ACIDISATION - BOREHOLE DEVELOPMENT AND REHABILITATION

Acidisation is a technique used to improve the performance of borehole s. It is used in the post-construction development of limestone-chalk boreholes and in the post-operation rehabilitation of boreholes clogge d by incrustations. The technique has been used widely, but not always with a full appreciation of the processes involved. Acidisation proce dures should be designed according to the required effects (e.g. slurr y removal, deep penetration of fissures, incrustation removal etc.). L aboratory studies and test-pumping indicate that the dominant effect o f acidisation in newly-drilled Chalk boreholes is to remove drilling s lurry, 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 vici nity of the point of injection, hence acid should be injected in the v icinity of major yielding horizons where known. For a typical public-s upply borehole in Chalk, it is believed that the current practice of i njecting up to 20 tonnes of concentrated acid is excessive and between a quarter and a half of that amount would probably suffice in many ca ses. Equivalent molar quantities of dilute acid should be equally sati sfactory if ease of handling is advantageous.