Proceedings World Geothermal Congress 2015

Melbourne, Australia, 19-25 April 2015

1

30 Years of Tongonan-1 (Leyte, Philippines) Sustained Production

Marie Hazel C. Uribe, Danilo B. Dacillo, Lorena M. Dacoag, Romeo P. Andrino, Jr. and Edwin H. Alcober

Energy Development Corporation, Brgy. Tongonan, Ormoc City, Leyte, Philippines

colo.mhb@energy.com.ph

Keywords: Sustainability, Production, LGPF

ABSTRACT

Tongonan-1 in Leyte, Philippines has achieved a milestone for sustainable production for 30 years. Reservoir management played a

key role in the continuous power generation and in conquering the challenges since the commissioning of the Tongonan-1 power

plant in 1983. The series of challenges in its 30-year history has been a source of lessons and the reason for the sound production

and injection strategies that have been implemented.

Cooler injection returns, blockages, erosion and pressure drawdown are among the challenges that have been met squarely by the

early and present resource managers. Technological knowledge and developments in field management, geochemistry and reservoir

engineering were crucial in analyzing and addressing these challenges. Likewise, experiences in targeting M&R wells proved to be

significant in achieving the sustained production.

1. INTRODUCTION

The Leyte Geothermal Production Field (LGPF), located in the province of Leyte in the Eastern Visayas region of the Philippines,

is known as one of the largest liquid dominated reservoir in the world. It is comprised of two distinct hydrothermal systems -

Tongonan to the north and Mahanagdong to the south, separated from each other by the low permeable Mamban block.

Figure 1: Location Map of the Leyte Geothermal Production Field

The Tongonan geothermal system encompasses four (4) sectors – Upper Mahiao, Tongonan-1, Malitbog and South Sambaloran,

over a total resource area of 15km2. The Tongonan baseline hydrological flow model identified a convective upflow centered in the

area bounded by wells in Upper Mahiao and Tongonan-1. Major outflow was identified towards the Malitbog sector and some

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minor outflows towards the north, northwest and northeast directions in Upper Mahiao. Noteworthy of the field is a natural

recharge in the form of a cooler fluid in the eastern region of the Upper Mahiao sector.

Figure 2: Tongonan Hydrological Flow Model

The experience of LGPF in the geothermal industry has come over from a long journey and can be considered one of the pioneers

in this ground. Field surveys started as early as in the 1950’s where the Commission on Volcanology completed a geothermal

feasibility study that led to conception of the PNOC-EDC in the 1970’s, tasked to develop the geothermal resources in the country.

This paper will focus on the journey of the Tongonan resource, where it all began for EDC, making the 30 years of Tongonan-1

Power Plant sustainable production possible – and the rest is history.

2. TONGONAN RESOURCE RESPONSE TO PRODUCTION

Commercial operation in the Leyte Geothermal Production Field started with the commissioning of the 112.5MW Tongonan-1

power plant in 1983. Twelve production wells were initially hooked - up to the system to provide the steam supply requirement of

the power plant. Mass extraction in the Tongonan resource further increased with the commissioning of additional power plants -

232MW in Malitbog and 132MW in Upper Mahiao during operational expansion in 1996 to 1997. This also leads to the increase in

brine injection to the resource.

Figure 3: Tongonan Production History

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Initial response of the Tongonan field after the start of large-scale production showed a large decline in the reservoir pressure

between the periods of 1996 to 2002. Likewise, discharge enthalpy of most production wells across the Tongonan field began to

increase, transforming liquid producing zones to highly two phase producers. However, discharge enthalpy declined in dry area

encroached with injection returns and is most notably at the wells in proximity to the injection sinks. Injection management

strategies were then implemented that stabilized field enthalpy in Upper Mahiao, Tongonan-1 and Malitbog sectors. Also, the

drilling of new deep low enthalpy wells affected field enthalpy trends.

