MODELING AND FORECAST OF THE EXPLOITATION

THE PAUZHETSKY GEOTHERMAL FIELD, KAMCHATKA, RUSSIA

A.V. Kiryukhin1, N.P. Asaulova2, T.V. Rychkova1, N.V.Obora2 , Y.F.Manukhin1 and L.A.Vorozheikina2

1- Institute of Volcanology and Seismology FEB RAS, Piip-9, P-Kamchatsky, Russia 6830062- Kamchatskburgeotermia Enterprize, Krasheninnikova-1, Thermalny, Kamchatka, Russia, 684035

Introduction, Conceptual Hydrogeological Model

3D numerical model setup & parameterization

Content:

Inverse modeling (iTOUGH2): natural state & exploitation

Modeling applications (TOUGH2, iTOUGH2) to exploitationreserves estimations

Input observation data

The Pauzhetsky geothermal field has been developed since 1966, when a 5 MWe power plant was put into operation.

Photo by V.M. Sugrobov, 1985

Conceptual hydrogeological model of the Pauzhetsky geothermal system assume cold meteoric water from regional recharge areas infiltrates through open fractures at 5-6 km depth in a high-temperature zone above 250оС, where heats up and upflows with enthalpies of 950-1050 kJ/kg through the base rocks to the volcanogenic-sedimentary basin, where production reservoir occurs.

46 years of exploitation with total extraction flow rate rangesbetween 180-260 kg/sduring 1975 – 2006 years.

Reinjection started in 1979

History of exploitation:

The initial 15 years of the exploitation at 160-190 kg/s show gradual temperature decline and chloride dilution of the production wells located near the natural discharge area,so new exploration wells were drilled, and exploitation gradually shifted away from the natural discharge area until wells were drilled into a central upflow zone located 1.5-2.0 km southeast from the old production field

History of exploitation:

(1) The Pauzhetsky hydrothermal reservoir is layered with an area of 2 2.5 km2 and an average penetrated thickness of 505 m connected at the bottom with the upflow zone.

Integrated analysis of the field data

(2) Well logging analysis show a double-porosity response of the reservoir, with a fracture volume fraction (FV) of 0.28 and an average fracture spacing (FS) of 105 m.

(Continue) Integrated analysis of the field data

(3) Natural thermal discharges include dominant hot boiling springs discharge with a measured rate of 31 kg/s, and minor heat discharge of steaming grounds (Verkhnee and East with a total discharge rate of 0.7 MWt).

(Continue) Integrated analysis of the field data

(4) Permeability-thickness and total production zones compressibility estimates show a kM range from 35 to 94 D·m

(5) Initial reservoir pressure is 34.5-35.5 bars at -250 m.a.s.l., and tends to increase in south-easterly direction (North site of the field).

(6) The production reservoir temperature is 180 – 220 оС; the upflow zone is restricted by a temperature contour within the drilled part of the field.

(Continue) Integrated analysis of the field data

(7) Enthalpy-chloride diagram show significant dilution due to meteoric water inflow (30 - 62% dilution).

(Continue) Integrated analysis of the field data

(8) Hydroisotope data (D, O18) and tritium data confirm significant meteoric inflow into hydrothermal reservoir.

2005

1983

(Continue) Integrated analysis of the field data

(9) Laumontite act as tracer of production zones in hydrothermal reservoir, whilecalcite-chlorite-smectites indicate low permeability zones (A.D. Korobov,1985). This is coincide with double porosity model approach.

(Continue) Integrated analysis of the field data

Hence, conceptual hydrogeological model of the Pauzhetsky geothermal reservoirbased on integrated analysis of the field data represented as a three-layer system which includes:(1) a middle layer hydrothermal reservoir at -250 m.a.s.l. with an average thickness of 500 m and area 4 * 5 km2; (2) an upper layer caprock with recharge\discharge windows; and (3) a base layer with feeding channels.

Conceptual Hydrogeological Model

3D numerical model include three layers: (1) a middle layer representing the hydrothermal reservoir at -250 m.a.s.l.; (2) an upper layer caprock with Discharge\recharge windows; and (3) a base layer hosted upflow zone.

Model Setup

Calibration points for natural state modeling include: (1) Vertically averaged temperatures in the mid-layer hydrothermal reservoir (52 T-points); (2) Pressures calculated at -250 m.a.s.l. (14 P-points); (3) Natural discharge rates (2 values). T=1-3оС, P=0.1-0.5 bars, Q=15-50% assumed.

Inverse modeling: Natural state

Model parameterization:(1) Hydrothermal reservoir (mid-layer) fracture permeability kr, m2(2) Mass flow rates at the bottom of the base layer Qb, kg/s

Inverse modeling: Natural state

Calibration data sets for exploitation modeling include: (1) Monthly averaged enthalpies in exploitation wells (10 E-datasets), (2) monthly averaged pressures at -250 m.a.s.l. (22 P-datasets), and (3) monthly averaged temperatures in the mid-layer hydrothermal reservoir (26 T-datasets). The total number of calibration points used was 13757,T=5оС, P=0.3 bars, h=20 kJ/kg assumed.

Inverse modeling: exploitation 1960-2006

(Continue) Model parameterization (exploitation):(3) Hydrothermal reservoir (mid-layer) compressibility Сr, Pa-1(4) Hydrothermal reservoir (mid-layer) fracture porosity f (5) Base layer compressibility Сb, Pa-1(6) Base layer porosity b (7) Base layer vertical permeability kb,m2(8) Upper layer North site “Hydraulic window” permeability kN, m2(9) Upper layer East site “Hydraulic window” permeability kE, m2(10) Upper layer West site “Hydraulic window” permeability kW, m2.

