Global occurrence of geothermal systems in different geologic settings: their identification and utilization

Transcription

1 Global occurrence of geothermal systems in different geologic settings: their identification and utilization Mar-2016 William Cumming Cumming Geoscience, Santa Rosa CA Office: Mobile: Skype: wcumming.com

2 Agenda Global Geothermal Occurrence of Geothermal Systems Thermodynamics implications for geothermal systems Classifications of geothermal systems What type of system is characteristic of the Western EARS? Implications of fault based systems for resource capacity Implications of fault-based systems for Western EARS Exploitation options for fault-hosted geothermal development Priorities for volcano-hosted and fault-hosted exploration Conceptual model elements Cumming (2013)

3 Geothermal Geoscience Conceptual Context Basic physics of permeable geothermal reservoirs (non-egs) Geothermal reservoirs lose energy to surface through any rock by heat conduction and through leaky rocks by buoyant advection of hot fluid In proportion to stored energy, a geothermal reservoir emits energy at a rate orders of magnitude higher than O&G reservoirs The geothermal emphasis on seeps does not indicate primitive technology relative to O&G but a difference in resource physics Implications for geothermal exploration strategy Geothermal reservoirs with vertical permeability leak heat upward, so hidden systems without near-surface manifestations are special Most cost-effective reduction of risk for geothermal resource with thick vertical permeability is to demonstrate permeability and temperature using water chemistry, if not from springs then from shallow wells Cumming (2013)

4 Geochemistry and Resistivity Methods Dominate Exploration For Permeable Geothermal Reservoirs Why? All geothermal systems with extensive vertical permeability leak hot water or heat to surface or near-surface For leaky systems, geochemistry cost-effectively indicates Likelihood of economic temperature? Significant permeability at that temperature? Resistivity detects the clay cap, cost-effectively answering What is the geometry of the reservoir top? How big might the reservoir be? What well casing design is optimum? Cumming (2013)

5 Geothermal Resource Setting Geologic setting Divergent (rift) Convergent (arc) Transform (pull-apart) Major volcanism Intracontinental rifts Moeck play type Magmatic volcanic Magmatic plutonic Extensional Non-convecting plays Others argue >230 C flash <180 C pumped 150 to 230 C gassy flash Moeck (2015, Geothermics)

6 Geothermal Resource Power Density

7 Best Geothermal Prospects Regional correlation with arcs, rifts, transform step-over extension, Recent volcanics, magma detected at depth by imaging, radiogenic heat flow, earthquakes, etc Power density >20 MW/km 2 correlates generally with Recent volcanics and specifically with water and/or gas geochemistry >240 C Hinz Extension rate decisive U. Wisconsin, 2015 < 240 C common and can be hidden, so look for extension

8 East versus West EARS Exploration Eastern EARS Gas chemistry shows >230 C on most current prospects Most targets volcano-hosted with significant Recent volcanism Recent volcanics tend to be thick Some reservoirs thinned by shallow magma and shallow 350 C isotherm (silica solubility) MT-TEM images clay cap with ambiguity from noise and limited smectite alteration in phonolite and trachyte Targeting general fault geometry favorable to sweet spots in distributed permeability of large reservoirs undergoing rapid extension Western EARS Water and gas usually ambiguous but more reliable indicators show <180 C Many targets more closely associated with structure than Recent volcanics Recent volcanics thin or absent Most Recent volcanics indicate deep eruption source, not shallow residence MT+TEM images possible clay cap and unsuitable host rocks - limited by noise and access Targeting specific fault geometry favorable to relatively low volume flow path in areas undergoing slower extension Cumming Geoscience

9 If West EARS is Fault-Based Geothermal Exploration and Development Strategy Find locations with a coincidence of: Relatively high temperature shallower than 5 km depth Tectonic setting with a high rate of extension favorable to open-space fracturing Rocks susceptible to open-space fracture permeability Targeting risk Most affected by hot water and/or gas with suitable chemistry High rate extension but faults are not enough, geometry and formation properties must be favorable If no springs, TGW best indicator of permeability hosting hot water flow Mostly <180 C so pumped/gas-flash production to binary generation

10 2-3 km Geothermal Systems, Settings, East EARS Development Analogy 5-7 km 350 C Melosh (2013, GEA)

11 Awibengkok Geothermal Field Awibengkok 377 MW Melosh (2013, GEA)

12 1 km 12 Geothermal Systems, Settings, West EARS Development Analogy BRADYS CROSS-SECTION 2 km Geothermex (2008)

