PROCEEDINGS, Thirtieth Workshop on Geothermal Reservoir Engineering Stanford University, Stanford, California, January 31-February 2, 2005 SGP-TR-176
REPEAT MICROGRAVITY AND LEVELING SURVEYS AT LEYTE GEOTHERMAL PRODUCTION FIELD, NORTH CENTRAL LEYTE, PHILIPPINES
Nilo A. Apuada, Rhoel Enrico R. Olivar and Noel D. Salonga
PNOC Energy Development Corporation (PNOC-EDC) Energy Center, Merritt Road, Fort Bonifacio Taguig City, Philippines e-mail: [email protected]
Bao valley. Two independent hydrothermal systems
ABSTRACT BILI RAN I SLAND
1238000 Upper Mahiao
Leyte Geothermal Production Field (LGPF) in North R I Upper Mahiao Tacloban L LGPF S 125 MWe P L Central Leyte is the largest geothermal area in the F H e I y L t . e F A G Ormoc U Philippines. It is comprised of two independent L u Upper Mahiao T Camotes Sea lf hydrothermal systems: the Tongonan Geothermal P R L E Y T E 0 50 Field (TGF) in the north and the Mahanagdong KILO METERS
MAHIAO F
W
Geothermal Field (MGF) in the south. 1235000 M Tongonan 1 SAMBALORAN Tongonan 1 P R R I South Sambaloran/Malitbog The commissioning of additional power plants Tongonan-1 MALITBOG 112..5 MWe P R beginning 1996 to harness the full energy potential of Malitbog the area led to a massive mass extraction from the 131 MWe South E field’s reservoir, which to date has incurred a Sambaloran/Malitbog MAMBAN FL R I Mahanagdong B cumulative net mass loss of 291 Mt in TGF and 120 MWe C 65 Mt in MGF. Microgravity measurements, in W F L F BAO tandem with precise leveling survey were conducted L over the area to determine the reservoir’s response to 1231000 Mahanagdong Mahanagdong A exploitation by detecting the minute exploitation- R I 60 MWe induced changes in gravity over time. The most Mahanagdong P R recent measurements were conducted in 2003. MAHANAGDONG Based on Gauss’s Potential Theorem and mass 0 .5 1 Km balance equation, the negative gravity changes from 1997 to 2003 indicate that the system (LGPF) has Mahanagdong NAA/tca, 2003 R I incurred a net mass loss of about 244 Mt and a 4580001227000 462000 466000 natural recharge of about 34 Mt or 12% of the total Figure 1. Generalized sectoral location of LGPF mass removed. production (PR) and reinjection (RI)
areas. The measured decreases in gravity were associated with the mass loss from the reservoir. The data was exist in LGPF: the Tongonan Geothermal Field also correlated with the pressure drawdown of about (TGF) in the north and the Mahanagdong Geothermal 4-4.5 MPa presently experienced over TGF and Field (MGF) in the south. MGF.
TGF occupies approximately 15 km2 of rolling to INTRODUCTION rugged topography and has three production sectors, The Leyte Geothermal Production Field (LGPF) is namely, Upper Mahiao (UM), Tongonan-1 (TGN-1) the largest geothermal area in the Philippines located and Malitbog-South Sambaloran (MB-SS) (Fig. 1). along the northwest trending structures of the TGN-1 in turn encompasses the Mahiao-Sambaloran Philippine Fault in north central Leyte (Fig. 1). LGPF production sector that supplies the first 112.5 MWe comprises six geographic sectors, namely, Mahiao, power plant that began in 1983. Full exploitation of Sambaloran, Malitbog, Mamban, Mahanagdong and TGF started in 1996 with the commissioning of additional 125 MWe Upper Mahiao power plant (1996), the 231 MWe Malitbog power plant (1996- 1997) and the 50 MWe SLI or steamline LEGEND: 5 interconnection (2000). The SLI pipes the excess + Extraction rate + Injection rate .
steam of TGF to the neighboring Mahanagdong O 4 M
Commissioning of more power geothermal field.
