The picture below beautifully illustrates the end result of a properly developed natural packed screen well.

Notice how proper development of the screen well above has removed all the near well fines thereby increasing porosity and hydraulic conductivity (permeability) in the zone immediately surrounding the screen. Note too that the screen slot size has obviously been chosen with proper attention to the particle size to be retained after development. In other words the driller of this borehole sized his screen after examining his cuttings from this sand and gravel water bearing formation.

If an artificial pack had been placed around the screen, the goal would have been the same. The grain size of the artificial pack and the slot size of the screen are matched to provide an area of much higher permeability in the zone immediately outside of the screen. This high permeability zone reduces the head losses necessary to get the desired flow and reduces the chances of incrustation

The artificial pack provides superior control over the movement of fine sand grains toward the well and permits a wider range of sand sizes to be screened. When surrounded by an artificial pack the screen may have a larger slot size and thus a larger open area. This larger open area allows for much more effective development.

Pre-pack screens are available and can offer advantages in certain applications. Research has shown that pack thickness of one inch or less is just as effective as packs of much greater thickness. However, the thinner pack must have consistency throughout. For this reason industrial vibrators are used when filling pre-pack screens to ensure the consistence of the pre-pack. Also, artificial packing material is sometimes used rather than natural gravel packs. Ceramic beads have proven to offer less resistance to flow and incrustation because of their uniformity of size and shape.

Types of well development techniques:

Chemical

Washing and Backwashing Mechanical Surging

Air Development Jetting

Chemical

Chemical agents are introduced into the development zone as solvents. Their action is intended to dissolve or loosen any clogging or blocking materials to make them easier to remove. The action of chemicals may also enlarge aquifer pores and improve permeability. Chemical based well development techniques can be gentle or violent in their action.

All chemical agents introduced into potable wells should be approved for such use by local authorities. Chemical methods are often used in conjunction with other well development techniques. This is particularly true when additional action is needed to break up mud cakes or flush out gelled muds. The chemical solution is allowed to stand in contact with the aquifer for the recommended soak period. After the soak period the solution is pumped or bailed from the hole. While well drilling fluids will break down naturally, the breakdown process may be enhanced by the use of chemical agents. Once degraded, the drilling fluids are much more easily pumped from the aquifer. Other chemicals may be used to break down clay smears and gelled bentonite. Chlorine breaks down polymers.

A tremie pipe can be used in conjunction with packing devices to isolate the areas of the borehole to be subjected to chemical treatment. Chemical treatment can be used to break down drilling fluids, clays and polymers. Acids are often used for improving the yield in limestone, dolomite and other calcium carbonate formations.

Washing and Backwashing

Drillers working in different regions have, through experience, come to rely on those well development techniques producing the best results in their areas. However, new techniques should always be considered and tried with the goal of obtaining the cleanest well with the best possible yield.

Overpumping is the simplest method of removing fine particles from formations. The theory is that if a sand free yield can be achieved by overpumping then a sand free flow will be the result when pumping at the normally expected lower rate. However, overpumping by itself is not considered the best well development approach. Overpumping is considered a limited approach to well development because water flows in a single direction only.

Backwashing reverses water flow and helps in the dilution, agitation and removal of sediment, fine particles and drilling fluids. Backwashing requires the introduction of water back into the well. If water taken from the well is to be reintroduced for backwashing, care must be taken to allow the settling out of particles from the removed water before reintroduction. Even so backwashing should not be the final step in the well development process; rather it may be an effective beginning or intermediate step. Washing and backwashing reverses the flow in the borehole during development. This reversal causes the collapsing of bridges in the particles of the near well area. This is desirable because collapsing these bridges further removes fines from the near well creating a cleaner flowing well.

Mechanical Surging

The forcing of water into or out of a well screen by use of a plunger type action is called surging. Surging tools can be used by both cable drillers and rotary drillers and can be used in combination with other development methods. Surging promotes a repeated change of direction in the flow of water in the well screen area. This repeated change of direction can produce good porosity in the near-well zone.

Mechanical surging is the first of two methods of well development that removes particles and clogging materials by the force of water impinging on them. A development method such as mechanical surging is a vigorous development method not suited to all aquifer types. However, mechanical surging has less potential for aquifer damage if a continuous flow of water into the well from the aquifer is maintained. Mechanical plungers may be fitted with one-way valves allowing them to lift water and fine sand out of the hole. Solid plungers do exist but have more potential to damage the aquifer. The results of mechanical surging should be measured by checking the well yield periodically, every hour after the process begins. Surge plunger should be a good fit in the casing. The plunger may be attached directly to the drill stem or operated by hand depending on well depth.

