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Geothermal Resources Council Transactions, Vol. 24, September 24-27, 2000 Interaction Between a Deep Geothermal Reservoir and a Shallow ~ r o ~ ~ d System: ~ a t The ~ r Case of the So~the~~ Negros ~ ~othe~~al Production Field and Metro Dumaguete Water District, Philippines ].A. Caranto', Pot. Pamatian', B.C. Vidal', R.R.A. Isidro', M.S. Ogena', R.V.J. Pascual', L.F. Bayrantel, S.E. Garcia' and Y. Yurtsever* PNOC EDC Geoscientific Department, Geothermal Division, Fort Bonifacio, Makati City 1 201 Philippines *Hydrology Section, f nternational Atomic Energy Agency, Vienna, Austria ABSTRACT The Southern Negros Geothermal Production Field has maximum reservoir temperatures greater than 300 C and fluids with baseline chloride concentrations of about 4100 ppm. Commercial operation of the field since 1983 resulted to major changes in the geothermal reservoir (Le. reinjection returns, pressure drawdown, inflow of cool acid fluids and mineral deposition). The isotopic composition of discharges from production wells indicates that the average infiltration altitude of meteoric water into the geothe~a~ system is at 1000 masl. The shallow groundwater in the SNGPF and Metro Dumaguete areas may be classified, based on stable isotope and geochemistry, as HC03-rich, Na+K-Cl+S04 type and Ca+Mg-Cl+S04 type. The isotopic composition of meteoric water indicate that the iso- tope altitude gradient for 6I8O and 62H are 0.28 and 2. I%o per 100 meters change in elevation, respectively. These correspond to a calculated recharge elevation of 1000-1400mas1, which appears too high for water sources located at 5-100 masl compared to the calculations by Pascual(l993). There is a general depletion of heavy isotopes in rainfall from 1993 to 1997 but this is not manifested in the major element chemistry of the groundwater. Seasonal variation, recharge of isotopically depleted waters and mixing of steam condensate are the possible causes of the decline in heavy isotopes. The natural outflow of the Palinpinon geothermal system extends further to the east as manifested by the presence of warm water seepages along the Okoy river valley and in the chemistry of four MDWD wells located 2-3 km east of the Palinpinon hotsprings. The changes in the chemistry and temperature of the Palinpinon springs and in the Chloride concentration of some groundwater wells are attributed to seasonal variations. Shallow wells located four kilometers to the north- east of Palinpinon thermal springs are already diluted by more than 50% in major ionic composition. Fluids are probably channeled through major northeast-trending structures. The mixing of mineralized fluids from the outflow of the Palinpinon geothermal system with some MDWD groundwater wells is a natural occurrence channeled through the east-northeast-trending Palinpinon fault. introduction The shallow groundwater systems in Dumaguete and surrounding areas were discussed in several hydrological and hydrogeological studies (e.g. A. Lazaro and Associates, 1991 ; Geotechnica, 1994). The present study focuses on the possible interaction between the shallow groundwater system and the deep geothermal reservoir. This study is partly funded by the International Atomic Energy Agency (IAEA) under the Co-ordinated Research Program (CRP) entitled "Isotope Figure 1. Simpli~ed geologic map and location of water sources of the Okoy and Banica watersheds. (Adapted from PNOC EDC, - and A. Lazaro & Associates, Inc., 19911. 339
Response of Hydrogeological Systems due to Long-term Exploitation". Isotope investigations in the Southern Negros Geothermal Production Field (SNGPF) were conducted since 1980 and in the Metro Dumaguete groundwater system starting 1993. Combined with geochemistry, this study also uses stable isotope in the characterization of the groundwater. The study area covers the Okoy and Banica watersheds which include SNGPF, Palinpinon, Sibulan, Dumaguete, Valencia and Banilad (Figure 1). A total of 45 samples were collected, out of which 28 were analyzed for isotope compositions. The objectives of the study are: (1) to determine the physical, chemical and isotopic characteristics of the deep geothermal and shallow groundwater systems and to establish a set of baseline data on the shallow groundwater; (2) to evaluate the dynamic changes in the deep geothermal and shallow groundwater systems due to long-term exploitation; (3) to ascertain any relationship between the deep geothermal system and the shallow groundwater system; and, (4) to model