13 Bradys Geothermal Field Bradys MW Geothermex (2008) 13

14 East versus West EARS Geothermal Systems, Settings, Development Analogies Awibengkok 377 MW Bradys MW Melosh (2013, GEA)

15 Western EARS Options Geothermal Systems, Settings, Geothermal Exploration and Development Consider full range of conceptual models, including faultbased as well as volcanic-associated models Use several resource conceptual models to illustrate uncertainty with focus on structural target in suitable rocks Use TGWs or slim holes to test multiple models at lower cost Investigate acquisition of more complete versions of data collected for other purposes, including reflection seismic, airborne gravity, regional MT etc Assess viability of outcomes consistent with resource types likely to be found Cumming Geoscience

16 Conventional Power Plants Dry Steam Single Flash 750 to 1300 MW Geysers Lund and Dipippo, 2003

17 Binary Power Plants Higher cost per MW Modular and faster to install Scalable 0.2 to 200 MW Used for all pumped production (<180 C) Used for special cases of flash production (>220 C) Pumped production requires economies-of-scale to support pump replacement Geothermal Systems, Settings, 1 MW Wabuska 38 MW Puna Lund and Dipippo, 2003

18 Geothermal Power Plant Types Lund and Dipippo, 2003

19 Direct and Cascade Geothermal Uses Reservoirs shallower and lower temperature, C, are used directly Spas, greenhouses, fish farms, industry and space heating Direct use since pre-history, power production 110 years Technology can add value, Existing users often resist sharing resource unless low impact can be guaranteed Cascaded use sometimes possible, but depends on chemistry, typically using heat exchange GeoHeat Center, 2005

20 Direct and Cascade Geothermal Use Geothermal use is temperature dependent Heat transportation limited longest in world is in Iceland, where high volume of 90 C water piped 27 km from Nesjavellir Field heat exchanger to Reykjavik for space heating Local resources must fit local needs For example, oil and mining applications must be near Cascaded use sometimes possible, but depends on chemistry GeoHeat Center, 2005

21 Agriculture and Aquaculture Increasing growth rate of flowers, vegetables, and other crops in greenhouses in cool climates Increasing growth rate and shorten time to maturity of fish, shrimp, abalone and alligators GeoHeat Center, 2015

22 Hot Spring Bathing and Spas (Balneology) Since pre-history, all regions in which hot springs are found have used them for bathing Japan is world leader in balneology, with Beppu alone having 4,000 hot spring baths serving 12 million tourists a year Romans built cities around baths supplied by hot springs, such as the city of Bath, still a major tourist center

23 Svartsengi Geothermal Field 77 MWe power generation 90 MWt district heating of Reykjanes suburbs of Reykjavik Blue Lagoon tourist attraction Lund and Dipippo, 2003

24 Geothermal Heat Pump also called Ground Source Heat Pump is not extracting heat from earth It makes energy use more efficient by heating or cooling more efficiently using the better heat exchange possible in saturated ground as opposed to in the air, as is done in conventional air conditioners. Ground source heat pumps are usually most effective in areas that use both heating and cooling cycles, that is, hot summers and cold winters There is almost no commonality between extractive and heat pump technologies Geothermal Heat Pumps Geothermal Systems, Settings, Lund and Dipippo, 2003

25 Exploration Data to Assess Geothermal Resource Uncertainty Is it there? POSexpl = Ptemperature * Pchemistry * Ppermeability Temperature: Water and gas geochemistry on all features Chemistry: Same as temperature but using process plots Permeability: Resistivity imaging to base of impermeable clay cap. Structural model. Map of thermal features and altered ground. Gravity and structure. If yes, how big is it? P 10, P 50, P 90 area Area: Conceptual model outlines from resistivity, geochemistry, alteration, structure, geology etc. Power Density: Analogous fields, plausible MW/km 2 Field analogies provide check on probabilistic approaches What is lowest cost exploration strategy Lowest cost well target order to failure or success Access and hazards: Review access and hazards Environmental etc: Assess risk for permit denial etc. Cumming (2013)