Figure 4: Tongonan Pressure and Enthalpy with Time

The pressure decline during increase in commercial operation resulted to secondary reservoir processes that affected steam supply

of production wells with time. These encouraged solids problem – erosion, surface scaling and wellbore blockage. It also

enhanced flow of cooler fluids from the outskirt to the production sectors such as brine-condensate returns in the north and

groundwater inflow in the east of the Upper Mahiao, brine from Tongonan-1 injection sinks, and groundwater inflow in the east and

west of MBSS sectors. Dried wells encroached with brine mists have silica deposition in the formation. Feed or output sharing

was also observed in some wells. Feed refers to the source which maybe a fault or structure intersected wherein geothermal fluids

enter the well thru the permeable zone/s for production. Feed or output sharing are observed in wells drilled in an area with close

distances from each other. Such is the incident in one of the production pads in the Malitbog-South Sambaloran sector wherein

after cut-in of a new well, existing online wells showed decline in their output. The observed total decline in output was almost

equivalent to the output of the new well suggesting output sharing between the wells in the pad likely due to the high intra-

formational permeability and close distances between their permeable zones. While in the Upper Mahiao sector, output sharing

was best observed between the two wells in the same pad. During output measurements, the output of one well will decline in an

amount equivalent to the increase in the output of the other well, and vice-versa. It was discovered during evaluation that the two

wells shared a common structure in the openhole section and were drilled relatively close to each other.

Below is the summary of reservoir process observed in the production wells of the Tongonan geothermal field.

Reservoir Process Tongonan - 1 Upper Mahiao Malitbog – South Sambaloran

Silica Scaling

Solids Problem

Brine Returns

Condensate Returns

Peripheral Groundwater Inflow

Feed / Output Sharing

Table 1: Summary of Tongonan Reservoir Processes

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Hydrological connection between the injection sink and the production sector become apparent in all the Tongonan sectors.

Recharge of cooler peripheral fluids has become prevalent in the east of the South Sambaloran and in the eastern section of Malitbog sectors. The western part of the South Sambaloran sector has also exhibited signs of relatively cooler fluid

incursion. Another reservoir process which contribute mostly to decline in production well steam flow is possible silica depositions either in the formation or wellbore. Majority of the production wells observed to have this condition are those located in the periphery of “dried-up” zone and injection sink. Silica deposition is enhanced by continuous flashing and casing breaks. Some dry wells began discharging significant suspended solids that caused erosion damage to some FCRS facilities. Cases of mixing production well with dry and watery discharges have caused deposition / scaling along their common branchline.

3. RESOURCE CHALLENGES AND RESOLUTIONS

Resource management in the Leyte Geothermal Production Field is a great challenge given the condition of the reservoir from long

term utilization. The major challenges encountered to sustain production are presented as follows,

(1) Steam Availability

(2) Injection Management

(3) Critical Wells

(4) Age Categorization of Wells

(5) M&R Pads Development and Planning

Some management strategies to address these concerns are the implementation of optimization of steam supply distribution to

maximize generation, brine injection transfer and conversion/development of non-utilized pads, monitoring of wells considered

critical due to casing break and erosive-corrosive nature, and sequencing of future M&R pads developments to be tapped for

production. Wells that have been in the service for above 25 years, with already minimal output and poor casing condition, have

also been identified for possible replacement of future drilling.

Generating the current temperature contours in Tongonan geothermal field was a challenge since most data from old wells were no

longer obtainable due to continuous expansion of the upper two-phase layer and inaccessibility to bottom due to wellbore

obstructions such as mineral or casing / liner damage. However, new wells drilled deep in the Tongonan-1 sectors have still tapped

the >300°C zone which is within the upflow region of the system. Latest temperature data showed that most of the production

wells in the Tongonan sectors are still within the 280°C contour. Temperature declines towards the identified outflows of the

resource and temperature dents observed at the injection sinks in Upper Mahiao, Tongonan-1 and Malitbog areas are now more

evident.

