Well Development Techniques Part (2)
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.
