Continuous Measurement of Geo-Gases for Effective Geothermal

Reservoir Management

Manuel S. Ogena1, Farrell L. Siega1, Danilo B. Dacillo1

Thomas Streil2 and Veikko Oeser2

1PNOC-Energy Development Corporation, Fort Bonifacio, Makati City, Philippines

2SARAD GmbH, Dresden, Germany

PNOCThe Energy Company

INTRODUCTION

PNOC-Energy Development Corporation and SARAD GmbH entered into a joint cooperation project which aims to study reservoir changes through continuous monitoring of the chemistry and physical characteristics of producing wells. These changes may result from mass extraction for production, waste fluid injection for disposal and pressure support, and from natural geologic processes, which could be correlated to the structures that controls the permeability of the reservoir. The project also aims to correlate the radon trends in well discharges with the seismic activities in the area and possibly use the radon concentration in geothermal gases as an earthquake precursor tool. Using this geochemical and geophysical information it is possible to optimize the recharging and production process.

The study area is in the Mahanagdong sector of the Leyte Geothermal Production Field (Fig. 1), one of the geothermal fields operated by PNOC-EDC. It is located in the island of Leyte, in central eastern Philippines about 600km southeast of Manila and occupies 107,625 hectares of geothermal reservations under geothermal service contract that PNOC-EDC got from theDepartment of Energy. Production came mostly from northeast trending faults, associated with the Philippine fault line, intersected by some 85 production wells drilled to a depth of between 1000-3000 mMD.

There are 5 main power plants and 4 optimization plants, generating around 700 MW gross power, installed in Leyte Geothermal Production Field. It accounts for 60% of PNOC-EDC’s geothermal installed capacity of 1150 MW and 37% of the entire Philippine geothermal power installed capacity of 1915 MW. It contributes heavily to the national grid and is the main interconnection supplier to two major Philippine islands, the Leyte-Cebu (200 MW) and the Leyte-Luzon (400 MW) grid interconnection.

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Manila

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Figure 1. Location Map of the Leyte Geothermal Production Field

MEDAS SETUP IN MAHANAGDONG

Pad Location and Layout

To achieve the objectives of the study, a Modular Environmental Data Acquisition System (MEDAS) has been installed at Pad MG5D in Mahanagdong sector (Fig. 2). This area in Mahanagdong is chosen because: 1) it is part of the largest geothermal field in the Philippines with five separate power plants and total installed capacity of about 700 MW, 2) it is bisected by the Philippine Fault, a major left-lateral transcurrent fault similar to the San Andreas fault, and 3) it has a dynamic geothermal reservoir, with wells on the same pad affected by several processes.

The MEDAS system at Pad MG5D is composed of four substations A, B, C and D, installed in each of the monitored production wells, namely, MG32D, MG19D, MG23D and MG18D, and a main station CPU at POS 200 building which controls the substations and stores all the measured data.

Figure 2. Location map of Pad MG5D and layout of MEDAS setup in Mahanagdong sector.

Substation Setup

Figure 3 shows the set-up in each of the four (4) substations installed at Pad MG5D wells. A Webre separator is used to separate the liquid and gas phases of the fluids from a geothermal well. The equipment has a built-in cooling system that cools the liquid stream to around 25oC. After cooling, the liquid stream goes directly to a water bucket with probes connected to a multisensor inside the electronic instrument housing for the analysis of the selected parameters. The gas stream on the other hand goes first to a separate gas cooling system where it is cool down to around 28oC before going to the gas sensors inside the electronic instrument housing. A 1000L-capacity water tank is also installed in each substation to ensure continuous supply of cooling water for about an hour, which is more than enough to shut down all 4 substations whenever supply of water from the main source stops.

MAHANAGDONG

TONGONAN

Figure 3. Substation setup at Pad MG5D wells.

Monitored Parameters and Data Acquisition

MEDAS is an innovative multi-parameter station, which can continuously record as a function of time up to more than 100 geochemical and physical parameters. A microcomputer system inside the MEDAS handles data exchange, data management and control and it is connected to a modular sensor system. The number of sensors and modules can be selected according to the needs at the measuring sites.

For the study in the Mahanagdong sector, each MEDAS substation installed in the wells has the following essential modules (Fig. 4):

a. Power Supply b. Data Logger with 8 analog inputs and 2 digital inputs connected to sensors/probesc. Radon Sensor with internal sensors for gas flow, gas temperature and relative humidityd. CO2 Sensore. Single Channel Analyzer coupled to the Radon sensorf. Multisensor for the analysis of water parameters, such as pH, conductivity, etc.

