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.