Inverse modeling: exploitation 1960-2006

Integrated results and physical meaning of inverse modeling (iTOUGH2): hydrothermal reservoir active volume capacity, basement upflow rate capacity and meteoric inflows rates.

10 parameters estimated, 3 of themstrongly correlated, so number of estimated parameters reduced to 7

Residual analysis shows reasonable standard deviations: enthalpy – 36 kJ/kg, pressure - 0.4 bars, temperature – 12oC.

Residual analysis shows reasonable standard deviations: enthalpy – 36 kJ/kg, pressure - 0.4 bars, temperature – 12oC.

Modeling applications: heat and mass balance during exploitation (2005)

Modeling applications (TOUGH2, iTOUGH2) to exploitationreserves estimations

In terms of Russia Federal Commission on thermal waters exploitation Reserves estimations (RFCR) requirements applied to Pauzhetsky:«Exploitation reserves – is sustainable production level of mass rate (steam, separate water) under specified separate pressure from existing and additional technically feasible production wells proved for 25 year exploitation period».

Category «A» – confirmed by stable rate during long term exploitation, and proved by heat and mass sources.Category «B» – confirmed by stable rate during 1 year flowtest, and proved by by heat and mass sources.Category «C1» – confirmed by stable rate during short-term tests, and proved by by heat and mass sources.

RFCR approval of «A+B» is needed to get long term license for geothermal field explotation in Russia.

Since there is no stable rate during long term exploitation of Pauzhetsky, we suggest to use TOUGH2\iTOUGH2 forecast as «A+B» estimator, based on current year exploitation flowrate value as initial. In this procedure iTOUGH2 used first to prove forecast is reliable, then, TOUGH2 used to run more realistic exploitation scenario (coupled wellbore-reservoir), which iT2 doesn’t have.

FORECAST OF RESERVOIR PRESSURE AND ENTHALPY DECLINE UNDER EXPLOITATION, 2007-2032 AT CONSTANT RATES LOAD (iTOUGH2)

Production rates by Nov. 2006(current year exploitation flowrate value):

264.9

iTOUGH2 error propagation analysis to enthalpy (well #106 ) show high reliabiality of the model for 1960-2032 year period forecast, since very narrow 95% confidence forecast interval (just 20 kj/kg).

FORECAST AT CONSTANT RATES LOAD

iTOUGH2 error propagation analysis applied to reservoir pressure (well #106 ) show high reliability of the model for 1960-2032 year period forecast, since very narrow 95% confidence forecast interval (just 1 bar).

FORECAST AT CONSTANT RATES LOAD

Really, exploitation of the Pauzhetsky geothermal field production wells conducted at constant wellhead pressures. Hence coupled wellbore-reservoir TOUGH2 option used for modeling 2007-2032 exploitation forecast scenario. Exploitation wells 103, 106, 108, 120, 121, 122, 123, GK3 and 131 were assigned in the model with wellhead pressures (WHP). Bottom hole pressures tables calculated for each well based on HOLA wellbore simulator. Exploitation wells production indexes PI were estimated directly in the model based on the current rates of production wells.

FORECAST OF EXPLOITATION 2007-2032 (CONSTANT WELLHEAD PRESSURES)

Four additional make-up wells were assigned in the model to maintain sustainable production 120A (turn on in 2008 year), 123A (2012 year), 107A (2016 year), 102A (2025 year).

It was also found necessity of switchWHP at well 131 to 3.0 bars (2020 year) and in well 122 to 3.5 bars (2028 year) to maintain sustainable production of those wells.

Show possibility of sustainable flowrate production in the range from 257 to 324 kg/s (average flowrate 287.4 kg/s) during next 25 years.

FORECAST OF EXPLOITATION 2007-2032 (CONSTANT WELLHEAD PRESSURES)

Show possibility of sustainable steam production in the range from 25.7 to 32.2 kg/s (average 28.9 kg/s) during next 25 years.

FORECAST OF EXPLOITATION 2007-2032 (CONSTANT WELLHEAD PRESSURES)

Modeling forecast (4 makeup wells in 25 years) correspond actual exploitation experience 1996-2006, when 2 additional wells (103 & 131)were put into exploitation during 10 years to support 25.6 kg/s average steam production rate.

Conclusions

TOUGH2\iTOUGH2 based numerical model of the Pauzhetsky geothermal field calibrated on natural state and 1960-2006 exploitation data. Seven principal unknown model parameters were estimated.

Numerical model generate reliable 95% confidence forecast for the enthalpy of production wells and reservoir pressure.

Modeling forecast at specified WHP of nine existing production wells and four additional make-up wells of the Pauzhetsky geothermal field confirm possibility average steam production 28.9 кг/с for the next 25 years. This steam production is sufficient to support 6.8 MWe of Pauzhetsky Power Plant and suggested to be used as A+B category of exploitation reserves (in terms of RFCR).

Heat and mass balances derived from the model explain the sources of exploitation reserves: upflow from base layer (40.6%), meteoric origin inflow through infiltration windows (30%), storage of hydrothermal reservoir (21.1%) and reinjection (8.3%).

Valuable help from Dr. S. Finsterle and Dr. K. Pruess (LBNL) is highly appreciated. This work was supported by SUE “Kamchatskburgeotermia”, FEB RAS project 06-I-ОНЗ-109 and RFBR project 06-05-64688-а.