WET News

It's rehab for water borehole • Overcoming a biofouling problem on a water production borehole proved challenging in more ways than one, not least a narrow window of opportunity. BES' Michael Bushby explains. camera could not pass into the drop set casing due to a dogleg which was up throwing the casing to the top side of the borehole and anything low- ered down was passing into the annulus between the two casings. Aer several attempts to enter the drop set casing, the consultant decided to leave the lower section until a later date due to the lack of time available. The 250mmØ casing was now scrubbed with a steel brush to loosen the biofilm. A special industry approved iron removal product, Boresaver, was introduced into the bore- hole at 128m and circulated before being air lied to settle- ment tanks and treated. Once the entire chemical was removed and the water chemistry had returned to its original state, the submersible pump was reinstalled and the borehole was put back into production. BES carried out simulation trials for gaining entry into the drop set casing at its Seaham depot and put forward propos- als to the consultant on how to tackle the cleaning of the lower section of the borehole when the opportunity arose. Aer carrying out simulated trials it was decided to run a 50mm pipe with a 'bent' lead length down to the top of the drop set casing. The CCTV camera would then be lowered to the base of the 50mm pipe and then, using the camera to guide the operation, turn the pipe until it lined up with the drop set casing. Once aligned, the 50mm casing would be lowered into the drop set cas- ing, thus gaining access to the bottom section of the borehole. It was also proposed to use a new chemical process to treat the biofilm. This was to inject chlorine dioxide into the borehole. Chlorine dioxide had a proven track record in other industries for the treatment and removal of biofouling mat- ter but BES had not used it in boreholes to a depth of 245m. BES' proposals were accepted by both the consult- ant and client. Loose debris An opportunity to carry out another cleaning operation arose in September 2017, and so the submersible pump and rising main were removed and once again were coated in a thick red-brown layer of biofilm. The borehole was scrubbed and airlied to the top of the drop set casing. The 50mm pipe with the bent leading length was lowered and entered into the drop set cas- ing with the aid of the camera. Now inside of the drop set cas- ing, the camera was lowered down the remainder of the borehole. At 214m the borehole was found to have filled up with debris and rust which was blocking most of the screened section. ONSITE ASSET MANAGEMENT A dosing pipe was installed to the base of the borehole and enabling chlorine dioxide to be pumped in The borehole was scrubbed and airlifted to the top of the drop set casing Once the submersible pump was reinstalled, the borehole was put back into production An air liing tube was now lowered into the 50mm pipe and the borehole was blasted with compressed air to remove this loose debris and to clean the screens. This operation was successful, although the bottom 10m of the casing could not be cleaned as the debris had solidified and could not be removed using this technique. The consultant confirmed B orehole Engineering Ser- vices (BES) is the borehole maintenance division of Drilcorp. It was recently com- missioned by a specialist hydro- geological consultant to over- come a biofouling problem on a major water production borehole. The works had been attempted by BES two years previously when there had been a narrow window of opportunity during a factory closedown period. An obstruc- tion within the borehole at 128m meant that only the top section from ground level to this depth could be treated inside the timescale available. The borehole was in a good productive state and too important for its owner to close down for further remediation. It was agreed that BES would put forward proposals for getting past the obstruction and cleaning the full 245m borehole when the next oppor- tunity arose. Biofilm The borehole had been drilled to 245m and 250mmØ mild steel surface casing was installed to a depth of 135m. A second 100mmØ casing was drop-set from 128m to 245m. This casing had a screened section covering 134m to 244m. Remediation works com- menced with a CCTV survey of the borehole which showed substantial build-up of bio- fouling matter. The CCTV 10 WET NEWS NOVEMBER 2017 that this small section was not important to the productivity of the borehole and instructed to halt this operation. The borehole was resurveyed and found to be clear to a depth of 235m. A 25mm dosing pipe was installed to the base of the borehole and 100 litres of chlo- rine dioxide was pumped into the borehole at a rate of 20l/hr.

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