the different shallow groundwater hydrogeological regimes using numerical simulation techniques. B r ief Geology The main lithology covering the SNGPF area are Quaternary volcanic deposits composed of andesitic lavas and dacite pumice flows which unconformably overlie the Southern Negros Formation (SNF) consisting of conglomerates, breccias, tuffs, pillow lavas and sandstones. Beneath the SNF is a thick sedimentary sequence of moderately to intensely altered calcareous siltstones, sandstones, and calcisiltites (with minor intercalations of andesitic lavas and breccias) of the Okoy Sedimentary Formation (OSF). Towards the east, the area is covered by Quaternary pyroclastic deposits. In the Lower Okoy valley where the Okoy and Banica river nearly converges, the rugged terrain breaks into a gentle plain. The lithology is composed mainly of Quaternary alluvial deposits of sands, clays and gravels with minor marine deposits of limestones and corals. The major structures in the SNGPF are the north-northeast and north-northwest-trending steeply-dipping normal and strike-slip faults that extensively crisscross the area (Hermoso and Mejorada, 1997). Inferred faults based on topographic maps were also observed in the eastern part of the watersheds (Figure 1). lithologic contacts, primary or in traformational permeability among pervious volcaniclastics of the SNF and along the chilled margins of andesitic dike intrusions within the SNF and the OSF (Hermoso and Mejorada, 1997). Meteoric recharge into the deep geothermal reservoir is limited in areas with intense fracturing, and the flow of the geothermal fluid is mainly towards the northeast and southwest where the hotsprings are found. The flow of the shallow groundwater is likewise towards the east following the topography. At the Lower Okoy valley, the Quaternary alluvium consists of highly permeable fine to coarse sand, gravel and boulder with occasional poorly permeable clay beddlenses and marine sediments (limestone, corals). Southern Negros Geothermal Production Field The Southern Negros Geothermal Production Field (SNGPF) hosts a geothermal system associated with the dormant, Quaternary Cuernos de Negros volcano. Operating since 1983, the field provides steam to the 112.5 MWe power plant in Palinpinon-I sector, and to four (4) units of 20 MWe modular power plants in Palinpinon-2 sector. Seventy-five (75) wells have been drilled with an average depth of 2900m and mostly deviated at an average throw of 1000m. Based on geoscientific and reservoir engineering evidences, hot geothermal brine with pre-exploitation temperature of 328 C and chloride concentration of about 4100 mg/kg, upflows in the vicinity of Laguna0 dome and flows outwards in the northeast direction towards the Lower Okoy valley and in the southwest direction to the Nasuji-Sogongon sector (Hermoso and Mejorada, 1997) (Figure 2). Isotopic evidences indicate that the parent geothermal fluid is a mixture of meteoric (80%) and magmatic waters (20%). Meteoric recharge into the geothermal system comes from an average altitude of 1000 mas1 (Gerard0 et al., 1993). H yd rogeology At the SNGPF area, the residual soil development on top of the volcanics are limited in depth due to the relatively fresh volcanics. The faults, fissures and joints as well as the degree of openings and in-fill materials in the joints play a key role in the movement of subsurface waters. Permeability is attributed to fracturing due to faulting, but significant permeability is also present along Figure 2. Conceptual Model of the Southern Negros Geothermal Production Field and the three types of waters at study area. 340
Characterization and Changes in the Deep Geothermal System Due To Exploitation The geothermal exploitation since 1983 have subjected the geothermal reservoir into four major processes, namely: ( 1) reinjection returns, (2) pressure drawdown, (3) inflow of cool acid fluids, and (4) mineral deposition (Orizonte et al., 1999). Reinjection (RI) returns were identified from the increase in C1, decline in C02 and C02/H2S ratio, decline in CI/Ca ratio and lowering of downhole temperatures and discharge enthalpies in the affected production wells. The RI fluid s rapid return, having a mean transit time of 5 to 18 days based on Iodine- 131 ( Inll) tracer, has resulted in severe thermal declines from 5 C to 30 C in most heavily affected wells (Urbino et al., 1986). Pressure drawdown of around 6 MPa at the production sector has enhanced reservoir boiling and expansion of the two-phase zone. This is evidenced by