| Abstract |
After two decades of study and monitoring, a mature conceptual model of the Bulalo geothermal reservoir has been developed which serves as the basis for ongoing numerical simulation. Bulalo is a hot, water-dominated, neutral-pH, low-chloride (2,800 mgkg) system that rapidly evolved into a two-phase system upon exploitation. This geothermal system has a structurally controlled core of relatively high permeability surrounded by hot, lower 'permeability rocks. The core region sits astride the 500 ka-old Bulalo dacite dome, which probably is a manifestation of more widespread shallow intrusion. Hot, lower permeability rocks in the west, east and south and low-temperature rocks in the northeast and southwest bound this nearly circular production area. Wells inside the high permeability core have high steam production, low annual production decline rates and fairly constant well chemistry. Noncondensible gas (NCG) content is generally low (0.5 wt%) but the southeast portion of the field is characterized by 2-3 wt% NCG in steam. Southeast Bulalo is also distinguished by abundant calcite deposition that is thought to be responsible for the relatively low permeability in this part of the field. In plan view, the production area appears relatively small but a broad basal region of deep high-temperature (>260░C) surrounds and underlies the main production area. The top of reservoir is shallowest (150- 300 m bsl') in the central core and deepens (21,200 m bsl) and broadens towards the edges. The geothermal system has a fairly well sealed reservoir top as indicated by the relative scarcity of surface thermal manifestations. Geochemistry and high measured temperatures have delineated two upflo w zones: a major Central upflow in the sweet spot and a ' bsl or below sea level minor one in the southeast. Stable wellbore chemistry in these two upflow areas suggests vigorous basal recharge during production. The lateral reservoir boundaries are partially open, and deep outflows (>1,200 m bsl) have been identified in the south and relatively shallow for the north and west. These outt7ows are now avenues of recharge of moderate enthalpy dilute fluids. |