Part One: Breaking down borehole damage.
By Thom Hanna, PG
As discussed in the previous column (“Goals of Well Development,” March 2023 WWJ, it does not matter what you use for well construction materials because if you don’t properly develop the well, it will be inefficient and have life-long maintenance issues and be expensive to own.
The process of well development starts during the design phase. We will cover the process of well development in two parts. In this first part, we will discuss methods that are generally used to put energy into the aquifer to break down borehole damage. Part two focuses more on methods that involve pumping in combination with methods to remove fine sediments from the well.
Different well development processes have evolved in various geographic regions to address the physical characteristics of aquifers present and the types of drilling rigs used.
Unfortunately, some development techniques are still used in situations where more recently developed procedures would produce better results. Effective development procedures should involve the initial breakdown of drilling fluids followed by the movement of fluids into and out of the screen and aquifer for the purpose of removing drilling fluids and fine sediment.
The first step in the development process, especially in wells that are drilled using drilling fluids, should be the breakdown of the drilling fluids and the suspension of fines to be removed that are near the borehole wall. This can be done while the screen is placed in the well as part of the filter-packing procedure if a filter pack is used.
Agitation of the filter pack and aquifer results in the removal of the finer fraction of sediment and rearrangement of the remaining aquifer particles (Figure 1). Bridges result when the fluid flows only in one direction, and reversing the direction of fluid flow results in the prevention of bridges in the filter pack that, if not removed, will result in sand pumping.
The action of putting energy into the aquifer breaks down bridging and rearranges the formation of filter pack, and the inflow portion then moves the fine material toward the screen and into the well for removal.
Multiple development techniques greatly enhance development. One of the development methods should move energy alternatively into and out of the aquifer, and the second should be a method (such as airlift pumping, bailing, or pumping) that removes fine-grained sediment from the well.
A good development program consists of the following steps:
- Break down and remove drilling fluids that are in the aquifer, filter pack, and well.
- Mechanically surge or jet the aquifer and filter pack to rearrange the filter pack and aquifer grains so that fines are brought into the well.
- Pump the well to remove fine sediment and establish high rates of water flow into the well.
Removing Drilling Fluids
The initial step in the well development process is the breaking down and removal of drilling fluids and natural fines from the aquifer and filter pack. If development is initiated by removal of the drilling fluids quickly, and without thinning the drilling fluids, there is the potential for collapsing the casing and screen.
If the fluids in the annulus are not properly broken down, they cannot enter the well as fast as fluids are pumped from the well. This can result in a fluid pressure in the annulus aquifer that is significantly higher than that inside the well. If the differential pressure exceeds the collapse resistance of the casing or well screen, the well can collapse.
Removing bentonite-based drilling fluids first requires breaking down the polymers that manufacturers add to bentonitic muds, which is usually accomplished by using chlorine. When using chlorine with clay dispersants, the manufacturer should be consulted as to what levels of chlorine are safe to use with its products.
In the past, polyphosphates (e.g., sodium acid pyrophosphate [SAPP] and sodium tripolyphosphate [STP]) were used to help remove bentonite and clays because the phosphates act as clay dispersants. However, phosphates are a potential nutrient source for bacteria. They are difficult to remove from wells because certain types of polyphosphates can precipitate, thus serving as an ongoing source of nutrients.
A polyphosphate precipitate also forms a glassy coating that can be difficult to remove. To eliminate the problems associated with polyphosphates, there are several non-phosphate dispersants used for removing silts and clays without introducing a food source for bacteria.
Adding a small amount of dispersant either before or during development also helps considerably in removing clays that occur naturally in the aquifer or which are part of the drilling fluid.
After the drilling fluids have been removed, the well can be mechanically surged and additional dispersants can be used to remove the filter cake and natural clays and fines that are contained in the aquifer adjacent to the screen, once the drilling fluids are completely removed from the filter pack.
