The process of drilling and installing a well --horizontal or vertical --into the subsurface leaves residual drilling fluid and drill cuttings in the hole and may also cause porosity and permeability loss in the surrounding formation. Both of these conditions will result in lost efficiency in the well, reducing the well yield, or in some cases, preventing the well from working at all. Well development is the process of removing this waste material and enhancing the flow of desired fluids (e.g., air, soil vapor, groundwater) into or out of the well, depending on its end use. Well development techniques have existed since the first hand-dug well was excavated. Some, like those developed for the petroleum industry, can be very complex. Others, used for years in the water well industry, are straightforward and require simple equipment. As effective as they are, most of these techniques have not yet been adapted to practical application in horizontal wells. Part of the difficulty in this transfer is the large difference in screened area between vertical and horizontal wells. The average vertical remediation well may have from 10 to 50 feet of screened sectionhorizontal wells often have hundreds of feet of screen, making development a time-consuming process. This chapter reviews the techniques that are commonly used or may be specified for horizontal well development. It also summarizes some of the important issues a driller should consider when bidding a project and choosing a development method. Rationale For Well Development The main objective of well development is to enhance the connection of the well assembly, to the maximum extent possible, to the soil formation that surrounds it. This involves several separate but related objectives:
How these objectives are met is dependent on the type of drilling mud used, the type of casing installed, the installation method, site constraints (water usage and discharge, construction area) and available development equipment. Groundwater Extraction Well Development Well Flushing or Jetting Most horizontal wells are developed using a combination of jetting, in which water or a chemical solution is injected under pressure through a nozzle that is moved back and forth within the well, and pumping, where injected fluids and groundwaterareremoved from the well either by suction or a submersible pump. If access is available at both ends of the well, a technique called overpumping may be used. In this technique, the pumping rate is adjusted to slightly exceed the jetting flow rate to assure that groundwater is being drawn into the well and subsequently removed. Tools and Accessories A wide range of jetting tools is available. The ones suitable for use in horizontal well development are generally from the water well drilling industry. Tools are typically of two types --those with several fixed nozzles or those with one or more nozzles attached to a spindle that rotates with water pressure. The main features of a jetting tool include:
The key points to consider in buying a jetting tool include the durability of the tool, the ability to replace the individual jets and the weight of the tool. Because the tool is often inserted into the borehole and cycled back and forth manually within short intervals, excess weight should be avoided. At this writing, most jetting tools are designed and sold for use in vertical wells, where tool weight is not an issue. The type of pipe used to run the tool in should also be considered. Several options are available:
Drill pipe are durable, and the rig hydraulics can be used to cycle the jetting tool back and forth within the well casing. However, there are some disadvantages to using this method. Because the rods are heavy and the drill rig is relatively powerful, it is possible to damage the well casing without any forewarning. This could happen, for instance, if the jetting tool snags within the well screen; the rig is powerful enough to break out a large section of the screen slots before the operator can react. Unless the contractor has a particularly generous project budget, developing with the rig is also costly. Most contractors would prefer to move the rig to the next drilling project and perform the development process less expensively. Threaded iron pipe shares some of the characteristics of drill pipe. It is durable, but heavy. If manual labor is used to trip the iron pipe in and out of the well and to cycle it within the development intervals, then this option is impractical except for relatively short holes. PVC pipe with threaded connections is a good choice for development pipe. It is relatively strong, lightweight to handle, buoyant in water and inexpensive. Threaded couplers are usually used to make up the pipe sections to simplify connection to the jetting pump with a short flexible hose. It is best to use Schedule 80 pipe for extra strength and stiffness during handling. Most available couplers have squared-off ends that must be chamfered with a file or grinder before use; otherwise they will hang up on the interior of the casing, near threaded joints or on the inner bead of an HDPE fusion weld. Various types of quick-connect couplings are available to speed up the development process. For instance, quick-connects can be assembled to thin-wall stainless steel tubing to make a lightweight, easy-to-handle jetting assembly. If this method is used, the contractor should thoroughly test the couplings; some types have been known to come apart at inconvenient times and places, leaving the contractor with the challenge of fishing for his lost development tools from within the well casing. Development Chemicals The fluid used for flushing may vary from clear water to a variety of chemical solutions, depending on the type of drilling mud used. Polymer drilling fluids Currently, polymer-based drilling fluids are the first choice of drillers installing horizontal remediation wells. The most common mud products on the market are either based on guar gum, from the guar bean, or xanthan gum, which is derived from corn. Both of these mud additives are food-grade, biodegradable and relatively easy to disperse.
