Abstract

Acidisation is a technique used to improve the performance of boreholes. It is used in the post-construction development of limestone-chalk boreholes and in the post-operation rehabilitation of boreholes clogged by incrustations. The technique has been used widely, but not always with a full appreciation of the processes involved. Acidisation procedures should be designed according to the required effects (e.g. slurry removal, deep penetration of fissures, incrustation removal etc.).

Laboratory studies and test-pumping indicate that the dominant effect of acidisation in newly-drilled Chalk boreholes is to remove drilling slurry, resulting in improvements in both the B, and especially the C, coefficients in the equation

s=BQ+CQ2

Caliper logging demonstrates that the greatest effects of acidisation are concentrated in the vicinity of the point of injection, hence acid should be injected in the vicinity of major yielding horizons where known. For a typical public-supply borehole in Chalk, it is believed that the current practice of injecting up to 20 tonnes of concentrated acid is excessive and between a quarter and a half of that amount would probably suffice in many cases. Equivalent molar quantities of dilute acid should be equally satisfactory if ease of handling is advantageous.

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