Mechanical Surging
Mechanical surging forces water to flow into and out of a screen by moving a plunger up and down in the casing and screen (Figure 2). Surge blocks are constructed so that the outside diameter of the rubber lips is equal to the inside diameter of the screen.
A heavy bailer can be used to produce the surging action but is not as effective as the close-fitting surge block. As is the case in all mechanical development processes, fine-grained material should be removed from the well as frequently as possible. The best results are obtained when this method is used in conjunction with pumping to remove fines from the well.
The initial surging motion should be relatively gentle, so that any material blocking the screen can break up, be suspended, and then be moved into the well. The surge block (or bailer) should be operated with care—particularly if the aquifer above the screen consists mainly of fine sand, silt, or soft clay that could slump into the screen if a filter pack is not used.
Surging should also be started slowly and relatively gently to avoid differential pressures that can cause collapse of the casing or well screen.
As water begins to move easily both into and out of the screen, the surging tool usually is lowered and positioned just above the screen. As the block is lowered, the force of the surging movement increases.
In a well that is equipped with a long screen, it might prove more effective to operate the surge block in the screen to concentrate its action at various levels. Development should begin above the screen and move progressively downward to prevent the tool from becoming sand-locked.
The force exerted on the aquifer depends on the length of the stroke and the vertical velocity of the surge block. The speed of retraction and length of pull are governed by the physical characteristics of the rig.
Surging should continue for several minutes, and the block should then be pulled from the well. Air can be used to lift the sediment out of the well if development is done with a rotary rig or if an air compressor is available. Sediment can be removed by a bailer or sand pump when a cable tool rig is used.
Care must be taken when swabbing wells that have plastic casing or screens. In particular, screens that have screen-slot sizes about 0.010 inch or smaller can become blocked. These wells and associated screens can be particularly troublesome because differential fluid pressures created by the swab by removing fluids from inside the well can collapse PVC casings.
Jetting with Water
The process of jetting with water consists of operating a horizontal jet inside the well screen so that jetting energy is directed out through the screen openings. The equipment required includes a jetting tool with two or more equally spaced nozzles; a high-pressure pump or compressor; hoses and connections; pipe; and a clean, potable water supply for jetting.
The jets force water through the screen openings, agitating and rearranging the particles of the aquifer surrounding the screen. Filter cake deposited during drilling is effectively broken down by fluid forces. Jetting is particularly successful in developing highly stratified, unconsolidated aquifers.
Figure 3 shows a jetting tool with five nozzle jets (four are diametrically opposed and one is on the bottom of the tool). The nozzles should be spaced equally around the circumference of the jetting tool to hydraulically balance the tool during operation. Horizontal holes drilled in a plugged pipe or coupling are reasonably effective, but the best results are obtained if the nozzles are designed for maximum hydraulic impact. The jetting tool should be constructed so that the nozzle outlets or holes are as close to the inside diameter of the screen as is practical (generally less than 1 inch).
Water used for jetting should never contain sediment. High concentrations of circulated sediment can damage screens and can cause erosion of screens if the jets are directed at one area for long periods. Thus sediment-ladened water should periodically be removed from the well. When jetting, the tool continuously should rotate and move vertically in the well.
The lowest nozzle velocity for effective jetting is considered to be approximately 100 feet/second. Better results are achieved with nozzle velocities of 150 feet/second to 300 feet/ second. Much higher velocities have been used successfully but care must be exercised.
Jetting pressures in screens constructed of PVC or other less abrasion-resistant materials (e.g., fiberglass) should typically not exceed 100 psi, although with some, low-flow, high-pressure jetting tools with higher pressures can be used. Table 1 provides data for nozzles of several sizes at different operating pressures.
The pipe that attaches to the jetting tool should have a diameter sufficient to minimize friction losses as water flows through the pipe, so that energy at the nozzle is as great as possible. Some standard pipe sizes are given in Table 2.
Jetting tool rotation is controlled by the rig. The tool is placed near the bottom of the screen and rotated slowly while pulling upward at a rate of 5 minutes/feet to 15 minutes/feet, depending on the nature of the aquifer.