physico-chemical indications such as increase in C02 in the total discharge and increase in discharge enthalpy. Cool acid inflow is a secondary type of recharge common among wells directed towards the high-gas upflow region. The sulfate-type acid- ity is derived from the oxidation of H2S in shallow groundwater forming an acid-so4 perched aquifer (Seastres et al., 1995), where the resulting acid is induced downwards along faults into producing horizons. Thus, acidity is commonly associated among wells with high-enthalpy discharges and high gas contents. Mineral deposition of silica (SO,), calcite (CaC03) and anhydrite (CaS04) as blockages in geothermal wells greatly reduces the capacity of the well. Silica reduced the observed reinjection capacity of the wells by as much as 50%. Calcite deposits, induced by boiling of fluids in the geothermal reservoir, were observed within the area of the flash point of the wellbore. Blockages of anhydrite were likewise observed in wells, normally just below the point of the acid-sulfate inflow (Orizonte et al, 1997). Little change in the chemistry of the thermal springs were observed from 1996 to 1999 possibly due to lower mass extraction at SNGPF. In a simple conceptual model, the Kaipohans and slightly acid to acid-so4/hco3 springs are located directly beneath the upflow region which are formed by steam condensation at near surface. Further down the slope from the upflow region the steam-heated HC03 springs are formed by heat transfer through low-permeable rocks. Further away the outflow of the geothermal brine mixes with groundwater forming the C1 to HC03-C1 springs. Shallow Groundwater System Field parameters, basic chemistry and isotopic compositions were used to delineate the different types of fluids in the study CATIONS ANIONS Figure 3. Piper diagram of shallow groundwater, springs and rivers at SNCPF and Metro Dumaguete. area. Field characteristics include conductivity, salinity and temperature while the basic chemistry includes the basic cations (Ca, Mg, Na and K) and anions (Cl, SO4 and HC03). There is an increasing trend in conductivity, salinity percentages and temperature from the south to the north towards Sibulan. Well 5554 consistently showed elevated values for the three field parameters mentioned. The piper diagram of the basic chemistry (Figure 3) and Schoeller diagrams suggest that the wells south of Banica show relatively low C1, SO4, Na and Li but elevated in HC03 relative to the wells to the north towards Sibulan. Four MDWD wells (5549,5553,5554 and 5555) show elevated C1, SO4, Na and Li which are located 2-3 kilometers east of the Palinpinon springs. Combining piper and Schoeller diagrams, the three groups of waters in the study area are: (I) Na+K-C1+S04 waters, (2) Ca+Mg-HC03+C03 waters and (3) Ca+Mg-Cl+SO, waters (Figure 2). At closer view the plots show a continuous trend from one group to another. It can therefore be assumed that there is a transition or mixing between the different water types. At the lower Okoy and Banica watersheds, there has been no observed regional decline in groundwater levels. Localized cone of depression of about 15 meters was observed within the immediate vicinity of the MDWD wells 5546, 5547 and 5548 due to the extraction of the MDWD (Geotechnica 1994). Along the Okoy river, the water level has considerably risen all the way to the edge of the volcanic terrain. The main recharge for both the Banica and Okoy watersheds are around the foothills of the volcanic terrane where lenses of the alluvial deposits drape into the volcanics. Recharge within highly fractured faults along 341
the Okoy and Banica river were also determined using f~cture-densi~ measurements (~eotechnica, 1994). 1400 1200 II 1000 400 Figure 4. Isotopic composition of different water types at SNGPF and Metro Dumaguete. Equation of line: Y = - 291.7 (del%) - 914.2-6.8-6.4-6.0-5.6-5.2-4.8-4.4-4.