The structural setting of the Macolod corridor is complex, but the earth’s crust in this area is “stretching” or extending due to the influence of regional strike-slip faults at its northern and southern margins. Within the Macolod corridor, the major features are NE-trending normal faults that have produced horst and graben structures and NW-trending left-lateral strike-slip faults that may bound them. There is also some evidence for older E-W directed structures. Alignment of scoria cones and maars in some areas, and andesite to dacite domes in others, also indicates that recent magmas most commonly reached the surface through NE-directed extensional structures. The hydrothermal system developed at Bulalo is associated with two dacitic domes (Mt. Bulalo and Mt. Olila) present on the SE flank of Mt. Makiling. Mt. Makiling is a larger Quaternary volcanic complex consisting of overlapping andesitic to dacitic stratocones and domes. Bulalo and Olila domes have been dated by the 40Ar/39Ar method at <20 thousand years old, providing evidence for sustained silicic magma intrusion into the upper crust, which is the ultimate heat source of the Bulalo hydrothermal system. A NE-trending line connecting these domes has the same trend as the dominant fault and fracture directions revealed by surface and subsurface geology. The spatial association of the highly permeable and productive portion of the Bulalo reservoir with the Mt. Bulalo dacite dome suggests that permeability is related to intersections of faults with its deep conduit system. The resistivity anomaly roughly coincides with the current production area, but extends about 6 km to the north along the eastern flank of Mt. Makiling. The Bulalo reservoir is a liquid-dominated, fracture-controlled hydrothermal system. The productive reservoir contains hot two-phase fluid and is approximately 7 km2 in area. It is roughly circular in plan view, and is bounded by hot, lower-permeability rocks to the NW and SE, and by lower temperature rocks to the NE and SW. Partially open boundaries or deep outflows have been identified on the N, E and W; the NW boundary of the resource appears to be the Cabulugan fault. The top of the reservoir occurs between 100 m (328’) and 1,250 m (4,100’) below sea level. It is shallowest near the field’s center, deepening gradually to the west and north, and deepening abruptly to the east and south. Upflow occurs in the central and SE portions of the reservoir. Relatively porous volcanic tuff units provide important fluid flowpaths at the reservoir top, especially on the western side of the main production zone. The bottom of the reservoir is unknown, but appears to be deeper than 3,050 m (10,000’) bsl beneath the central production area. The reservoir fluid is a neutral-pH sodium chloride liquid with low total dissolved solids and low gas content relative to other geothermal systems in the Philippines, making it especially attractive as an alternative energy source to fossil fuels. The fluid salinity is also low, with an average Cl concentration of 2800 mg/kg. The average reservoir temperature is 280°C, and the maximum temperature is around 300°C in the SE sector of the field. 
As of 2003, 109 wells have been drilled throughout the field. Of these, 71 wells provide current steam requirements, 15 wells are used for brine and condensate re-injection, while the remaining wells are inactive, plugged and abandoned, or unproductive. The deepest well has a measured depth of 3,625 m (11,890’) while the shallowest is 655 m (2,148’). The average well depth is about 1830 m (6000’). Average steam and brine flow rates are 13 and 14 kg/s (102 and 111 klbs/hr), respectively from a design pressure of 0.7 MPa (95 psig) down to pressure below atmospheric (vacuum). Sub-atmospheric pressure at the turbine outlet is maintained by direct-contact condensation of the expanded steam. This process, which occurs at the condenser, liberates the non-condensible gases that were fractionated into the steam phase during the steam/brine separation. To prevent pressure gas build-up in the condenser, equipment is employed which diminishes carbon dioxide and hydrogen sulfide gas present in the steam. Chevron’s philosophy is to carry out essential reservoir monitoring needed for field management on a cost-effective basis. We are committed to developing and using innovative technologies where possible, but most reservoir monitoring techniques employed by PGI are conventional “tried and true” technologies. These core monitoring activities supply the necessary information to reliably provide geothermal energy at a competitive price. The primary measure of reservoir performance is steam deliverability. This is established from knowledge of reservoir permeability, reservoir pressure, and individual well production. Reservoir pressure is directly measured downhole across known permeable zones using heat-resistant pressure and temperature registering tools. Permeability is estimated using production and injection test methods in conjunction with Enthalpy/Flow tests, various types of downhole logs. Accounting of individual well geochemistry, production rate and enthalpy values are determined through chemical analysis and flow measurements using immiscible tracer chemicals (tracer flow testing or TFT). The tracer chemicals are injected at the wellhead and samples are taken downstream along the production line. Since the reservoir produces two-phase fluid, both pressure and enthalpy are needed to establish the thermodynamic state of the fluid and properly apportion steam and brine in the total flow. To explore the development potential and/or limitation of the resource, numerical simulation of the reservoir is employed. Detailed simulations matching field production history including gravity changes have been accomplished and are periodically updated. Numerical simulation is an important tool in validating both the tactical field operational plans and the reservoir development strategies to maximize the benefit of exploiting the resource. Geochemical monitoring is among the most important means of quickly understanding and responding to changes in the geothermal system. The monitoring program at Bulalo encompasses quarterly sampling of production wells and injected brine, semi-annual sampling of steam lines, and daily sampling of steam entering the power plants. All analyses are currently performed at the ISO 25 certified onsite laboratory facility. These data are used along with flow rate data to “take the pulse” of producing wells. Geophysical monitoring provides data critical for management of geothermal reservoirs. The most important geophysical monitoring method has been precision gravity and leveling surveys, done annually or bi-annually since 1980. Gravity models have therefore been utilized in reservoir simulation based on well data as a “matching or calibration tool,” or independent check of results. Gravity modeling for the period 1980 to 1984 suggested that known mass withdrawals did not require extensive recharge to the main area of production, however, recharge may have increased to about 30% of the total mass extracted during the period from 1984 to 1991. Efforts to integrate more recent gravity survey with reservoir modeling are currently underway. Bulalo is a mature geothermal field having produced for over two decades. The field has remained relatively problem free aside from the need to move injection outward on the western side. PGI, in cooperation with NPC, has responded to overcome this problem in the early 1990’s. The decision to push through and accelerate the development of the indigenous resource was most astute and beneficial to the Philippines at that time. When the NPC-PGI service contract was signed, oil price was less than US$4/bbl. By the time commercial operation commenced, prices had quadrupled. A financial study conducted by ELC-Electroconsult for the Asian Development Bank showed that the net cash flows including avoided oil cost had paid for the project in 1986. The center of production will remain the currently developed area, although new wells will tap deeper sources of steam. PGI’s strategy is now focused on deep development to maximize the generation potential of the field and ensure its long-term sustainability. Rehabilitation of the power plants will improve steam usage, improve reliability and increase capacity. Currently available steam based on current usage rate of the power plants is equivalent to about 290 MW. Ongoing projects should increase steam availability to 402 MW at the steam usage rate of the rehabilitated plants by January 2005. |
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