What is Borehole Rehabilitation?

Rehabilitation is the action taken to repair a borehole whose productivity has declined or that has failed through lack of monitoring and maintenance of the pump and/or well structure. This is often a financial problem, or a logistical one – a function of remote location and, possibly, of conflict preventing easy access. Surface pumps, such as wind-pumps or hand-pumps, often fail for purely mechanical reasons – broken rods or corroded risers, for instance – and disused boreholes silt up or have objects dropped into them. Unfortunately, if a borehole has become tightly blocked by hard debris, such as stones and pieces of metal (a not uncommon occurrence), it is probably totally lost. Existing boreholes are likely to be well sited in terms of usage, since they must originally have been drilled for a purpose. Therefore, it is almost always advantageous to rehabilitate them.

It can be reckoned as a rule of thumb that a simple rehabilitation (no casing replacement) will cost around 10% of the price of a new borehole.

When to Rehabilitate

All pre-existing boreholes within a project area should be inspected for the possibility of rehabilitation, unless they are on privately owned land. The extra water might not be needed, but as boreholes provide access to groundwater, they could be used as observation holes for monitoring local water levels. Abandoned boreholes may act as pathways for the contamination of an aquifer, or enable the mixing of ground waters of differing quality from separate aquifers. They might also
present a physical hazard to, say, local children, especially if they are of large diameter and open. Redundant boreholes are potentially useful as groundwater monitoring points, even if they cannot be rehabilitated for production; but holes that are beyond repair should be backfilled using clean,
inert, non-polluting materials such as gravel, sand, shingle, concrete, bentonite, rock, or cement grout.

A borehole that has stood unprotected – by a top casing cap or a surface installation – for some time will almost certainly have been lost because of, say, objects being dropped into it by children. If a blockage can be reached from the surface it should be probed with a strong metal bar to get an idea of its solidity. Loose fine material might be removable using compressed air (see below); if this can be done, full rehabilitation might be a possibility. If the borehole was protected by a cover and is apparently clear, it should be checked for depth by plumb-line dipping, for static water level by dip meter, and for method of construction and internal condition by means of down hole camera. Before carrying out rehabilitation, it is advisable to sample and analyses the local groundwater (if possible) to ensure that it is not unduly chemically aggressive.

Rehabilitation Methods

The basic rehabilitation process should consist of the following principal stages in this order:

• 1. Collection of archives and information (from water authorities, drilling companies, aid organizations, etc.) on the borehole design
• 2. Inspection by down hole camera
• 3. Breaking-up of clogging deposits and incrustations
• 4. Removal of silt and debris by surging and airlift clearance pumping
• 5. Borehole disinfection
• 6. Step-drawdown test

I. Inspection by down-hole camera
Prior to commissioning camera inspection, efforts should be made to locate borehole design and construction details as that may save a lot of time. However, in Somalia archives of borehole design might be hard to find.

Typically, rehabilitation might consist of an initial camera run before de-silting by conventional air surging. A second camera inspection should then be carried out to check the efficacy of the de-silting operation and to obtain a clearer picture of down-hole conditions. All camera runs should be logged in detail and videotapes of the inspection retained for future reference.

A survey video enables full inspection of the inside of a borehole to be carried out, from top to bottom, in ‘real time.’ Side views allow casing or screen condition to be observed at accurate, recorded depths. With information of this quality, problems can be identified and complete rehabilitation of a borehole planned. Construction details can be observed directly and compared with the original log, if one is available. Objects or debris dropped into a hole can be inspected and the possibility of removal assessed. Water cascades, and to a certain extent, water quality (chemical precipitates, turbidity), can be viewed on a television monitor.

II. Breaking up of clogging deposits and incrustations
It is usually difficult – if not impossible – to remove old casings or screens to clean or replace them, so other methods often need to be used. Screens can be cleaned using a rotating wire brush or scratcher, but they may have been weakened by corrosion, so care should be taken not to worsen their condition. Borehole restoration methods are similar to those used in development, except that incrustations have to be broken up and removed.

a. Water jetting (Also Know As: BLOWING)
If it is done systematically, water jetting at high pressures can be a particularly effective means of de-clogging and cleaning the internal surfaces of boreholes. A jetting nozzle on the end of a length of high-pressure air hose or pipe is required. Test trials have shown that nozzle exit pressures of 17,000 kPa (for a 1.5 to 2″ nozzle, positioned about 1″ from the screen) will be effective on most occasions. In unlined boreholes, the jetting pressure limit is around 40,000 kPa. To avoid damage to plastic screens, pressures greater than 20,000 kPA should be avoided, because very high pressure jetting (greater than 30,000 kPa) can cut through plastic casing. Steel casing can withstand pressures of up to at least 55,000 kPa, and the screens that best respond to jetting treatment are those with high open areas and continuous slots, such as wire-wrap types like Johnson screens.

