| Abstract |
Core analysis integrated with other data indicates that the top of the Bulalo geothermal reservoir is a corrugated surface controlled by the first point of downward continuous open space in linked fractures that terminate upward in ductile clay-rich lithologies or are blocked by partially sealed veins. Fault-valve action likely mediated cycles of upward growth and resealing of such fracture systems at initial conditions. Declining fluid pressure and minimum stress influence the depth and integrity of the reservoir top seal once fluid production is underway. The presence of large fractures that are locally open, the declining minimum stress due to fluid production, and thermal affects associated with pumping cold fluid in a hot borehole all impact the ability of seal rocks near the reservoir top to maintain fluid pressure during leakoff tests (LOT). Considering these factors, LOT are of limited value in determining if well design criteria have been met. Plug porosity (4 to 14%) and permeability (<0.001 to 0.1 md) are typical of reservoir rocks, but permeability ranges to higher values (0.1 to 0.2 md) for samples that have well-oriented, partially filled fractures parallel to the plug axis. Horizontal plugs from the ash-flow tuff have higher permeability than vertical plugs from the same rock indicating welding and hydrothermal processes promote higher horizontal permeability. Porosity and vertical permeability of whole core is similar to that measured on plugs, but permeability measured in the horizontal plane ranges to higher values (4 to 9 md) due to sampling of larger more partially open fractures proximal to faults. |