Advanced & Emerging Topics

Subsurface Thermal Energy Storage (STES)

Store excess heat underground for seasonal use — integrates with geothermal and district heating systems:

Types of STES

• Borehole Thermal Energy Storage (BTES): Array of boreholes store heat in rock/soil — 10-100 GWh capacity
• Aquifer Thermal Energy Storage (ATES): Two wells — hot and cold — in same aquifer — cycle seasonally
• Combined ATES + heat pump: Moderate temperature storage with heat pump amplification
• Underground Thermal Energy Storage (UTES): Generic term covering all subsurface thermal storage

Underground Hydrogen Storage

Store green hydrogen in depleted reservoirs or salt caverns — emerging technology for energy storage:

Storage Options

• Salt caverns: Most suitable — impermeable rock salt, self-sealing, proven technology
• Depleted gas reservoirs: Existing infrastructure, proven geology — but gas contamination risk
• Aquifers: Most abundant — but microbial activity may consume hydrogen, water chemistry changes
• Abandoned mine shafts: Low-cost infrastructure — but limited capacity and containment concerns

Hydrogen in Aquifers

• Injectivity: Similar to natural gas — water-wet formation requires displacement
• Storage efficiency: 10-30% of pore volume — lower than gas storage due to water displacement
• Microbial consumption: Hydrogenotrophic microbes may consume 0.1-1% of stored hydrogen per year
• Geochemical reactions: H₂ reacts with minerals — can alter porosity and permeability
• Recovery rate: 60-80% — makeup injection needed to maintain cushion gas

Carbon Capture & Storage (CCS) in Saline Aquifers

Inject CO₂ into deep saline aquifers — largest potential for permanent carbon storage:

CCS Process

• Capture: Separate CO₂ from flue gas — post-combustion, pre-combustion, or oxy-fuel
• Transport: Pipeline or ship — supercritical CO₂ at >73 bar
• Injection: Deep well into saline aquifer — typically 800-2,500 m depth
• Storage: CO₂ trapped by structural, residual, solubility, and mineral trapping

Well Design for CCS

• Casing: Full-length steel casing — CO₂ is corrosive, requires corrosion-resistant alloys
• Cement: CO₂-resistant cement — standard Portland cement degrades in CO₂ environment
• Injection interval: Permeable zone below confining layer — maximizes storage security
• Monitoring: 4D seismic, pressure monitoring, groundwater sampling — verify containment
• Well integrity: Regular log surveillance — detect leaks early

Mining Pit Lakes

Post-mining groundwater rebound creates pit lakes — potential water resource or environmental liability:

• Formation: When mining ceases, groundwater rises and fills the pit — creates permanent water body
• Water quality: Often poor — acid mine drainage, heavy metals, high TDS
• Treatment options: Aeration, chemical treatment, constructed wetlands, blending with clean water
• Beneficial use: If treated — recreation, aquaculture, irrigation, industrial supply
• Management: Long-term monitoring required — water quality may change over decades

Coastal Managed Aquifer Recharge

MAR in coastal settings specifically targets saltwater intrusion prevention and freshwater storage:

• Hydraulic barrier: Injection wells create freshwater ridge — prevents saltwater advance
• Extraction-injection cycle: Extract brackish water, treat, reinject as freshwater
• ASR for coastal areas: Inject during wet season, extract during dry — buffer against sea-level rise
• RBF near coast: River bank filtration induces freshwater flow toward coast — natural barrier

Design Considerations

• Freshwater-saltwater interface: Model using Ghyben-Herzberg — design injection to maintain interface
• Injection rate: Must exceed extraction rate — net positive freshwater balance
• Water quality: Pre-treat to prevent aquifer clogging and chemical reactions
• Monitoring: Network of piezometers and salinity sensors — track interface movement
• Climate adjustment: Account for sea-level rise — interface may advance 10-50 m per decade