b. Acidization
For seriously affected boreholes, a combination of physical and chemical methods might be most effective. Acidization can remove carbonate incrustations and ferric hydroxide deposits in their early non-cemented stage. Hardened iron deposits would require physical breaking up by the methods described above. A 30% sulphamic acid solution to the volume of the screened or open section to be cleaned can be used for 15 to 24 hours, with the water in the borehole being periodically agitated by air that is blown in.

c. Hydrofracturing
Old boreholes drilled into low-yield formations, such as Precambrian crystalline rocks, can be stimulated by a process known as hydrofracturing. The technique can be applied only to open, uncased sections such as might occur towards the bottom of a hole. First, inspection by down-hole camera or down-hole geophysical log must be run to assess the suitability of the borehole to such treatment. The section to be worked should already be fractured to a certain extent, and must be isolated using some kind of packer. This might consist of a series of rubber seals that can be expanded in the borehole by a hydraulic ram or by compressed air from the surface. An injection pipe runs down the center of the packing system. High-pressure water is injected into the borehole in order to create or enlarge the fractures. Sand can be added to the water to keep open (‘prop’) newly developed fractures. Reports indicate that yield increases of 20 to 80% have been achieved using hydrofracturing. Depending upon the nature of the formation, injection pressures of 35 (soft) to 140 (hard) bar are used. After treatment, water and debris are air-lifted out in the normal way.

III. Relining
A borehole seriously affected by corrosion – that is now pumping out sediments – can be restored only by partial or total relining. The necessary course of action may be decided only after a borehole camera survey, or a geophysical logging, has determined the extent of the damage or deterioration. Down-hole logs might contain indications (water temperature, conductivity, flow, resistivity, or casing collar logs) of holes in casing.

Any new casings or screens that are installed should be of corrosion-resistant materials to avoid a repetition of the original problem. A new lining will be of smaller diameter, so the new pump will have to be chosen with this in mind. Corroded screens should not be relined if at all possible, because concentric screens create turbulence and abrasion, and fragments of corroded metal could be sucked into the borehole during pumping.

Although it can be extremely difficult, corroded screens should be removed and replaced by new corrosion-resistant materials. Any attempt to do this would involve bringing a large drilling rig on site and using its pulling power to remove the old casing string. Any lost gravel pack material can be blown out. With new casings installed, the borehole can be developed in the usual way. New casings and screens can be protected from corrosion under water by means of sacrificial electrodes (cathodic protection). Sacrificial anodes of a metal that is higher in the electromotive series (relative tendency to oxidation) than steel – such as magnesium and zinc – are attached and

corrode in preference to the protected metal of the casing. Such systems are used to protect ships, underwater pipelines, and pump installations, but are rarely applied to borehole casings or screens. Techniques for the removal of clogging deposits and incrustations include high pressure surging, jetting, air-lift pumping, air-bursting, and chemically assisted dispersion. Periodic rehabilitation, which ought to be carried out on a regular basis, can remove deposits before they harden with age.
In addition to air surging, a drilling rig can be used to redesign an uncased borehole or one from which linings have been pulled out. A borehole can be reamed or deepened to intersect more of the aquifer or to provide greater available drawdown.

Shallow dug wells can also be rehabilitated using a drilling rig. When the water level drops below the bottom of a well, the well runs dry. If the structural integrity of the well (its sidewall and surface structure) is sound, a rig can be brought in to drill a borehole through the base of the well and further into the shallow aquifer (or even deeper, but it may be better to drill a completely new borehole if this is desired). The borehole can, if necessary, be cased and screened by the methods described above, and a hand-pump mounted on the dug well slab with risers extending down into the borehole.

IV. Borehole sterilization
Boreholes affected by iron incrustation should be sterilized by chlorination between the clean-out pumping and the step test, to destroy ubiquitous iron bacteria to and delay re-infection of the well. Granular HTH can be dissolved and added so as to leave about 50 milligrams per litre of residual free chlorine in the borehole water. Mixing the solution in the borehole can be done by blowing with the airline used for the air-lift pumping. The concentration should be monitored using a water testing kit. The borehole can then be pump tested

V. Step-drawdown testing
A step-drawdown test will indicate whether rehabilitation has been successful; it can also serve as a new baseline against which future well performance can be measured. Test pumping of a rehabilitated borehole will also help to re-establish normal groundwater flow and remove remaining silt particles.

VI. Mechanical repair
Many boreholes lie disused because pumps have broken down or because of the lack of necessary expertise or spares. In the case of hand-pumps, this can be relatively easy to fix: all that is needed is a set of standard tools with which to remove the pump handle, chain, riser pipes, rods, and piston, for inspection and repair or for replacement. Pumps and risers on deeper boreholes might require a tripod and a vehicle with a winch for removal. If at all possible, a borehole should be inspected by video camera once a pump has been removed.