A NEW method of installing deep gas wells that is claimed to improve gas production levels has been used on a recent remediation project in the north west of England.
The technique uses continuous flight auger piling equipment fitted with a modified hollow stem auger to install the pipework for the gas wells.
Bill Bould of piling contractor Westpile, which developed the idea, says that in the past many wells have not achieved predicted levels of gas production.
The problem, he explains, occurs in driven wells in which the fill around the well is compacted during installation. This, together with the smearing effect of driving, inhibits gas flow to the well.
The new technique ensures that the ground immediately surrounding the well is disturbed enough to allow gas to flow through the soil. To minimise the amount of spoil produced, only the bottom 3m of the auger stem is fitted with flights.
The 1.5ha site is in an old sandstone quarry backfilled with landfill which is up to 45m deep. The local authority had received a number of complaints about strong odours from householders whose gardens border the site, and decided that action had to be taken. The total volume of polluted air space, classed as 'special waste', is 300,000m3.
The local authority secured funding from English Partnerships and commissioned Applied Environmental Research Centre to design a gas remediation system consisting of vent wells and carrier pipework. The strategy called for some 70 wells to 30m, but did not specify the method of installation.
Westpile won the contract through its proposed use of the new technique, which it claims is quieter and causes less vibration than conventional methods.
Boreholes were drilled with the modified auger to a maximum depth of 30m or until refusal. The auger was then pulled out 300mm and a heavy weight dropped down the hollow stem to punch off the auger cap. The slotted gas well HDPE pipe could then be slid down the stem and the well capped, with the auger spun out in reverse and used to create a void around the top 1,250mm of the well. Bentonite grout was then used to seal it.
Although boring conditions were highly variable across the site, an average of four wells were installed every day. Once finished, horizontal carrier pipework was laid in trenches and the whole system sealed with a geosynthetic clay liner.