During development, the driller's ob jective is to break these molecular chains into smaller segments that are more easily flushed from the borehole. Two chemical solutions are typically used to do this:
Sodium and Calcium Hypochlorite. Sodium hypochlorite, the main ingredient of household bleach, is often specified to break down polymer muds. Calcium hypochlorite, the "chlorine" used in swimming pools and spas, is also frequently used. Either of these reagents are usually mixed at a concentration of 1000 parts per million (ppm) and are injected at a flow rate sufficient to assure ample mixing of the solution with the drilling mud in the well bore. Table 1 lists the quantity of concentrated reagent, available at various concentrations, needed to make up 1000 gallons of development solution at a 1000 ppm dilution. The hypochlorite solution is usually jetted into the well and allowed to react with the drilling fluid for several hours; then the well is developed using clear water. Table 1: Concentrations and Dilutions for Typical Hypochlorite Solutions
Drillers should be aware of some safety, regulatory and chemistry issues involved in the use of sodium or calcium hypochlorite. Both of these chemicals are strong oxidizers when concentrated and can cause severe chemical burns if the material contacts the skin, or in particular, the eyes. When mixing development solutions, workers should wear appropriate protective clothing, including chemical-resistant gloves and splash shields. The proper MSDS documentation must also be on hand when these chermicals are in use. From a regulatory standpoint, it is important to note that some states and localities restrict the usage of these chemicals in wells. In some areas, it is acceptable to use easily obtained household bleach or pool chemicals to develop a well; in others, chemicals cannot be introduced into an aquifer unless they are of a higher grade or purity, e.g., "technical" or "reagent" grade. Chemicals of this purity are more expensive and less likely to be available locally, so it is wise to inquire about possible restrictions during the bidding process and budget and schedule accordingly. From a chemical standpoint, not all reagents behave alike, even if they have the same basic composition. In particular, some pool chemicals may have additives to buffer the oxidation process so the chlorine lasts longer in a pool or spa. These additives will interfere with the breakdown of drilling mud, so the use of buffered calcium hypochlorite should be avoided. Some household bleaches may contain perfumes (e.g., "lemon-scented") in addition to the active sodium hypochlorite and should not be used for development. Finally, the driller should be aware that calcium and sodium hypochlorite are not absolutely interchangeable. Some groundwater treatment systems may not be compatible with the sodium or calcium ions released during development, so designers may specify a particular development chemical. Do not substitute one for the other without inquiry. Enzymes. An enzyme is a chemical compound that hastens the transformation of one specific organic compound into another. The enzyme can be likened to using a key to open a lock, rather than opening that same lock with a sledgehammer. The use of chlorine is more akin to the sledgehammer approach to well development --relatively crude, but effective. Enzymes are used in extremely low concentrations, on the order of a few ppm. A quart of enzyme for breaking down a guar gum drilling fluid costs approximately $1000, but will treat an entire year's worth of drilling mud for the average contractor. Enzyme treatment is also advantageous since it does not kill native microbe populations, which will consume any residual drilling mud that might be left in the formation after development is completed. Currently, enzymes are only effective on guar gum-based fluids. Other products are in development for use on xanthan-based drilling fluids. Bentonite Bentonite drilling muds are difficult to disperse and remove from a well bore because the Bentonite clay materials are relatively sticky and tend to cling to the well bore and casing. For this reason, most horizontal environmental wells are not drilled with bentonite; in fact, most specifications prohibit its use.
