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Stray Gas Impacts: Sources and Mechanisms of Migration
Lisa Molofsky, Ann Smith, John Connor
GSI Environmental Inc.
Houston and Austin, TX (713) 522-6300
GSI ENVIRONMENTAL INC.
Stray Gas Impacts: Why do we care?
Hydrocarbon Gas Composition: >95%: Methane <5%: Ethane, Propane, Butane, etc.
Methane
Does methane pose a health hazard? Methane is not toxic below the lower
explosive limit of 5% (50,000 ppm). No long-term health effects
Key Problem: Methane is EXPLOSIVE in concentrations
above the lower explosive limit. Methane can act as an asphyxiant at high
concentrations.
http://www.energy.alberta.ca/NaturalGas/723.aspa
House Explosion, Bradford County, PA
Natural Sources
■ Swamps, lakes, and glacial sediments
■ Shallow gas-bearing deposits
■ Coal seams
Methane in Groundwater: Anthropogenic or Natural Sources?
Source:http://en.wikipedia.org/
Anthropogenic Sources
■ Gas pipelines ■ Subsurface storage
reservoirs ■ Coalbed methane recovery ■ Municipal waste landfills ■ Oil and gas wells
Source:http://www.pnl.gov/science
Methane in Groundwater: Natural Sources
http://www.nature.com/news/2010
http://www.nicerweb.com http://spectregroup.wordpress.com/2010/06/10/methane-based/
http://rtec-rtp.org/tag/eternal-flame-waterfall/
■ Swamps, lakes, and glacial sediments
■ Shallow gas-bearing deposits
■ Coal seams
http://blogs.agu.org/magmacumlaude/2010/
Swamps, Lakes, Sediment
Natural Gas Seeps
Bubbling Springs/ Ponds
http://nature.nps.gov/nnl/site.cfm
Methane in Groundwater: Anthropogenic Sources
■ Gas pipelines ■ Coalbed methane recovery ■ Municipal waste landfills ■ Subsurface storage
reservoirs ■ Oil and gas wells
Landfill Gas
http://www.budgetgreenliving.com/
Coalbed Gas
http://www.humanevents.com
Natural Gas Pipeline
http://www.pnl.gov/science/images/highlights/
Gas Storage Field
http://barryonenergy.wordpress.com
http://texasvox.org/2011/04/11/
Oil and Gas Extraction
Methane in Groundwater: Sources Associated with Oil and Gas Extraction
Similarities: Well Construction Same casing & cementing design/ practices Differences: Extraction Technique Hydraulic fracturing is used to extract shale gas
http://www.glossary.oilfield.slb.com
http://www.fracfocus.com
Surface casing
Inter-med. casing
Production tubing
Cement casing
Drilling mud Production
casing
Well pad
Shale Gas Concerns: Subsurface Impacts
? Subsurface breach
Surface casing vent
Failure of Surface Casing or Cement Rupture of surface casing can inject natural gas into aquifers and water wells.
1
Concern that fractures propagate upward to drinking water zones, contaminating wells with injection fluids.
Uncontrolled Fractures
2
1000
2000
3000
4000
5000
6000
7000
8000
9000
Uncontrolled Fractures: Impact on Drinking Water ?
Mapped Fractures in Marcellus Shale
SOURCE: Pinnacle, 2011
Depth of Hydraulic Fracture Zone
DEP
TH
IN
FEET
■ Microseismic mapping of hydraulic fractures shows no impact on DW aquifers.
■ Fracture migration limited by “bleed out,” stratigraphy, laws of physics, etc.
> 1000 m Depth of Drinking
Water Wells
GROUND SURFACE 0
Conductor Hole Casing
Surface Casing
Intermediate Casing
Production Casing
Perforation Blasts
Intermediate Gas-Bearing Zone
Target Producing Zone
Fresh Water Aquifer
Drill Hole Casing Pipe Cement
Fracturing Fluid
Hydraulic Fractures
Shale Gas Well Construction
Seal
Shallow Gas- Charged Lens
Perforation Blasts
Intermediate Gas-Bearing Zone
Target Producing Zone
Fresh Water Aquifer
Hydraulic Fractures
Open Hole (No Cement)
Seal Cement or Casing Integrity Breach
Pressure Builds up in Casing String
Shale Gas Well Construction: Potential Issues
Shallow Gas- Charged Lens
Un-Conventional vs. Conventional Gas: New Challenges
Major Oil-Producing Statesin U.S. (> 50 Mill bbls/yr) Principal Shale Gas Basin
SOURCE: www.eia.gov/oil_gas/rpd/shale_gas.jpg
New Geology: New areas of exploration with complex geology that is not always well understood
Bakken
■ Faults and fractures ■ Depth to water ■ Presence of shallow
gas-bearing deposits
How do you characterize gas sources? Microbial vs. Thermogenic Gas
Formed via biodegradation of organic material in shallow subsurface
Formed via breakdown of organic material under high temps. and pressures at depth
Thermogenic Microbial
KEY POINT: Geochemical data can narrow the list of potential sources.
Anthropogenic Natural
■ Deep Oil and Gas Deposits
■ Coalbed Extraction ■ Natural Gas Systems ■ Pipelines ■ Subsurface Storage
Reservoirs
■ Shallow Gas Bearing Units
■ Coal Seams
Thermogenic
■ Municipal Waste Landfills
■ Wetlands (swamps and lakes)
■ Glacial Till and Alluvium
Microbial
Thermogenic and Microbial Gas: Can Originate from Both Anthropogenic and Natural Sources
Pre-Drill Sampling
Method: Establish Baseline Groundwater Quality Samples are collected from water wells within an established distance (e.g., 2500 ft.) of a proposed gas well prior to drilling. Purpose: Detect Groundwater Impacts Potential impacts to groundwater quality can be identified based on the change in groundwater quality from pre-drill to post-drill sampling.
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