26 Geothermal Exploration of Rift and Arc Systems Top Data Priorities For Decision Risk Analysis Base Maps Old Boreholes Geochemistry Active Alteration Geology MT (+-TEM) Structure MEQ GPS + DEM Cumming 2013 Digital high resolution satellite 50 cm color images georectified and integrated with an SRTM DEM. In some areas, Google Earth is adequate. Digital topographic maps with culture at resolution better than 1:50000 All images and maps should be in UTM projection with datum specified Temperature logs, water/gas, cuttings descriptions, cuttings clay analyses Water chemistry of all hot springs and water wells and gas chemistry of all fumaroles, acid-sulfate features and boiling springs support with CO2 flux Based on visual review of <50 cm color images, identify candidate features and ground check for alteration type and surface temperature Standard exploration geology: volcanic history, expected reservoir rock, formation map, heat source, exposed brittle rocks like rhyolite dome as cold water source, basin structure, sediments, hydrothermal processes, alteration mapping (not just active alteration detection), eruption breccias, etc. Remote reference robust MT from Hz, better Hz QA using D+ editing, 1D Occam inversion AVG, likely 3D inversion Add TEM if likely to detect top of conductor at <30% cost increment Map lineaments, ground check geometry and rate, review evidence of extension. Implications for permeability of the interaction of structure with reservoir leakage, vertical stress, formation properties, alteration, irregularity, etc. Microearthquake monitoring for >3 months to detect magma below active basalt shield or for hazard assessment of an active andesite volcano (<4 to >100 Hz) GPS makes other methods more cost-effective if quality is controlled. Check datum and coordinate quality. Use DEM or dgps for elevation.

27 Geothermal Exploration of Fault-Based Systems Top Data Priorities For Decision Risk Analysis Base Maps Old Boreholes Geochemistry Active Alteration Structure Geology MT (+-TEM) TGW (or slim hole) Digital high resolution satellite 50 cm color images georectified and integrated with an SRTM DEM. In some areas, Google Earth is adequate. Digital topographic maps with culture at resolution better than 1:50000 All images and maps should be in UTM projection with datum specified Temperature logs, water/gas, cuttings descriptions, cuttings clay analyses Water chemistry of all hot springs and water wells and gas chemistry of all fumaroles, acid-sulfate features and boiling springs support with CO2 flux Based on visual review of <50 cm color images, identify candidate features and ground check for alteration type and surface temperature Map lineaments, ground check geometry and rate, review evidence of extension. Implications for permeability of the interaction of structure with reservoir leakage, vertical stress, formation properties, alteration, irregularity, etc. Standard exploration geology: volcanic history, expected reservoir rock, formation map, heat source, exposed brittle rocks like rhyolite dome as cold water source, basin structure, sediments, hydrothermal processes, alteration mapping (not just active alteration detection), eruption breccias, etc. Remote reference robust MT from Hz, better Hz QA using D+ editing, 1D Occam inversion AVG, likely 3D inversion Add TEM if likely to detect top of conductor at <30% cost increment If the geochemistry or old boreholes do not indicate commercial temperature Gravity +- magnetics Gravity maps structure if geometry simple, like sediments over crystalline rocks Incidental Cumming 2013 If available, use aeromagnetic and reflection seismic data but probably not costeffective to acquire.

28 Western EARS Options Geothermal Systems, Settings, Geothermal Exploration and Development Consider full range of conceptual models, including faultbased as well as volcanic-associated models Use several resource conceptual models to illustrate uncertainty with focus on structural target in suitable rocks Use TGWs or slim holes to test multiple models at lower cost Investigate acquisition of more complete versions of data collected for other purposes, including reflection seismic, airborne gravity, regional MT etc Assess viability of outcomes consistent with resource types likely to be found Cumming Geoscience

29 Generic Geothermal Geothermal Systems, Settings, Conceptual Model Elements Distributed Permeability Upflow Small Outflow Single Fault Zone Upflow Large Shallow Outflow Cumming (2013) 29

30 Geothermal Conceptual Model Elements Hydrology, especially deep water table but also perched aquifers Isotherm pattern consistent with pressure and permeability Heat Source Deep benign hot buoyant upflow in fractures Formations and alteration favorable to open space fracture permeability (and often primary permeability at shallower depths) Smectite Clay Cap (commonly combined cap, rarely, non-smectite cap) Faults creating permeable zones, flow barriers and field boundaries Reservoir temperature outflow with buoyant flow updip below clay cap (in liquid systems) Sub-commercial outflow with buoyant flow updip below clay cap Cold meteoric water flow down-dip into reservoir Cumming (2013)

31 Rationale Anomaly Hunting Works by analogy Pitfalls Conceptual relevance to new targets not considered, just outcomes Other data not conceptually integrated Not directly tested by wells Drill a 5 ohm-m anomaly and it remains 5 ohm-m Remedy Use for early and low cost decisions For high cost decisions, use conceptual models to support team risk assessment Cumming (2013)

32 Conceptual Models Geothermal Systems, Settings, Rationale Decisions based on analogous experience Conceptual differences considered Directly tested by wells Pitfalls Who can integrate geophysics, geochemistry, geology, reservoir engineering A single model is always wrong Multiple models require risk assessment Proposed Remedy Training on building conceptual models and assessing risk using case histories Cumming Geoscience