R plants & SLI E
P 3 S
Initial extraction rate started at 0.5 to 1.1 Mt per N O T month from 1983 to 1989 (Fig. 2). Of the total 2 C I amount, about 0.1 to 0.5 Mt were injected back into R 112.5 MWe PP T E 1 deep wells. With the commissioning of the additional M power plants, the monthly extraction rate increased abruptly from 1.3 Mt in 1995 to 4.5 Mt in 1998 and 0 further rose to around 5 Mt when SLI was put online. '83 '85 '87 '89 '91 '93 '95 '97 '99 '01 '03 Figure 2. LGPF cumulative monthly mass The cumulative net mass loss from 1983 to present is extraction rate from 1983 to 2003. about 291 Mt.
On the other hand, MGF, located 8 km south of 400 TGF, is divided into two sectors: Mahanagdong-A (MG-A) and Mahanagdong-B (MG-B). MG-A 300 constitutes the 120 MWe main plant and 12 the MWe 0 .5 topping cycle plant, while MG-B supplies 60 MWe to 16 3 the main power plant and 6 MWe to the topping = ed ct cycle plant. Commercial operation of MGF ra 4 200 xt .5 E 2 commenced in 1997. Since the full exploitation of s 21 as s = M os MGF, the cumulative extraction started at 9 Mt in e s L tiv as 1997 and increased to 192 Mt in May 2003 (Fig. 3b). Metric Tons la M 6 u et 103.9 100 m N d = Of the total amount, 127 Mt were injected back into um cte C nje ss I deep wells incurring a net mass loss of about 65 Mt. ve Ma mulati C u m To date, both TGF and MGF experienced pressure 0 drawdown. For twenty years of continuous fluid 1997 1998 1999 2000 20001 2002 2003 extraction, TGF has experienced pressure drawdown Time (Year) of 4 to 4.5 MPa affecting the Upper Mahiao and a) Tongonan Geothermal Field South Sambaloran production fields (Fig. 4). Consequently, various physical and chemical changes 200 occurred in the reservoir mainly in response to field 160 utilization. Such changes include the rise in enthalpy, 2 19 the lowering of the water level, which resulted in the = d te 5 lateral and vertical expansion of the steam zone and 120 ac 6 tr = ex ss the decline of brine discharge (Dacillo and Siega, s lo as ss 7 2003). m a 12 80 e t m = Metric Tons e d tiv N cte la nje u s i um as As early as 1998, pressure drawdown was detected in 40 C m ive MGF as evidenced by declining reservoir chloride lat mu concentration, silica geothermometer and well output, Cu which decreased from 192 to 135 MWe. This 0 1998 1999 2000 2001 2002 2003 pressure drawdown induced the inflow of cooler Time (Year) peripheral waters that has swamped the reservoir (Herras and Siega, 2003). Downhole pressure b) Mahanagdong Geothermal Field measurements recorded the highest drawdown of Figure 3. Cumulative mass extraction from 1997 to about 4.5 MPa in the vicinity of MG-28D, -30D and 2003; a) Tongonan Geothermal Field, b) -31D (Fig. 4). Mahanagdong Geothermal Field.
Repeat gravity measurements in other part of the The purpose of the survey is to demonstrate the world have become a standard geophysical method in importance of repeat gravity survey with regard to monitoring the response of geothermal reservoir with the field management of geothermal reservoirs. exploitation. This technique in tandem with precise leveling survey could yield valuable information on the causes of gravity change between surveys. Legend: Welltrack 308D Wellname TGN-1 Power Plant Upper Mahiao Power Plant 1,240,000 Malitbog Power Plant
0 -1 Gravity contour ( microgal )
- 4
- - 2 1 -3
- - 3 -1 2 -4 1,235,000
0 1. 0 .0 1 .0 - .0
-2
3 0
- .
4 - 1,230,000
-3.0
Wells color code: Tongonan 1 Upper Mahiao S. Sambaloran Malitbog Mahanagdong
460,000 465,000 1,225,000 Figure 4. Pressure drawdown from 1996 to 2002.
