EFFECTS OF GAS-LIFT PUMPING ON BOREHOLE HYDRAULIC CONDITIONS AT FINNSJON, SWEDEN

The recorded volumes of flushing water used during the drilling of cored boreholes, and the amounts recovered at the surface, have been used to estimate the potential quantities of flushing water and drilling debris injected into fractures. The results show that significant amounts of flushing water and drilling debris may be injected into fractures during drilling. For cored boreholes penetrating major fracture zones, only about 7-8% of the total volume of flushing water used was recovered at the surface. A total loss of flushing water occurred in all boreholes during penetration of these fracture zones. The recovered amounts of drilling debris were also low. Higher percentages of drilling debris were recovered from the air-flush percussion boreholes but, on the other hand, larger total amounts of debris were lost to the rock. During gas-lift pumping in borehole KF110, only very small estimated amounts of the flushing water (3-4%) and drilling debris (approximately 1%) lost to the rock were recovered. The gas-lift pumping was also shown to have a minor influence on the borehole hydraulic parameters calculated from hydraulic tests before and after pumping.