33 Generic Geothermal Geothermal Systems, Settings, Conceptual Model Elements Distributed Permeability Upflow Small Outflow Single Fault Zone Upflow Large Shallow Outflow Cumming (2013) 33

34 Geothermal Conceptual Model Definition by Isotherm Pattern The natural state isotherm pattern is essential component of a geothermal resource conceptual model and the initial constraint for numerical reservoir simulations. For simulation models, the isotherms constrain: the overall reservoir permeability pattern, bulk permeability values and boundary conditions the thermodynamic state of the reservoir including phase (boiling), upflow, outflow etc The isotherm pattern is constrained by the geoscience data and is mutually constrained by the interpreted reservoir elements: Isotherms arch upwards where water, gas and heat leak through the clay cap to surface manifestations based on surface temperature, geochemistry, alteration and resistivity Isotherms are close together in the clay cap due to the impermeable clay from resistivity imaging separating cool meteoric water from buoyantly rising hot reservoir water In systems interpreted to be hydrostatically pressured (water dominated) based on geochemistry and resistivity, the resistivity and/or geochemistry should detect a buoyant outflow that should be correlated with laterally elongated isotherms dipping upward The isotherm pattern differs for distributed permeability and single fault-hosted systems Arrows aid isotherm interpretation and are essential for out of section flow description Cumming (2013)

35 Geothermal Conceptual Model Elements Hydrology, especially deep water table but also perched aquifers Isotherm pattern consistent with pressure and permeability Heat Source Deep benign hot buoyant upflow in fractures Formations and alteration favorable to open space fracture permeability (and often primary permeability at shallower depths) Smectite Clay Cap (commonly combined cap, rarely, non-smectite cap) Faults creating permeable zones, flow barriers and field boundaries Reservoir temperature outflow with buoyant flow updip below clay cap (in liquid systems) Sub-commercial outflow with buoyant flow updip below clay cap Cold meteoric water flow down-dip into reservoir Cumming (2013)

36 Geothermal Conceptual Model Elements Hydrology, especially deep water table but also perched aquifers Isotherm pattern consistent with pressure and permeability Heat Source Deep benign hot buoyant upflow in fractures Formations and alteration favorable to open space fracture permeability (and often primary permeability at shallower depths) Smectite Clay Cap (commonly combined cap, rarely, non-smectite cap, very rarely for commercial systems, uncapped) Faults creating permeable zones, flow barriers and field boundaries Reservoir temperature outflow with buoyant flow updip below clay cap Sub-commercial outflow with buoyant flow updip below clay cap Cold meteoric water flow down-dip into reservoir Cumming (2013)

37 Geothermal Permeability Davatzes et al (2010) and Lutz et al. (2010)

38 Smectite Clay Interpretation Model for Resistivity in a Geothermal Context Hydrated smectite alteration is created over almost all Arc and Rift geothermal systems due to gas loss from hot water and it is deposited in sedimentary shales and siltstones Hydrated smectite causes low bulk permeability Hydrated smectite causes the lowest resistivity detected in all commercial geothermal systems. Archie s Law is for clay-free rock. Assumption that low resistivity implies high temperature is incorrect in a geothermal context Smectite is temperature sensitive, converting to illite clay at higher temperatures and is complete near C Low resistivity correlates with low permeability cap over high resistivity, high permeability, high temperature reservoir Cumming (2013)

39 Geothermal Conceptual Model Elements Hydrology, especially deep water table but also perched aquifers Isotherm pattern consistent with pressure and permeability Heat Source Deep benign hot buoyant upflow in fractures Formations and alteration favorable to open space fracture permeability (and often primary permeability at shallower depths) Smectite Clay Cap (commonly combined cap, rarely, non-smectite cap Faults creating permeable zones, flow barriers and field boundaries Reservoir temperature outflow with buoyant flow updip below clay cap (in liquid systems) Sub-commercial outflow with buoyant flow updip below clay cap Cold meteoric water flow down-dip into reservoir Cumming (2013)

40 Standard Geoscience Plan >200 C Arc or Rift Geothermal Exploration Gas and fluid geochemistry for existence and conceptual target MT to map base of clay cap Maybe TEM for MT statics Geology, alteration and structure for context Shallow hydrology for context Cumming (2013)

41 Global occurrence of geothermal systems in different geologic settings: their identification and utilization Mar-2016 William Cumming Cumming Geoscience, Santa Rosa CA Office: Mobile: Skype: wcumming.com