( Weighted Mean del '80 Figure 5. isotope attitude gradient for del '*O. Stable Isotope and Chemistry With Time The Local Meteoric Water Line (LMWL) was updated using available precipitation isotopic data ( 1991-1 997). The excess deuterium was recalculated and reduced to 12.5%0 from the previous value of I4%o (Gerard0 et al., 1993) making the equation of the line 8H = 8 H80 + 12.5. Isotopic compositions of the deep geothermal fluids prior to the commercial operation in 1983 are also plotted for reference (Figure 4, overleaf). Using a plot of Cl concentration versus 6I80 values, the water sources could be classified into having low, moderate and moderately high chloride values. The sources classified as having low Cl are the wells characterized as Ca+Mg- HC03+C03 type, while the moderate and moderately high C1 are the Ca+Mg-Cl+SO, type and Na+K-Cl+SO, type of waters, respectively. Isotopic composition of the groundwater for the years 1993, 1995 and 1999 show significant decreasing trends of isotopic compositions. The dep~etion in isotopes could be attributed possibly to: (1) increase in recharge of isotopically depleted waters, (2) seasonal variations, and (3) steam addition. Additional sets of data will likely verify if there is indeed an effect of seasonal variation to the isotopic compositions. The weighted averages of the isotopic composition of rainwater at the four monitoring stations in Ticala, Puhagan, Balas-balas and Nasuji are defined by the equation h = - 347.3*~~~ - 1266 and k = - 47.52*62H - 857.24 with correlation coeficients of 0.90 and 0.82, respectively. The value of h is the mean elevation of meteoric water recharge. Based on the equations, the average isotopic composition of rainfall at sea level is -3.65%0~180 and -8.O4%0 6*H corresponding to a decrease of 0.28%0 and 2.1%0, respectively, for every 100 meters increase in elevation. Figure 5 shows the isotope altitude gradient for 6W. All available rainfall isotopic data with deuterium excess of IO- 14 was used in the calculation of the elevation of recharge. Based on the equations above, the calculated elevation of recharge is from 1000 to 1400 masl, which fall within the area of Nasuji and Sogongon at SNGPF, Mt. Talines, and the slopes of Cuernos de Negros volcano. The calculated recharge elevation appear to be too high relative to the calculations by Pascual (1993) for wells that are located only at about 5-100 masf. Only the cold springs 5524 and 5544, located at 342
relative to the waters in well 5554. This suggests that the geothermal outflow from the Palinpinon area mixes with meteoric water and flows towards the northeast to Sibulan through major structures. It appears that slightly mineralized fluids are being channeled along the Lower Okoy Splay fault through the Lower Okoy Fault and manifest itself in the area where Mainit well is located (Figure 6). The geothermal fluids from Palinpinon, therefore, mixes and forms an interface with the shallow groundwater towards Camanjac. In the Mainit well area, the re-emergence of less mineralized geothermal fluids creates another boundary, the extent of which could possibly be just within the immediate vicinity of the well. Figure 6. Postulated interface of the geothermal outflow and the shallow groundwater. 7 10 and 3 10 masl, respectively, may be receiving meteoric water recharge from 1000 masl, but this may not hold true for the other water sources. There are noticeable depletion of isotopic values from 1993 to 1997 but the major element chemistry of the shallow groundwaters do not show the same trend. This general decreasing trend possibly accounts for the variance in the calculated meteoric water recharge values of Pascual(1993) and this study. Some significant chemical parameters (e.g. C1 and Li) monitored from the Metro Dumaguete wells reveal that there is a noticeable increase in C1 and Li concentration from December 1996 to May 1997 from 60 to about 100 ppm and from 1.0 to 1.3 ppm, respectively. The variations could possibly be attributed to seasonal variation as the Dumaguete area receives less amount of precipitation fkom January to May based on a 44- year precipitation data. This suggests that the well receives considerable recharge amount from precipitation. Inferred Interface Between the Geothermal System Outflow and Shallow Groundwater Along the Okoy river valley where the Okoy and Banica rivers nearly converge before branching out in different directions, the natural geothermal outflow is physically manifested in the form of warm springs (e.g. 5540 and Pal 3). The geothermal fluids are probably channeled through the Odlumon, Palinpinon and Odlumon Splay A Faults and rise up close to the surface before the contact of the volcanic terrain and the Quaternary alluvium. These fluids cross the geologic contact through the Palinpinon Fault and contribute to the relatively elevated Na, Li, B, Cl(5554) and As concentrations in wells 5549,5553, 5554, and 5555. Waters in well 5532 located 3.75 kilometers away from well 5554, is already diluted by more than 70% in As, Li and B, and about 50% in C1, Na and K concentrations Conclusions The SNGPF hosts geothermal fluids having temperatures of more than 300 C and