| Abstract |
The resevoir rocks of the Leyte Geothermal Field consisf largely of highly fractured andesitic lavas and pyroclasfics intercalated with minor /enses of terrestrial to marine sedimentary rocks. Pasf drilling operations using conventional drilling technology in the reseruoir rocks have indicated that significant formation damage occurred due to a combination of mechanical and chemical damage effects assocrafed with /oss of drilling fluids into the formation. Well 409, a 12- MWe well in Upper Mahiao ceased discharging after an adjacent well 419D was drilled blind for four days. Another well, 307D, likewise ceased discharging after numerous cement plugs were conductedin adjacentw ell 313D. To minimize formation invasion-related problems, PNOC-EDC adopted the use of Underbalanced Drilling (UBD) technology. Aside from ,fs widely recognized benefit of reducing damage caused by invasion of drilling fluid into the formation, additional benefits are seen due to reduction in drilling time, early detection and dynamic testing of permeable zones while drilling, minimizing circulation /oss and increasedp en e trationr afes. Well 422D rs the first well drilled using UBD or air drilling. The well was drilled safisfactorily, being drilled 1 10 meters short of its programmed total depth of 2700 meters and 10 days more than its programmed completion date. Possrb/e drilling interference to adjacent wells was measured through water sampling and analysis for Total Suspe nded So/ids [SS/. ISS monitoring resu/fs among adjacent wells yielded positive resu/fs, with filtered samples possibly consisting of rock cuttings and formation, but their levels were generally insignificant. Steamflow measurementasm ongf hese wellsi ndicatedg enerally stable outputs. Another well that was drilled using airdrilling technology was well 315D. The well encountered multiple permeable zones and a highly fractured formation that made it difficult to esfab/rsh returns resulting to poor hole cleaning. Adjustments in mud properties had to be made which was aggravated by pump mechanical problems. lnterference monitoring in adjacent wells yielded relatively high ISS levels with recovered samples akin to the mud components used. |