The parameters currently monitored both for the liquid and gas streams are tabulated in Table 1. Additional sensor probes for silica, calcium and ammonia analysis in the water stream as well as sensors for ambient temperature, pressure and relative humidity measurements are being proposed for installation. A micro-seismograph is likewise planned for installation in order correlate the trends of the Radon gas in the area.

COOLED GAS

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Figure 4. Essential modules and probes installed in each substation.

Table 1. Parameters monitored in each substation.

p HC h lo rid eR e d o x P o te n tia lC o n d u c tiv ityW a te r T e m p e ra tu re

W A T E R S tre a m

R a d o nC O 2

G a s F lo wG a s T e m p e ra tu reG a s R e la tive H u m id ity

G A S S tre a mP A R A M E T E R S

p HC h lo rid eR e d o x P o te n tia lC o n d u c tiv ityW a te r T e m p e ra tu re

W A T E R S tre a m

R a d o nC O 2

G a s F lo wG a s T e m p e ra tu reG a s R e la tive H u m id ity

G A S S tre a mP A R A M E T E R S

All substations (A, B, C, D) are connected to the main station about 50 m away where data retrieval from all operating data loggers is conducted (Fig. 5). The main station has the following components:

1.) CPU module which has a time step controller operating system for data acquisition2.) an extender module and memory card for data storage and downloading 3.) a control module for the power supply and operation of the system in each substation4.) telemetry or data transfer module (modem and other accessories) for the remote

reprogramming and control of the MEDAS and downloading of data.

Set-up for a remote access and downloading of data from the main station CPU through a GSM dial-up connection is still on-going. At present, downloading of data stored in the memory card is conducted through direct cable connection of a laptop computer to the main station CPU.

Data and Results

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Figure 5. Schematics of remote data acquisition for the MEDAS

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Data and Results

The results of an investigation in Marmara region, Turkey, suggested that basement geology, the existence of deep water circulation as in geothermal activity, and structural features of the area have prime influence on the Radon gas variation, specially prior to the occurrence of an earthquake. In the case of seismically active region of Vogtland/NW Bohemia, high gas emission rates of free CO2 were recorded at springs accompanied by seismotectonic processes such as the Vogtland/NW Bohemia swarm quakes.

In Mahanagdong, the CO2

and Rn gas levels of the monitored wells were likewise measured and correlated with the recorded local seismic events. Based on the CO2

and Rn trends shown in Fig. 6, wells MG18D, MG19 and MG23D have relatively stable CO2 and Rn gas levels. The fluctuations observed in the data may be correlated to the local seismic events within the project site of PNOC-EDC. No major events were observed within the period September to November 2005 thus correlation of Rn and CO2 gas trends during major earthquake event could not be established.

Figure 6. Correlation of Radon and CO2 gas trends with the recorded local seismic events.

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Figure 7. Correlation of well fluid pH, as measured on-line using MEDAS vs. as determined during monthly production well chemistry monitoring.

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Figure 7 shows the trends in pH for wells MG18D, MG19 and MG23D. The discharge fluids from these wells have pH ranging from 6.0-7.0, as determined through monthly collection of water samples. The pH level measured on-line through MEDAS generally gave similar values in all three wells. Fluctuations in the readings (pH<4.0) given by the MEDAS, as observed in wells MG18D and MG19 in November 2005, indicate the need for periodic recalibration of the pH electrodes to pH standard solutions. The lower pH values between 4.0-5.0 for MG18D in October reflected the unstable fluid pH of the well after its bullhead acidizing in September 2005. This trend was also indicated in the regular well chemistry monitoring wherein MG18D’s fluid pH decreases from stable pH of 6.5 to 5.8 in October 2005.

Figure 8 shows the Cl trends of geothermal fluids from wells MG18D, MG19 and MG23D as measured on-line using the MEDAS. The measured Cl levels in these wells are not comparable to the actual Cl levels determined through the regular well chemistry monitoring (Table 2). In well MG19, Cl level measured is 6000-6500 ppm, which is at least two times the actual Cl level. In well MG18D, the measured Cl level is significantly lower, less than 1000 ppm, while in MG23D, significant fluctuation in Cl from less than 1000 ppm to as high as 3000 ppm can be observed. The reason for these differences in Cl levels could be attributed to not sufiicient calibration, because the calibration solution was only 15 and 300 mg/l. A recalibration is necessary. The relative fluctuations are of a big interest.