salinity of about 4100 ppm Cl. These fluids upflow in the vicinity of the Laguna0 dome and flow outwards to the northeast in the Lower Okoy valley and to the southwest to the Nasuji- Sogongon sectors. Isotopic studies suggest meteoric recharge from an elevation of about 1000 masl. Fifteen years of geothermal exploitation resulted to four major changes in the geothermal reservoir: (1) reinjection returns, (2) pressure drawdown, (3) inflow of cool acid fluids, and (4) mineral deposition. Based on stable isotope and major element geochemistry, the shallow groundwater in the Lower Okoy valley is grouped into three major water types: (1) Na+K-Cl+SO, waters, (2) Ca+Mg-HC03+C03 waters and (3) Ca+Mg-Cl+SO, waters. An updated precipitation isotopic composition was used in recalculating the deuterium excess of 12.5, making the equation of the LMWL &H = 8 + 12.5. Recalculation of the isotope altitude gradient revealed a decrease in PO and BH of 0.28%0 and 2.1%0, respectively. The calculated elevation of recharge ranges from 1000 to 1400 masl, which appear too high for water sources located about 5 to 100 masl. Seasonal variation in C1 and Li concentration were observed in MDWD well 5549. This signify that the aquifer in the vicinity of the well allows for faster rate of infiltration and percolation into the groundwater aquifer. There are significant decline in isotopic compositions in wells 5503, 5504, 5507, 5546 and 5547. The depletion in the isotope values could be attributed to seasonal variations, recharge of more isotopically depleted waters, or steam addition. The geothermal fluids from the natural outflow in Palinpinon extends further to the east as manifested in the chemistry of wells 5555, 5549, 5553 and 5554. Some of the mineralized fluids cross the contact of the volcanic terrane and the Quaternary alluvium, mixes with the groundwater and flows on the subsurface towards the northeast. 343
There has been no change in the physical and chemical characteristics of the thermal manifestations since 1996 probably due to the lower mass extraction at SNGPF. The observed changes in the temperatures of the springs are mainly due to seasonal variations. "he mixing of mineralized fluids in the ~~~D wells 5555, 5549, 5553 and 5554 possibly through Palinpinon Fault is a natural occurrence extending from the geothermal outflow in Palinpinon. Recommendations and Future Work Plans I. Continue with the numerical simulation using MODFLOW and later using FEFLOW, a program capable of simulating groundwater flow, cont~inant transpo~, heat transport and fluid density affected transport processes. 2. Conduct sampling on selected shallow and MDWD wells for tritium analysis. This will supplement the isotope data and will provide information on the level of tritium in wells in the study area relative to the 1993 tritium data. 3. Continue sampling for chemistry and isotope analyses for two more seasons (1 year) to fully deter~ine the processes affecting the isotopic compositions of the shallow wells and MDWD wells. References Gerardo, J.A., Nuti, S., D'amore, E, Seastres Jr., J.S., and Gonfianti, R., 1993. Isotopic evidence for magmatic and meteoric water recharge and the processes affecting reservoir fluids in the Palinpinon geothermal system, Philippines. Geothe~ics Vol. 22, No 516. Pp. 521-533. ~eotechnica Co~oration, 1994. Final report, Integrated watershed development plan and hydrological study for Okoy and Banica watersheds, SNGP. Vol. 11. Hermoso, D.Z. and Mejorada, A.V., 1997. The Palinpinon 1 production field: a case study for reinjection breakthrough. Geological Society of the Philippine Convention, 1997. Orizonte, R.G., Amistoso, A.E. and Aqui, A.R., 1999. Reservoir management during 15 years of exploitation; Southern Megros Geothermal Production Field, Philippines. Proceedings WGC 2OOO. Pascual, R.V.J., 1993. Isotopic investigation of ~oundwater in Okoy-Lower Banica watersheds, Southern Negros Geothermal Project. PNOC EDC Internal report. Seastres, J.S., Hermoso, D.Z. and Gerardo, J.Y., 1995. Application of stable isotopes in evaluating the reservoir changes at Palinpinon Field during exploitation. Proceeding of the 16" Annual PNOC EDC Geothermal Conference, pp. 43-53. Urbino, M.E.G., Zaide, M.C., Malate, R.C.M. mdbueza,e.l., 1986. Structural flowpaths of injected fluid based on tracer tests-palinpinon I, Philippines. ~~ceedings, 8fh New Zealand Ge~ihe~~al ~ ~ r ~ h ~ p, A ~ ~ pp ~ 53-58. u ~. Vidal, B.C. and Pamatian, PI., 2000. SNGP thermal springs update for years 1998 and 1999. PNOC EDC internal report. 344