GRAVITY DIFFERENCES obtained by correcting the measured gravity differences for the gravitational effects of vertical The main causes of gravity differences at the same ground movements, changes in groundwater level point between surveys are vertical ground and changes in base value. movements and net mass loss from the geothermal field (Hunt, 1977). Other factors that affect gravity For convention, a decrease in gravity is referred to as differences are: changes in ground water level, a negative change and an increase a positive change. changes in saturation (soil moisture content) in the Negative changes imply net mass loss and positive aeration zone, local topographic changes, horizontal change imply net mass gain. ground movement and changes in gravity at the base station. Except for the change in ground water level and the changes in gravity at the base station, all DISCUSSION OF RESULTS other factors affecting gravity differences are The uncorrected gravity changes between 1997 and negligible. Gravity values may also vary with time 2003 (Fig. 5) indicate that there was a decrease in the (in million years) as a result of deep seated regional value of gravity with a maximum difference of –160 mass movements (active volcanism) but because µgal in the Upper Mahiao and TGN-1 sectors. The geothermal fields generally occupy a relatively small greatest decrease in gravity value occurred within the area, and the difference in time between surveys is main TGF production field where it coincides with short, the gravity effects of such movements are the location of the ≥ –100 µgal contours. Away from usually small and can be neglected. the production field, the gravity differences become smaller. Whilst in MGF, the production sector falls The gravity effects of mass movements in the within the region where gravity generally decreased geothermal reservoir, called gravity changes, are by about –40 µgals, which coherently centers on the Legend: 0 -8 Welltrack 308D Wellname TGN-1 Power Plant 100 - Upper Mahiao Power Plant 1,240,000 Malitbog Power Plant -120 -140 0 -1 Gravity contour ( microgal )
60 -80 -1 -60
1,235,000 ( ) -40 -200
(+ ) 40 20 0 1,230,000 -40 2 0 ) Wells color code: (+ Tongonan 1 Upper Mahiao 2 (+ ) 0 S. Sambaloran 0 Malitbog Mahanagdong
460,000 465,000 1,225,000 Figure 5. Uncorrected gravity changes from 1997 to 2003. main production area, extending from MG-23D, -19, mass loss of 212 Mt of geothermal fluids (Fig. 3a). -30D, -28D, -26D, and -14D. A 20-µgal increase in Pressure drawdown contours from 1996 to 2002 gravity can also be seen in the southern reinjection (Fig. 4) indicate that the highest drop in pressure are area near wells MG-5RD, -6RD, -7RD, -8RD, and located in Upper Mahiao and South Sambaloran -9RD. The production area itself does not lie directly (> 4.0 MPa) areas, which also coincide with the over the zone with the largest change in gravity, but location of the highest negative gravity changes rather, along the gradient where the gravity changes (≥ –100 µgals). Similarly, the depressurized 1 bar from 0 to –40 µgals. The centers of the largest gravity PCO2 area has extended from Upper Mahiao to South decrease form two separate “sinks”, one at the Sambaloran sectors (Fig. 7), coinciding also with the southeastern and the other at the western fringes of –100 µgal contour. This depressurized zone, likewise, the production boundary. corresponds to the areas where production wells are already discharging dry steam. The gravity changes between 1997 and 2003 corrected for elevation and base changes are depicted Beginning 1997 when Mahanagdong became online in Figure 6. The values used for vertical gravity for geothermal power production until 2003, the gradient in elevation correction and base correction sector has lost a net total of 65 Mt of mass from the are –298 µgal/m and 24 µgal, respectively. They reservoir as a consequence of exploitation (Fig. 3b). show no significant differences in pattern with that of The production sector this time is roughly defined the uncorrected gravity changes. However, the by a decrease in gravity of 20 to 40 µgals (Fig. 6). gravity values of about –100 µgal are expectedly Compared with the uncorrected data, the region lower because they were corrected for elevation and where there is a decrease in gravity (negative values) base changes. From 1997 to 2003, TGF yielded a net is more extensive, and broadens out to include most
iue6 Corrected gravity changes from 1997to 2003. Figure 6. 1,225,000n 1,230,000n 1,235,000n 1,240,000n Wells color code: Upper Mahiao Malitbog S. Sambaloran Tongonan 1 Mahanagdong Figure 7. 1,230,000 1,235,000 1,240,000 Wells color code: Legend:
Malitbog Upper Mahiao 1 S. Sambaloran Tongonan 1 2 . .0 0 308D 455,000 Malitbog Power Plant PowerMalitbog 1996 PCO contour in bar in contour PCO 1996 02PO inbar contour 2002 PCO Upper MahiaoPower Plant TGN-1 Power Plant Welltrack Wellname