Mechanical surging does have potential to damage the aquifer and should be done with aquifer. The force exerted during mechanical surging depends on the length of the stroke and the vertical velocity of the surge block. Swabbing is another variation of surging. Swabbing does not depend on reversing flow into the well. Rather the swab is slowly lowered to the desired depth and then drawn upward. Swabbing creates a pressure differential below and above the swab during the up stroke. This differential creates a powerful action which draws fines from the near well area into the bore hole for removal.

Air Development (air surging and pumping)

Several techniques for the air development of wells exist. However, all inject air into the borehole such that aerated slugs of water are lifted irregularly out the top of the well casing. Air pressure may be cycled on and off to create a surging action desirable in well development. Sufficient air pressure will result in a continuous flow of aerated water out the top of the well, removing sediment and fine particles from the borehole.

For small wells, air may be injected down the drill stem into the formation. For larger diameter wells a separate airline and eductor pipe are inserted into the borehole. The size of the eductor pipe and airline depend on air pressures and volume available as well as the casing diameter. Numerous sources caution drillers that under some conditions the use of air development approach can create aquifer air locks, in such cases a development with water is a wiser choice. Even so air as a development is probably the most popular and widely used method of well development today.

The type of discharge produced from a well during air development depends on the air volume available, total lift, submergence, and annular area. In practice, two different flow conditions can be recognized when air is used when air is used for water well development although other flow regimes may exist at much lower or higher velocities in smaller diameter pipes. The picture above provides an illustration of how multiphase flow (water and air) occurs in the casing during air development. The percent submergence, total lift, and capacity of the compressor will control the relative proportion of air and water for a particular well.

A. Introduction of a small volume or air under high head causes little change in the water level in the well. In this case, the air pressure available is just sufficient to overcome the head exerted by the water column.

B. As air volume increases, the column becomes partly aerated. Displacement of the water by the air causes the water column to rise in the casing. Drawdown does not change because no pumping is occurring.

C. Further increases in air volume cause aerated slugs of water to be lifted irregularly out the top of the casing. Between surges, the water level in the casing falls to the near the static level.

D. If enough air is available, the aerated water will continually flow out the top of the well. With average submergence and total lift, the volume of air versus water is about 10 to 1. Higher air volumes may increase the pumping rate somewhat, but still higher rates may actually reduce the flow rate because flow into the well is impeded by the excessive air volume.

Well Jetting

Development by high velocity jetting may be done with either water or air. A jetting tool is attached to the lower end of the drill string and lowered to the bottom of the well screen. Rotation is controlled by the rotary rig. The jetting tool activated by either air or water forces high-pressure fluid out the nozzles of the tool very effectively, developing the formation. Because of the high pressures used damages to the well screens may result through improper use of jetting tools. However, jetting is seen as possibly the most highly effective development technique in terms of well yield after completion. The essential point to be made is that yield depends to a great extent on the development method used. Particles loosened by jetting tools may be later removed by pumping or bailing.

Aquifer Development Techniques

To this point, our discussion has been centered on development techniques in the near-well area. However, this discussion would not be complete without some mention of aquifer development as distinct from well development.

In regions where ground water comes from bedrock, when the volume of water is inadequate, aquifer development techniques may be effectively employed. Aquifer development or aquifer stimulation can increase well yield far beyond those discussed above.

In limestone or dolomite aquifers, acids can be beneficially used to open up the formation around the borehole. The acid dissolves calcium carbonate thereby increasing hydraulic conductivity.

Hydrofracturing

In Hydrofracturing, the production is isolated from the rest of the well using inflatable packers lowered in place and inflated. Once the production zone is isolated, high-pressure water is inserted in the isolated production zone at pressures up to 10,000 psi. These pressures are sufficient to fracture most formations causing small type breaks to open up and spread rapidly. Often these artificially created fractures will reseal unless artificially propped open. Often sand or plastic beads are forced into the newly created fractures as part of the hydrofracturing process. Hydrofracturing can be a useful technique in low yield formations for increasing yield and reliability of supply.

Liquefied CO2 Injection

A further method to open rock fractures and stimulate production is the injection of Carbon Dioxide CO2. One approach is to inject liquid CO2 into the producing formation, which has been isolated by inflatable packers. The introduction of the liquid CO2 freezes the surrounding water, opening nearby fractures. As the liquid CO2 becomes a gas, it expands into the formations, opening them further. This method is an improvement on an earlier method inserting dry ice into the production zone, which had been isolated.

Date: 2016-03-03; view: 939