Grouts, well seals

If bentonite pellets or chips are to be used instead of grout, the bentonite pellets can be placed in shallow wells (within 15.0 m of ground surface) by pouring them through the annular space from above in much the same manner as the filter pack is placed. If this installation procedure is chosen, sufficient potable water must be added in conjunction with the bentonite to hydrate the individual pieces. Hydration causes the pieces to swell and form a highly impermeable seal. 

Remediation commenced with the interception of the recoverable gasoline. One 4-in.-diameter vapor extraction well and three 2-in. vapor monitor wells were installed. Each well was constructed with 4 ft of well screen placed and filter packed across the unsaturated portion of the shale. The annulus of the wells was sealed to the surface with bentonite and cement grout according to local standards. The monitor wells were located in a triangular pattern around the vapor extraction well at distances of 5.3, 10.6, and 25.4 ft. 

When it is required to seal the steel casing of gas and oil wells to the walls of the borehole and to seal porous formations, a grout mixture is used. Both Portland and pozzolanic cements are used as setting agents. The latter consists of slaked lime and a pozzolan. The setting time of the pozzolanic cement is shortened by the addition of an accelerator (e.g. soda ash which is immediately causticised by the lime) and by the relatively high temperatures at depth [32.27]. 

Reservoir sites in limestone terrains vary considerably in their suitability. Massive horizontally bedded limestones, relatively free from solution features, form excellent sites. On the other hand, well-jointed, cavernous and deformed limestones are likely to present problems In terms of stability and watertightness. Serious leakage usually has taken place as a result of cavernous conditions that are not fully revealed or appreciated at the site investigation stage. Indeed, sites are best abandoned where large numerous solution cavities extend to considerable depths. Where the problem is not so severe, solution cavities can be cleaned and grouted (Kannan, 2003). In addition, reference has been made by Milanovic (2003) to the application of a blanket of shotcrete to seal areas of karstic rock in reservoir basins. However, wet rock surfaces are not suitable as far as the application of shotcrete is concerned and neither is it wise to allow groundwater pressure to build up beneath shotcrete. 

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