| Abstract |
PNOC-EDCÆs geothermal well designs primarily consider secondary permeability from fault structures in determining parameters to be used as basis for drilling. Primary permeability from lithological contacts and intra-formations only contribute a minor part to the total permeability of the geothermal reservoir in the Leyte Geothermal Field, except at the Malitbog injection sector where there is proven lithological permeability. This is the area where 5R12D was drilled. The uncontrolled blow-out of this well necessitate accurate designing of the relief well 5R13D. This well considered the permeability attributed to Bayabas and Ding Faults, the inherent permeability within the andesitic tuff breccias of the Mamban Formation, the formation temperatures at shallow depth and the shallow two-phase zone that apparently exists in the sector. There were limited options for the pad location of the relief well since the area is bounded on the west by steep terrain and to the east by the Malitbog River. The base of the slide area located 170m north of well 5R12D was the best possible option and the design would necessitate shallow kick-off at 120m, a high build-up rate of at least 5░/30m, and the steep drift angle of 39░ to target the depth of 458mVD at a limited distance of only 170 m. These parameters would then have to be obtained only through the use of conventional downhole mud motor with single shot surveys. Actual drilling proved to be harder than expected, as a correction run was necessary since the programmed DA was unattainable. But even with the relief well hitting 11 meters below target, the drilling fluids was able to cool down the fractured reservoir that eventually led to the ôdeathö of 5R12D. The geoscientific assessment of the reservoir, precision in the geological prognosis and sound understanding of the reservoir characteristics paved the way to a scientifically based well design. |