The petroleum industry has developed acid-flushing methods that are very effective at flocculating the Bentonite and removing it. These methods have been adapted and successfully used for development of a few horizontal remediation wells. The process begins with a clean water flush to remove most of the mobile drilling mud and cuttings within the well bore and casing. Then an acid solution, which could be composed of hydrofluoric, citric or a more complete organic acid, is jetted into the well bore. This solution will flocculate the bentonite clay particles from the well bore and turn them into a curd-like form which may be more easily removed from the well, assuming that the screen size is not too fine. The final cleanup requires water to flush out the flocculated bentonite. The process works best when used on larger installations that require the use of a carrier casing. Since the purpose of a remediation well is to remove contaminants from the groundwater not to introduce new ones, most clients are reluctant to allow the use of acid flushing for well development. Mixed Metal Hydroxides Drilling muds using mixed metal hydroxides for a base have also been used on a few horizontal well installation projects. These muds are fairly exotic and expensive; those intending to use them should consult directly with the manufacturer for advice on how to break them down. Jetting and pumping, as for other drilling muds, is the preferred method for introducing development solutions and removing spent fluids. The Jetting Process Jetting is usually completed through a series of passes through the screened or slotted section of the well. For instance, in a 100-foot screened section, typical specifications may call for the following: 1. Jet in a 1000 ppm solution of calcium hypochlorite at a rate of five gallons per foot. 2. Let the solution stand overnight. 3. Jet with clear water at a rate of 8 gpm dividing the screened section of the well into 20 five-foot sub intervals end jetting within each interval for a period of four minutes with constant, back-and-forth agitation. Do this three times. 4. Overpump at a rate of 10-12 8pm. Specifications such as these can be easy to dismiss as a minor part of the project during the bidding process, but it is important to consider the time and effort involved. At 8 8pm, it will take over an hour to jet in 500 gallons of calcium hypochlorite, and more than five hours to complete the clear water jetting. Each additional 12 feet of well screen will add an hour to the jetting requirements. It becomes clear that well development can quickly become more time consuming than the well installation itself. Pumps and Pumping In addition to the jetting process, which breaks down and liquefies drilling mud and cuttings, groundwater well development also requires pumping to remove the mobilized waste from the hole. The choice of pump to perform this duty is partly dependent on the depth of the well and partly on the nature of the cuffing to be removed. At shallow depths, trash pumps or diaphragm pumps are a good choice. Both types of pump are designed to handle heavy loads of solids, but diaphragm pumps are particularly good in this respect. If the pump capability is sufficient, it is possible to simply connect the pump intake directly to the projecting end of the well casing and use suction to remove the water. At slightly greater depths, it will be necessary to insert a suction hose down the well casing. In deeper wells it is generally more practical to use a submersible pump for well development. Submersible pumps used in this manner will likely require more maintenance because of increased wear from transported sand and grit. Just as the contractor should consider the time-consuming aspects of jetting estimating well development costs, the costs associated with pumping and associated waste handling should also be evaluated. To avoid misunderstanding in the field, it is important to determine in advance what the requirements are for pumping volume. Environmental Protection Agency guidelines call for purging from three to five well volumes from vertical monitoring wells. This guideline has also been proved to be generally appropriate for horizontal extraction wells. The contractor must resolve if a "well volume" is the volume of the well bore or the well casing; there can be substantial difference in the amount of water that must be pumped between the two measurements. Other Methods Swabbing, Surging Swabbing or surging is sometimes specified for development in horizontal well designs that have specifications "borrowed" by the engineer from vertical well drilling contracts. These development methods are accomplished by dropping a piston-like arrangement with rubber wipers (a swab) or a tightly tiffing solid block (a surge block) down the well, then quickly withdrawing it. One-way valves close on the "up" cycle. The result is a piston pump action; which pushes a column of water out of the hole above the swab, and a vacuum pump action beneath the swab, which pulls water out of the formation. This vigorous water movement is effective at cleaning silt, clay and drilling mud from the well's sand pack. In a vertical well, this is an effective development method that is easily accomplished with the wireline on a typical auger or mud rotary