Getting underneath the problem

There are a number of techniques for rehabilitating contaminated land but many are not only highly expensive but are only applicable to sites that have been cleared of the processing or storage plants that have caused the pollution reports Chris Francis.

Horizontal or directional drilling is a means of reducing contamination in polluted soils and groundwater by providing a conduit for sub-surface fluids. The technique consists of using directional drilling rigs so that curved wellbores of almost any trajectory can be created under the contaminated site to remediate plumes of contaminated groundwater. This is dependent on characterising the site hydrology for the well positioning together with determining the geology and geo-chemistry of the location. The technique has been widely used in the US where pollutants such as ethylene dichloride, monochlorobenzene, lime (with leachates of 12 to 13 pH) mixed with chlorinated asbestos and lead, chlorofluorocarbons, TCE, raw petroleum and petroleum hydrocarbons have been removed and/or treated. In an application involving petroleum hydrocarbons, directional wellbores were used to prevent their migration into the water table.

The great advantage of the technique is that it can be used to decontaminate land on which buildings still exist. Curved wellbores can be driven under site buildings or drilled under structures surrounding cleared sites. Horizontal drilling is also able to remove pollutants from the much wider area that has been contaminated outside the site boundaries.

The Charles Machine Works Inc, makers of the Ditch Witch trenchless equipment, has developed a specialist horizontal drilling rig that consists of a carriage spindle which provides the thrust and torque to a dual pipe system to create the wellbores. The rigs are marketed in the UK by the Swansea company Euro-Equipment.

The outer (or directional control [DC]) pipe, which is connected to the outer casing (or steering tool) of the bottom hole assembly (BHA) controls hole deviation and the weight on bit. The inner pipe provides torque to the drill bit. Thus the spindle carriage can turn the two pipes in opposite directions at variable speeds to guide the BHA which drives the drill bit via the inner pipe. To vary direction, the rotating DC pipe is stopped, the steering tool is used to set the direction and drilling continues with the rotation of the DC pipe only being resumed when the boring is to continue in a straight line.

Although, blind wellbores can be formed with horizontal drilling, the continuous wellbore is more often preferred as the screens can be pulled back though the adit from the opposing breakthrough position. The trajectory is continuously monitored by sensors to ensure that it is accurately positioned within the plume and to ensure that the continuous wellbore breaks the surface at the required position. Location and guidance systems consist of electronic packages behind the cutting head which enables the end of the drill to be located, to provide the azimuth and inclination of the BHA and the orientation of the drill face.

Various systems exist including one that is based on three magnetometers to measure the position (or azimuth) of the earth's magnetic field and three accelerometers to measure the inclination of the tool within the earth's gravitational field. Data is processed at the surface to give real-time location of the BHA. A second system is based on three gyroscopes which are aligned to true north prior to drilling so that they can then detect any deviation for the surface computer to calculate azimuth, inclination and drilling tool orientation. The third method relies on a battery-powered sonde transmitting radio signals to the surface where a hand-held unit can determine the position of the electronic beacon, calculate its depth and display drill-face orientation.

Once the wells are in place, various methods of remediation can be used. Air can be injected into the contaminated underground water via the screens. This volatises the contaminants and removes them to a specific zone known as the Œvadose zone' for treatment or removal. This procedure, known as Œair sparging', is often used in conjunction with vapour extraction to draw out contaminated air. The horizontal wells can also be used to supply nutrients to micro-organisms injected into the contaminants to render them harmless as a means of bio-remediation. More simply, the contaminated fluids can be pumped, via the screens, treated and returned to the environment when remediation is complete. A recent technique has been to inject carbon dioxide via the screen which creates a triple reaction: The gas expands and causes agitation near the well screen which forces the CO2 into the polluted plume; the water temperature drops and carbonic acid is formed.

Although most of the technology for horizontal drilling has come from the US, one development that was evolved in the UK was the Terrafilter screen patented by the British company Drilling Equipment Manufacturing Company (Demco). Demco originally developed the Terrafilter for collector wells in Africa and the Far East to exploit water from wadis and alluvium flood plains but it has been adapted for directional drilling by Bedrock Enterprises in the US where it is known as Hydroquest.

In conventional boreholes, screens are surrounded by a gravel pack which is simply poured into the borehole and sinks under gravity to act as a sand filter. The Demco solution was to design a filter which could be attached to the screen and therefore inserted horizontally. These filter mesh screens provide large open areas to let in the maximum amount of water while preventing the ingress of fine particles.

They can be fitted to any type of base pipe although plastic with an 1/8 inch or 120 slot width over 20 per cent open area is normally used. The three stage filter consists of inner and outer coarse meshes sandwiching the filter mesh geo-fabric. The inner coarse mesh acts mainly as a conducting system for the water filtered by the fabric layer. This facilitates the movement of water from the filter fabric interface to the receiving slots in the base pipe. The integral ribs in this layer of plastic material form a vast network of water conducting channels oriented in such a way as to ensure equal distribution of water to the receiving slots. The outer coarse mesh merely protects the two inner meshes from damage during transportation and installation.

The heart of the filter is the filter mesh fabric, chosen from sieve analysis, to suit the particle size of the aquifer. Even very fine fabric meshes have open areas greatly exceeding the 20 per cent open area of the base pipe. Finally, a heat-shrink seal prevents the three-layered mesh from migrating along the base pipe when it is transported or installed.


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