ngawha reservoir model - jogmecgeothermal.jogmec.go.jp/report/file/session_160602_06.pdfngawha...
TRANSCRIPT
GNS Science
Ngawha Reservoir Model
John Burnell
JOGMEC-GNS Workshop, Tokyo
2 June 2016
GNS Science
Outline
• Purpose
• Conceptual Model
• Model Structure
• Model Verification
• Expansion Scenario
• Surface Features Model
Slide 2
GNS Science
Purpose
• To develop a numerical reservoir model of the Ngawha Geothermal Field for consenting purposes
• Objectives
– Obtain an acceptable match to historical data
– Use a dual porosity model to more accurately model
injection returns in the reservoir
– Consider effects of possible future expansion
– Predict impact on surface features
Slide 3
GNS Science
Conceptual Model
Slide 4
GNS Science
Reservoir Characteristics
• Reservoir hosted in Waipapa Greywacke between 500m and 1500m depth
– Low porosity
– Permeability due to fractures and faults
• Low permeability siltstones form a cap
– Some transport through faults and fractures
Slide 5
GNS Science
Resource Areas
Operations area
Slide 6
GNS Science
Reservoir Characteristics II
• Geology
– Permeable greywacke
– Cap with small channels connecting to surface
• Temperatures in permeable reservoir are ~230 °C
• Hotter temperatures in the north
• Pre-production testing suggested fractured
reservoir with high permeability
• Hot upflow > 300°C, located in north < 10 kg/s
• CO2 concentrations ~1.5%
Slide 7
GNS Science
Hydrological Model
Slide 8
GNS Science
Model Validation Data
Slide 9
• Pre-production temperature and pressure measurements.
• Measured changes in reservoir pressure due to production.
• Changes in production enthalpies.
• Changes in CO2 concentrations in production wells.
• Measured pressure changes in the 1983
interference tests.
• Travel times from the 2011 tracer test.
GNS Science
Dual Porosity Numerical Reservoir Model
Slide 10
GNS Science
Dual Porosity Model
Matrix
Fracture
Used to represent flows in fractures• Fast flow paths• Small volumes
• Model cooling
Slide 11
GNS Science
Model Grid
Slide 12
GNS Science
Geology and Resource Boundary
Permeable inner region
Slide 13
GNS Science
Upflow
Upflow = 10 kg/s
Slide 14
GNS Science
Model Verification
Slide 15
GNS Science
Methodology
• Run natural state model
– Compare to measured temperatures and pressures
• Run historical production
– Compare to measured pressure changes, enthalpies and CO2 concentrations
• Add tracer injection.
– Compare to measured tracer
• Run production and injection for 1983
interference test
– Compare to measured pressures
Slide 16
GNS Science
Natural State Temperatures
Slide 17
GNS ScienceSlide 18
GNS Science
Some Improvements Still Needed
Slide 19
GNS Science
Natural State Pressures
Slide 20
GNS Science
Changes Due to Development
Slide 21
GNS ScienceSlide 22
GNS Science
Pressures in Injectors Don't Match as Well
Slide 23
GNS Science
Production Enthalpies
Slide 24
GNS ScienceSlide 25
GNS ScienceSlide 26
GNS Science
Issues with Enthalpies
• Inconsistent with measured tempertures
– NG9 temperatures give enthalpies of 980 kJ/kg
– Initial data – 920 kJ/kg
– Also inconsistency between well enthalpies and station enthalpies
– In other fields it is suggested that measurement uncertainties are ~100 kJ/kg
– If this is the case then the model agrees within
measurement uncertainty.
Slide 27
GNS ScienceSlide 28
GNS Science
CO2 Concentration
• CO2 gives information about long term connection between injectors and producers
Slide 29
GNS ScienceSlide 30
GNS ScienceSlide 31
GNS Science
Tracer Test – Injection into NG16
• Arrival times give information about short term transport between injectors and producers
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
Co
nc
en
tra
tio
n (
ug
/L)
NG16 Tracer
NG4
NG9
NG12
Slide 32
GNS Science
Model Results
Arrival time
Slide 33
GNS ScienceSlide 34
GNS Science
Interference Test – October 1983
• Pressure propagation across the reservoir
Slide 35
GNS Science
Overall Match
• Ngawha Numerical Reservoir Model is well-calibrated
• Matches data from many different aspects of the field
• Based on physical understanding
– Reduces uncertainty
Slide 36
GNS Science
Expansion Scenarios
Slide 37
GNS Science
Expansion Scenarios
• Two scenarios
– Scenario A: 25 MWe expansion
– Scenario B: 50 MWe expansion
• 25 MWe stage from 2020
• 50 MWe stage from 2023
• Adjusted injection rates to match observations
– Loss of gases (currently 0.5%)
– Other loses/spillages (0.35%)
• Used supplemental injection to control reservoir pressures
Slide 38
GNS Science
Scenario Pressures
Slide 39
GNS Science
Some pressure changes
Slide 40
GNS Science
Enthalpies Decline
Slide 41
GNS Science
Will need some plant changes
Slide 42
GNS Science
Surface Feature Modelling
Slide 43
GNS Science
Surface Feature Modelling
• Ngawha Springs are valued highly by local community
• Ngawha Springs have low flow rates
• Reservoir model is too coarse to capture changes in flow to surface features
• Developed a separate model of the area around the surface features
Slide 44
GNS Science
Schematic of Surface Feature Model
Deep Reservoir
Surface Waters
CaprockPermeable
Conduit
Slide 45
GNS Science
Model Grid
Slide 46
GNS Science
Vertical Extent
Slide 47
GNS Science
Natural State Temperatures
Slide 48
GNS Science
Natural State Pressures
Slide 49
GNS Science
Surface Feature Model Setup
• Fixed pressures and temperatures in the bottom boundary
– Corresponding to measured reservoir data
• Fixed conditions in atmosphere
• Flow driven by boundary conditions
– Adjust conduit permeability to match flow
– Modelled flow was 1.3 kg/s
Slide 50
GNS Science
Response to Production
• Used output from numerical reservoir model expansion scenario
– Reduced reservoir pressure by 0.3 bar
– Reduced reservoir temperature by 1 °C
– Reduced reservoir CO2 mass fraction from 1.3% to
0.1%
Slide 51
GNS Science
Surface Feature Model Results – Flow to
Surface
Slide 52
GNS Science
Surface Feature Model Results – Steam
Fraction
Slide 53
GNS Science
Surface Feature Model Results – Temperature
Slide 54
GNS Science
Surface Feature Model Results – CO2 in
Discharge
Slide 55
GNS Science
Chemistry Calculation for Surface Features
• Ran a calculation with GeoChemist Workbench to obtain spring chemistry
• Reduced the CO2 content and re-calculated
• The main changes are an increase in pH (6.2 => 6.95) and a reduction in HCO3-.
Slide 56
GNS Science
Conclusions
• Well-calibrated model for consenting purposes
• Matches many different aspects of flow in the
reservoir
• Pressures can be controlled for 50 MWeexpansion scenario
• Impact on surface features is minor
Slide 57