Lines-of-Evidence Approach to the Evaluation of Stray Gas Incidents

Transcription

1 Lines-of-Evidence Approach to the Evaluation of Stray Gas Incidents Lisa Molofsky, Ann Smith, John Connor, Shahla Farhat GSI Environmental Inc. Tom Wagner, Albert Wylie Cabot Oil and Gas Corporation GSI ENVIRONMENTAL INC. Houston and Austin, TX (713) CABOT OIL & GAS Pittsburgh, PA

2 Methane in Groundwater: Anthropogenic or Natural? GAS WELL MIGRATION NATURAL GAS SEEP House Explosion, Western PA KEY POINT: Eternal Flame Falls, NY Multiple sources of methane in shallow groundwater. Source identification challenging.

3 Practical Guidelines for Evaluation of Methane Impacts on Groundwater Case Study: Marcellus, NE PA Problem: Asserted that Marcellus shale gas extraction was causing stray gas impacts on water wells Lessons Learned: Lines of Evidence for Stray Gas Studies Susquehanna County PENNSYLVANIA

4 Methane Impacts on GW in Northeastern Pennsylvania: Regional Survey of Pre-Drill Methane in Water Wells Pre-Drill Methane in >1700 Water Wells, FINDINGS 78% of water wells contain methane Higher concentrations in valleys vs. uplands

5 Mechanism for Methane Accumulation in Valleys? Methane from Shallow Subsurface Valley Faults and Fractures

6 Underlying Geology of Northeastern PA: Non-Producing Gas-Bearing Units Key Strata Geologic Cross Section: Susquehanna County Glacial Till: Contains Microbial Gas Catskill & Lockhaven Formations: Thermogenic gascharged sandstone deposits (with extensive fracture networks) Marcellus shale: Gas at ~6000 ft below surface.

7 Underlying Geology of Northeastern PA Key Point Organic seams in Catskill formation are potential source of thermogenic gas

8 Fault and Fracture Networks Seismic Line: Susquehanna County, NE PA Large-Scale Faults Tully Limestone Question: Barriers to gas migration from deeper units or structural connections?

9 Historic Locations of Gas Shows: 200 years Key Point Shrewsbury Gas Field, 1969 Lovelton Gas Field, 1965 Salt Springs, pre-1800 Shallow gas shows observed in water and gas wells for over 200 years Harveys Lake Gas Field, 1956

10 Salt Springs State Park, PA: Historic Bubbling Spring Susquehanna County, PA Inscription on rock wall behind spring: JMK 1862

11 Water Well Construction and Aquifer Dynamics in NE Pennsylvania WATER WELLS: Most wells penetrate the Catskill Sandstone Open hole completions, draw water from fractures GAS WELLS: All pass through Catskill and Lockhaven Surface casing Open annular spaces Casing integrity Source: PA DCNR, 2012

12 Isotope Analyses: Source Gases Identified by Pennsylvania DEP and Cabot Upper and Middle Devonian Gases Above Marcellus Marcellus Shale Gas Source Gases Marcellus Production Gas (PA DEP & Cabot) CO 2 Reduction Microbial Gas Thermogenic Gas Fermentation Upper / Middle Devonian Gas Above Marcellus (PA DEP) Dissolved Gas Identified as Upper/Middle Devonian (PA DEP) KEY POINT: Local Marcellus shale gas may be distinguished from shallower overlying Upper and Middle Devonian gases.

13 Isotope Analyses: Water Well Gases Sampled by PA DEP, Cabot Oil, and Duke Univ. Upper and Middle Devonian Gases Above Marcellus Marcellus Shale Gas Water Samples Water Wells: Dissolved Gas (PA DEP & Cabot) CO 2 Reduction Microbial Gas Thermogenic Gas Fermentation Osborn et al. Study: Dissolved Gas In Water Wells Spring: Salt Springs State Park (Cabot) KEY POINT: Dissolved gases from local water wells are most consistent with microbial or shallow thermogenic sources.

14 Lessons Learned: Lines of Evidence for Stray Gas Investigations Geology/ Hydrogeology Presence of gas-bearing strata, fault and fracture systems, aquifer dynamics History Regional Patterns Well Construction Prior oil and gas activity, stray gas incidents and evidence of naturallyoccurring gas seeps. Regional patterns of methane occurrence and composition. Construction and completion details for affected water wells and nearby gas wells. Geochemical Fingerprinting Analyses of composition and isotopic signatures of gas sources.

15 OBJECTIVE: Effective approach for evaluation of potential impacts of shale gas development on GW resources. Credit: Bennett V/Flickr