drill rig. This method is nearly impossible to accomplish in horizontal drilling because the rapid surging motion of the swab cannot be easily duplicated. Swabbing the bore with drill pipe and rig hydraulics is too slow to effectively stress the aquifer and remove fluid from the well. Even assuming that this kind of swabbing is technically feasible, another difficulty would arise in fusion-welded HDPE wells, a common horizontal installation. In these wells, the interior bead, formed during the fusion process, projects into the interior of the well and would interfere with the passage of tools through the well bore. In general, contractors should question any specifications that call for this mode of development. Air Sparge Well Development Considerations for Air Sparge Wells Air sparge wells may be developed in a manner similar to groundwater extraction wells, but it is difficult to develop them to the same degree. The reason for this lies in the limited amount of open area within the perforated section through which the contractor can directly affect the surrounding well bore. Well screens may have from 30 to 60 percent open area within the screen section; an air sparge well may have one, 1/8-inch perforation every three or four feet. Jetting has somewhat limited value in this situation. The danger in performing incomplete development on a sparge well is that fine-grained material may enter the well in the future if the sparge air is turn off for any reason. If enough material enters the well, it will eventually plug up. Pumping the well, combined with a limited amount of jetting, is usually the best bet for sparge well development. Soil Vapor Extraction Well Development Because they are installed above the water table, soil vapor extraction wells have less opportunity for a thorough development cycle than do their submerged cousins. Water that is injected during the jetting process will tend to drain down away from the well, preventing its removal (along with suspended drill cuffings) by pumping. Because there is also no groundwater entering the well, it is more difficult to mobilize and remove waste material. Fortunately, air is much less viscous than water, and soil vapor extraction wells may still work efficiently even if the development cycle is less than ideal. One adaptation the contractor can make is to move the pump inlet and the jetting tool closer together, if possible. This will give the development water less opportunity to drain from the well before the pump captures it. If sufficient water is available, it is also possible to successfully develop a soil vapor extraction well without extensive jetting by introducing water from one end of the well through a PVC pipe and sucking it from the other end, either with a diaphragm pump or a vacuum truck attached to the well casing riser (as described above). Because microorganisms grow quickly in the environment surrounding a soil vapor extraction well, purging the well with a chlorine solution after development in order to sterilize the vicinity is worth consideration. This will help prevent the excessive growth of microorganisms which could eventually plug the well, a process known as biofouling. Developing Wells With Integrated Filters Recent innovations in well screen design have resulted in screens that are more resistant to plugging and silting. Most achieve this objective by reducing the effective slot size of the screen, maintaining the same open area in the screen by using very fine-diameter materials. Some screen materials use multiple layers of fine mesh within a protective, perforated casing; others use a single layer of geotextile; and others use a sintering to bond resin beads into a porous casing wall. Although all of these designs make the casing much more resistant to silting, they also confound any attempt to mobilize drilling mud or cuffings that may remain in the hole, once the casing is installed. The use of carrier or washover casings for installation can reduce this problem, but will not eliminate it. Jetting and overpumping, although of limited effectiveness in removing larger soil particles, can be effective in removing finer-grained material and initiating the creation of a graded filter between the well screen and the surrounding formation. Rather than simply follow a predetermined recipe for development times, it is best to monitor the turbidity of the development water, which should progressively clear up as development continues. It is more productive to make several passes of short duration than to attempt to completely develop the well in a single pass. Summary Well development is a significant part of any horizontal well installation project. Drilling the well bore and pulling in the casing may go very smoothly, but unless drill cuffings and residual drilling mud can be successfully removed, the well will be useless to the end user. It is important to consider the drilling fluids and techniques to be used in terms of the development techniques that must be used to counteract any harmful effects on the formation surrounding the well bore. Most wells respond to a combination of jetting and pumping, using development solutions that are specifically intended for the type of drilling mud that was used. |
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