#;a piece of metal with a diamond tip&#;that is attached to the end of a drilling pipe.

A hydraulic system feeds the pipe down into the ground until the drill bit reaches the water that is under your property. As the drill bit turns, dirt and rocks will make their way up to the surface. Professional well drillers often use drilling fluid to reinforce the stability of the drill hole.

2. Auger Water Well Drilling

If your property is sandy, you might end up having to use the auger method. Basically, the auger drill is a large tool that burrows directly into the ground. It carries the sand to the surface, which makes it easier to drill the second part of the hole.

Often, well drilling professionals will install a drill casing in order to keep the hole from collapsing in on itself. If your water table is 50 feet or less under your property, the auger drilling method will be the most common.

3. Drive Point Drilling

Drive point drilling has a lot of similarities to oil drilling. You have to make sure that the pipes are large enough to penetrate the ground and let water in, but small enough to keep the hole intact.

You can hand-drive a well, but you&#;re better off contacting a well drilling professional. Once your pipes are driven down to the level of the water table, water will flow through the pipes. The drive point method is often used for water tables that are 25 feet or less below the surface.

Finding a Well Drilling Professional

Installing a water pump is one of the most important things you can do for your property value. When it&#;s time to find a well drilling professional, you should look for companies with many years of experience and outstanding customer reviews.

Water tables can get contaminated by so many things: flowing through landfills, leaky septic tanks, and pesticides. When you do drill a well, make sure that you check your well regularly.

You can talk to your well drilling company about the best place to install and locate the well. Do you want it in your backyard, out of the way, or would it be more effective in the front or side of your home?

When you have your well dug and ready to go, give us a call or drop us a line and we&#;ll schedule a consultation for your residential or commercial property well water maintenance program.

Contact us to discuss your requirements of Water Well Drill Pipe. Our experienced sales team can help you identify the options that best suit your needs.