Horizontal collector wells for landfill gas extraction

Steve Cox

Landfill gas (LFG) is a product of the degradation of biodegradable wastes in a landfill site. Typically it is a mixture of up to 65% methane and 35% carbon dioxide plus trace gases. Methane is explosive at concentrations between 5 and 15% by volume in air. Without proper management the migration of gas from a landfill can give rise to the risk of fire and explosion.

 

Typically, LFG collection is achieved by sinking vertical wells within the waste mass, but the effective length of the wells is limited to the depth of the unsaturated zone below the landfill cap. This often restricts the efficiency of such wells and so several are required to cover a small area effectively.

Other disadvantages of vertical wells include:

the installation of each well gives rise to a potential weakness in the cap,

air ingress can result in aerobic conditions developing in the landfill, perhaps causing landfill fires and/or the development of explosive gas concentrations within the gas extraction infrastructure. To aviod air ingress, the upper 2-3m of the well is sleeved, thereby reducing the effective length of the well,

each vertical well requires expensive pipework for connection to the extraction system,

the volume of gas produced by each well is variable and in order to achieve control of the subsurface gas pressures each well needs to be balanced - this is operationally labour intensive,

and,

vertical boreholes produce a large quantity of spoil when they are drilled; disposal of spoil can be problematic, especially if it is contaminated.

 

It is thought that horizontal collector wells could be used to improve the efficiency of LFG extraction systems from low yielding, shallow and laterally extensive landfill sites. LFG extraction using horizontal well technology has been applied in the past but in most instances the extraction wells have been installed as vent trenches in virgin ground beyond the periphery of the landfill site, or as gas drains within an operational landfill as filling progresses. These installations have been constructed using conventional installation methods, principally trenching and excavation. Such methods are undesirable in closed landfills because the integrity of the engineered cap is compromised and a large volume of spoil is produced.

However, horizontal collectors may offer several advantages over vertical wells:

the larger zone of influence of horizontal wells may increase gas yields and/or reduce the number of wells required. This advantage should be particularly evident at shallow sites,

fewer wells will require less pipework and less balancing,

and,

the length of sealed sections at the entry and exit points of the well can be increased without significantly reducing the effective length of the well.

It is considered that the disadvantages associated with conventional installation methods can be overcome by installing the horizontal collector wells using directional drilling technology:

by penetrating the landfill cap at the points of entry and exit only, damage is minimised,

and,

drilling using low volumes of drilling fluids at a high pressure has been shown to minimise, or eliminate, spoil returns from the borehole.

 

In a controlled trial, a horizontal gas collector has been installed within a shallow, laterally extensive landfill site for the extraction of landfill gas. The Metallic Tile Landfill Site near Stoke on Trent has been selected for this purpose. Vertical wells installed at this site do not remove landfill gas efficiently.

Research undertaken at this site aims to investigate the suitability of horizontal wells for landfill gas control, and answer the following questions:

does the horizontal well yield more gas than a series of vertical wells in a shallow, laterally extensive landfill site,

what effect does the horizontal well have on gas pressures within the site, gas migration and gas quality (in comparison with vertical wells),

and,

which well type incurs the higher costs, in terms of installation, connection to the extraction system, running and maintenance, and the environment?

Project Status May, 2000. A 100m long horizontal well has been installed on site by directional drilling. The well now requires monitoring, analysis and reporting.

Research results will be posted to this site as they become available.

Cleanaway Ltd is supporting this work
through the Landfill Tax Credit Scheme

Comments about this web site, and any queries regarding SUnRISE activities, should be made to
Dr. Anne Stringfellow, SUnRISE Research Co-ordinator at :
e-mail:as10@soton.ac.uk      
telephone : +44 (0) 23 8059 3988      fax: +44 (0) 23 8067 7519

SUnRISE is based in the Department of Civil and Environmental Engineering
at the University of Southampton

All rights reserved. No material from pages on this website may be reproduced in any form, or by any means without the permission in writing from the University of Southampton.