PCO2 contour from 1996 to 2002. 6,0e465,000e 460,000e 2 2
( ) 460,000
2 1
. . 0 0 +) (+ Legend: 308D -1 0 Malitbog Power Plant Upper Mahiao Power Plant TGN-1 Power Plant TGN-1 Power 0 Gravity contour microgal ( ) Welltrack 465,000 Wellname 1 2
3 Km NAA/tca2003
Legend: Welltrack 308D Wellname TGN-1 Power Plant Upper Mahiao Power Plant 1,240,000 Malitbog Power Plant
0 -1 Elevation contour ( cm )
4
1,235,000 0 1
- 2 2 1 - 0 1 - 8 - 6 -
4 -
0 2 - 2 1,230,000
Wells color codes: Tongonan 1 Upper Mahiao S. Sambaloran Malitbog Mahanagdong
460,000 465,000 1,225,000 Figure 8. Elevation changes from 1997 to 2003. of the eastern and the northeastern side of the northwest to southeast orientation and approximately production area. A well-defined 20-µgal increase in follows the trace of the Philippine fault, which gravity encloses the southern and western reinjection transects the area. The subsidence was minimal with areas. only a maximum of about 20 mm and dips towards the northwest in the direction of TGF, where the The ground vertical movements (elevation magnitudes are relatively larger and are probably differences) observed on the network from 1997 to more significant. 2003 were mostly negative (Fig. 8). The highest recorded subsidence (180 mm) occurred at TGN-1 INTERPRETATION production sector 1 km north of well 1R8D. Significant gravity differences at LGPF occurred at
TGF during the survey period 1997 to 2003. The Generally, the amount of subsidence is relatively large negative gravity difference was primarily small since full exploitation of TGF commenced only caused by the net mass loss of 212 Mt of fluid and in 1998, hence, no significant physical evidence on steam (Fig. 3) from the geothermal reservoir due to the surface could be found. Furthermore, the very exploitation. At present, the effect of vertical ground small variance in elevation between 1997 and 2003 movement is still minimal. However, the effect of may be attributed to the zero-waste disposal scheme changes in shallow groundwater level cannot be of the company, which reinjects back a substantial discounted. amount of mass into the reservoir.
The large negative gravity changes at TGF correlate In MGF, an apparent subsidence occurred between well with the liquid pressure drawdown from 1996 to 1997 and 2003. The subsided region has a general 2002 (Fig. 4) with corresponding lateral and vertical 4R9D Inflow of Cooler Fluids 4R10D 4R5D 4R12D 4R11D Original Two-Phase Region
4R3D 405 4R4D 416D 411D 4R6D 417D
408 414D
415 D 407
409 403 Expansion of Two-Phase 410 404 406 Region 4R1 412D 108 4R2D 402 401 110D 106 413D 418D 505D A 109D 111D 311D 102 308D Expansion of Two-Phase 103 208 516D Inflow of Cooler Fluids 101 209
Region 105D 212 2R2 213 307D 309D 515D 215 1R3 202 306D 2R4D 214 310D 302 514D 1R10 2R3D 305D 503 1R8D 301D 304D 513D Expansion of Two-Phase
1R5D Region 1R4DA 511D 303
508D 501 0 1R6D 5 2A' 510D 5R6D 509 506D 5R8D Shallow Two-Phase 5R4 5R5D Outflow 1R7D 5R1D 5R11D 5R10D 5R7D
5R2D
5R9D 5R3D Extension of Shallow Two-Phase Outflow Wells color codes: w/c reverted back Tongonan I to single phase Upper Mahiao S. Sambaloran Malitbog
1230000 1232000459000 12340004620 123600000 1238000 465000 Figure 9. Lateral expansion of the 2-phase zone (after Sta. Ana, 2002). expansion of the 2-phase zone (Figs. 9 and 10). the microgravity measurements by a corresponding Likewise, the depressurized 1 bar PCO2 area where it decrease in gravity values. As shown in Figure 6, up extended from Upper Mahiao to South Sambaloran to greater than 20-µgal decrease in gravity was sector (Fig. 7) corresponding to the areas where observed occurring at the central portion of the production wells are discharging dry steam fall production sector since 1997. The area includes well within the > -100 µgal contour. This also depicts the MG-23D in the southwest, wells MG-30D and MG- lateral expansion of the shallow steam cap towards 28D at the central part, and well MG-24D in the South Sambaloran and the decline of the reservoir northeast. Comparing this figure with the plot of pressure (Dacillo and Siega, 2003). pressure drawdown contours (Fig. 4), it is plain that the maximum drawdown was experienced roughly The exploitation of MGF since 1997 has incurred over the same area, which also borders around wells pressure decline in the central part of the reservoir. MG-30D and MG-28D. This induced the inflow of cooler peripheral waters, particularly from the northwest and has presently On the other hand, the positive gravity change swamped the reservoir thereby reducing the in-situ experienced in the vicinity of wells MG-5RD, -6RD, steam supply. Power output is now at about 135 -7RD, -8RD, and -9RD, which was as high as about MWe, which went from a high of 192 MWe in 1998 20 µgal could possibly be due to reinjected fluids, (Herras and Siega, 2003). with the area being a reinjection sink.