Table 2. Actual Cl levels of the monitored wells.

Nevertheless, the Cl data obtained may still be use to check the reservoir processes which may have affected these wells. Though, care should be taken in interpreting these data, and correlation with other parameters should be conducted. Below are some of the observations deduced from the Cl plots above (Fig. 8) and the CO2 plots (Fig. 9) below.

1. The decreasing trend in Cl of well MG18D between September to October may reflect the unstable condition of the well after its bullhead acidizing, which also coincided with the lowering of the fluid pH during this period. Although this is not reflected in its CO2 trends.

2. The general increase in Cl level of well MG23D in November as well as the slight decline in CO2 level may indicate recovery from effects of cooler, degassed and low saline fluids mixing. The inflow of this type of water was the dominant reservoir process observed for this well.

3. In well MG19, no clear observation can be forwarded due to limited data in Cl. The Cl sensor installed in this well may have been defective as most of the data are erroneous (negative values). The increasing CO2 trend could be an indication of boiling in the well bore.

Well Cl (ppm), as analyzed

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Figure 8. Fluid salinity trends (in terms of chloride) as measured on-line using MEDAS.

The redox potential may be use as indicator of the influx of oxygen-saturated waters. In the case of well MG18D (Fig. 10), declining trend in redox potential from September to November was correlated to influx of the oxygen-saturated drilling waters used in MG34D. This was confirmed with the trends being shown in the chemistry of well MG18D which showed decline in reservoir chloride concentration and quartz geotemperature by as much as 400 ppm and 15oC, respectively (Fig. 11). The subsequent recovery of MG-18D after the drilling of MG-34D was reflected in the plot by an increase in the redox potential for the period December 2003.

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Figure 9. CO2 gas trends as measured on-line using MEDAS.

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Figure 10. Redox potential plot for well MG18D.

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Figure 11. Geochemical trend of MG18D

Same can also be observed in the redox potential trends for well MG23D (Fig. 12). The declining trend in the redox potential between September to November maybe correlated with the influx of condensate injected at MG4DA as well as the cooler peripheral waters inflowing into the southern part of Mahanagdong. The stabilization in redox potential trend for well MG23D towards the end of November could also be an indication of recovery from these reservoir processes. The regular production well chemistry monitoring in well MG23D indicated this influx of cooler, dilute waters since year 2004 (Fig. 13).

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Figure 12. Redox potential plot for well MG23D.

Figure 13. Well chloride trends for well MG23D.

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SummaryThe MEDAS system was able to demonstrate its capability to collect and record data continuously from the monitored production wells. Moreover, no major events were observed within the period September to November 2005 thus correlation of Rn and CO2 gas trends during small micro-seismicity event could be established.

As for the second objective of monitoring on-line the chemistry and physical characteristics of producing wells to evaluate reservoir changes, initial data indicated good correlation with the pH, Cl, CO2 and redox potential. The pH level measured on-line through MEDAS generally gave values closer to the actual fluid pH of the three monitored wells. Fluctuations though in the readings (pH<4.0) given by the MEDAS indicate the need for periodic recalibration of the pH electrodes to pH standard solutions.

In the case of the Cl, values measured by MEDAS in the monitored wells are not comparable to the actual Cl levels determined through the regular well chemistry monitoring. The reason for these differences in Cl levels could be attributed to a calibration problem. This possibly affected its capability to accurately measure the actual Cl levels of the wells, which is in the range of 3000-3500 ppm. Nevertheless, the trends observed in the Cl, when correlated with the CO2, could still be used to check the reservoir processes which may have affected these wells.

The redox potential may be use as indicator of the influx of oxygen-saturated waters. This parameter indicated response in well MG18D which was affected by influx of the oxygen-saturated drilling waters used in MG34D drilling and in MG23D, which was affected by influx of condensate injected at MG4DA as well as cooler peripheral waters inflowing into the southern part of Mahanagdong.

Recommendations

To improve further the MEDAS set-up currently installed in Mahanagdong, the following are recommended:

1. installation of additional sensors such as Ca, NH3 in water and SiO2 to increase further the capability of the MEDAS in real-time monitoring of the well discharge chemistry

2. recalibration of Cl electrodes, which could measures Cl level up to 5000 ppm.

3. installation of a micro-seismic equipment and sensors for the ambient temperature, pressure and relative humidity in order to have better correlation of Radon data with local seismicity and ambient condition in the area