Figure 5: Tongonan Temperature Contour

160

170

180

190

200

210

220

230

240

250

260

270

280

290

300

4 5 7 0 0 0 4 5 8 0 0 0 4 5 9 0 0 0 4 6 0 0 0 0 4 6 1 0 0 0 4 6 2 0 0 0 4 6 3 0 0 0 4 6 4 0 0 0 4 6 5 0 0 0

EASTING (m)

5R45R5D

5R6D

5R1D

5R7D5R2D

5R9D

5R8D

5R3D

5R10D

5R11D

TGE6

TGE1

TGE11

TGE4

LM1

TGE5A

LM2

MN3RD

4R7D4R10D

4R9D

4R14R2D

408

405

4R4D

4R3D

4R5D

4R6D

4R12D4R11D

4R13D4R14D

109D

214

110D

111D

215

217D

218D

219D

115D

221D

223D

208A

202

' 209A105D

108

106

101

103

102

114D

222D

113D

213212

212AD

216D

112D220D

411D

402

414D

410

401

403

406412D

404409

407

415D

416D

421D

413D

428D417D

423D

427D

424D418D

422D 425D

426D

419D

420DA

1R3

2R4D1R10

1R4DA

1R6D

1R7D

1R8D2R3D

2R2

1R9D

1R11D

1R5D

301D 304D

303

302

312

318D

306D

305D

314D 315D317D

307D309D

310D

308D311D

313D

316D

511D

513D

514D

501

505D

506D

518D

519D508D

510D

503

509

515D

517

520D

502

516D

U p p e r M a h ia o

T o n g o n a n -1

M a litb o g

S o u th S a m b a lo ra n

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In the same way, update on the pressure contour has shown lowest downhole pressure at ~5MPa to enclose the central production

sectors of Tongonan-1 and South Sambaloran. High downhole pressures are still observed in northern periphery of the Upper

Mahiao sector and along Malitbog wells where recharge is available either in the form of injection returns or groundwater inflow.

Figure 6: Tongonan Pressure Contour

4. FUTURE OUTLOOK

Continuous extraction and commercial expansion of the Tongonan Geothermal Field resulted to decline in pressure and boiling of

the central production field. At the start of utilization of the three sectors, boiling occurred in the reservoir as evidenced by the

sudden surge of steam flow as initial response and then followed with a sharp decline. Field-wide extraction has enhanced inflow of

peripheral cooler fluids, brine returns and mineral deposition in the drawn-down area which contributed to the steam flow decline.

However, the rate of decline was arrested when mitigating measures such as optimized brine loading and strategic injection

locations were implemented.

Aside from the strategic injection utilization, drilling of M & R wells and work-over of old wells were also undertaken to sustain

steam supply to the power plants. Amongst the latest interventions is the Leyte Steam Augmentation Project (LSAP). LSAP started

in 2008 with the aim of drilling 19 production wells and 2 injection wells to replace steam decline and augment steam available.

Drilling of LSAP wells with a target output of 105.5MW achieved 98.4% success after obtaining a total of 103.8MW after

completion in 2011.

Figure 7: Tongonan-1 Field Enthalpy Trend

1980 1985 1990 1995 2000 2005 2010

1200

1600

2000

2400

2800

Enthalpy

(kJ/kg)

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In Tongonan-1 alone, the field experienced initial sudden surge in enthalpy during commercial expansion in 1996-1997 due to

pressure decline which resulted to initial increase in steam fraction from drying up of the wells. The increase was not sustain due to

secondary reservoir processes such as blockages and cooler fluids inflow which in turned decrease the steam production. However,

when resource management strategies were implemented beginning of 2002, the Tongonan-1 sector was able to sustain steam

supply requirement to the power plant making it operational until now even after about thirty years of utilization.

REFERENCES

Dacillo, D.B., et. al.: Tongonan Geothermal Field: Conquering Challenges of 25 Years of Production, Philippines, Proceedings,

World Geothermal Congress, Nusa Dua Bali, Indonesia (2010).

Sta. Ana, F.X.M., et. al.: Increased Production Load in Tongonan Geothermal Field, Philippines: Reservoir Response and Field

Management Strategies, Proceedings, 24th Workshop on Geothermal Reservoir Engineering, Stanford University, Stanford,

CA (1999).