The pressure decline as a direct consequence of mass According to Gauss’s Potential Theorem (Hammer extraction is very apparent in the MGF. In the MG- 1945; Hunt 1970), DL area, for example, extensive drawdown since 1998 has resulted to a decline of almost 5 MPa in ∆M = Σ∆g∆s/2πG reservoir pressure in the central part of the production field. This decrease in subsurface mass is reflected in 1000
800 NW SE 502 600 202 402 102 215 511D 105D 5R2 400
200
) 0 m (
- 200
H Original 2-Phase
T - 400 Region P
E - 600 Gas Region
D Expansion of 2-Phase Region 1996 Expansion of 2-Phase D - E 800 Region 1998
C Expansio U -1000 n of 2- P Ex hase Re D p g 98 ansion 2000 ion 19 Expansion E 200 -1200 1 R
-1400
-1600 Reverted to liquid Reverted to liquid due to RI Returns in due to RI Returns in 2002 ? 2001 ? -1800
-2000
0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 DISTANCE (m) Figure 10. Yearly vertical expansion of the 2-phase zone zone along line A-A’(after Sta. Ana, 2002). where: ∆M is the net mass loss from the system, ∆g incurred a net mass loss of about 244 Mt and a is the corrected gravity difference for area ∆s, and G natural recharge of about 34 Mt or 12% of the total is the gravitational constant (6.67 x 10-11 N m2 kg-2). mass removed. The integrated sum of the corrected gravity difference (Σ∆g∆s) between 1997 and 2003 obtained At TGF, the main causes of gravity changes resulting from the contours in Fig. 6 is about –10205 µgal km2, from mass changes in the reservoir are the liquid hence, the resulting net mass loss from the system drawdown and the saturation changes in the 2-phase zone. This extraction, with a recharge deficit of about (∆M) is about 244 Mt. Using the mass balance 212 Mt reflected a 4.5 MPa drawdown in reservoir equation, pressure, which in turn induced the migration of
cooler peripheral fluids towards the production ∆ (Mw + Ds) – (R + I) = M reservoir, thereby lowering its steam capacity.
where: Mw is the mass withdrawn from the wells, Ds The pressure drawdown experienced in the MGF is a is the surface discharge, R is the natural recharge, and direct consequence of the massive mass withdrawal I is the injected fluid. The natural recharge (R) is 34 from the geothermal reservoir. Likewise, the field has Mt or 12 % of the total mass removed from the lost a total of 65 Mt of water and steam to the system. atmosphere since its exploitation in 1997.
Examination of the contour map of gravity changes It was presented that the trends observed in the (Fig. 6) shows that the gravity changes are generally various reservoir monitoring methods were similarly negative, but the greatest is in TGF area. This is attained using microgravity data. With several sets of where the greatest degree of mass withdrawal microgravity data, trends can also be established that compared with replacement is occurring, probably as will allow tracking of the path taken by reinjected a result of lateral and vertical expansion of the 2- fluids within the reservoir. Reservoir properties, such phase zone (Figs 9 and 10). The negative gravity as porosity and saturation can also be estimated using changes in the reinjection areas indicate that more the method, and will be an invaluable aid in testing fluid is still being withdrawn than being injected. numerical models of the reservoir performance. Given the versatility, the microgravity method could CONCLUSIONS be considered as an alternative if not a better method Based on Gauss’s Potential Theorem and mass in monitoring reservoir processes. balance equation, the negative gravity changes from 1997 to 2003 indicate that the system (LGPF) has REFERENCES Tongonan Geothermal Field, Leyte, Philippines." PNOC-EDC internal report, 72pp. Allis, R.G. and Hunt, T.M. (1986). "Analysis of
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