Loosened material accumulates at the bottom of the screen as the jetting tool is raised. Slow rotation and upward movement assures treatment of the entire surface of the screen. Jetting should be continued until the amount of additional material removed is negligible. To avoid screen erosion and to expedite development, the jetting tool should never be operated while it is in a stationary position.
High-Velocity Water Jetting
High-velocity water jetting (Figure 4) is a relatively new well development technique. Pressures as great as 10,000 psi and velocities of up to 1000 feet/second have been utilized success¬fully for well development of wells with steel screens.
It is important to note that a 10,000 psi pressure is the gauge pressure and represents the pressure inside the jetting tool and piping. The screen and casing are not sub¬jected to this pressure because the hydrostatic head and frictional losses tend to decrease the pressure at the face of the screen. Pressures of 3000 psi have been used in PVC. The technology comes from cleaning pipelines. It is high velocity but low flow.
It is critical that the high-pressure jetting package be designed specifi¬cally for the particular screen-slot size, well diameter, screen material, and aquifer type. The pump and nozzle configuration should maximize the downhole pressure and focus the energy appropriately.
It also is imperative that jetting tools be rotated and moved vertically to eliminate screen damage from abrasion.
The high-velocity jetting technique is an effective way to break up encrustation and remove fine sand material. Jetting the well casing as the tool is removed also can be an effective way of revealing any weaknesses in the casing. It is a great tool for well rehabilitation and cleaning off incrustation and scale from the inside of a well.
Gas Impulse and Percussive Methods
High-pressure gas impulse and the resultant percussive wave generation are methods used to generate rapid and high-energy pulses downhole. These methods use high-pressure injection of gas to generate acoustic waves and high-energy pressure pulses (Figure 5).
A small volume of highly compressed air or inert gas is stored within the tool, and when released, generates beneficial well development and cleaning energies through created vibrations and high velocity movement of the water within the borehole.
The tool can be placed into selected zones and fired, creating vibrations and surging action when the energy is released. Initially, it generates a shock wave, which creates high velocity movement of the water surrounding the tool. The impact of the energy release facilitates the breakdown of hard mineral scale that collects on the well screen, filter pack, and in the near-well geological formation.
The released energy can penetrate several feet beyond the well screen, therefore making it an efficient and effective process. The number and pressure range created per minute varies with the specific tool. Some tools can be adjusted from the surface without removing the device from the well, allowing energy discharges to be varied to suit specific well characteristics.
Use of non-reactive gasses is advised to limit impactions on the well and aquifer. The use of nitrogen limits the introduction of oxygen that can impact metals oxidation within the well in addition to the potential stimulation and growth of aerobic bacteria.
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As mentioned earlier, it takes several methods of development to remove borehole damage in wells with longer screen intervals. Although wells with 5 feet to 10 feet of screen might be developed by only airlift pumping, there will be times when that might not be enough to adequately develop the well and other techniques will be required to fully develop the well.
In part two, which will be in the July 2023 WWJ issue, we will discuss methods that are generally used to pump to remove fines from the well.
References
Sterrett, R.J. 2007. Groundwater & Wells, Third Edition. Johnson Screens: New Brighton, Minnesota.
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It features 20 years of advancements in technology and knowledge gained in the field. Included are new borehole geophysical techniques, information on underbalanced and horizontal drilling, new approaches to well development, as well as details on ASR, groundwater monitoring and remediation, prepack screens for water interactive well design and well maintenance programs for water well contractors and design engineers.
Click here to order it, call (800) 551-7379, fax (614) 898-7786, or email customerservice@ngwa.org.
Thomas M. Hanna, PG, is a technical director of water well products/hydrogeologist for Johnson Screens where he works in areas of well design, development, and well rehabilitation. He is a registered professional geologist in Arizona, Kentucky, and Wyoming and has worked for several groundwater consulting firms. Hanna can be reached at thom.hanna@johnsonscreens.com.