Contamination of Aquifers by Overpressuring the Annulus of Oil and Gas Wells

Abstract

Medina gas wells and oil wells in northwestern Pennsylvania, northeastern Ohio, and western New York create a potential for contamination of the fresh‐water aquifers that overlie the production zones of these wells. Most of these wells are constructed in a manner which results in an open annulus which is a few hundred to a few thousand feet long below the surface casing of the well. This annulus is a potential avenue of migration of contaminants from strata of higher hydrodynamic pressure into formations of lower hydrodynamic pressure. If gas from the strata exposed to the annulus is not permitted to escape to the atmosphere, the annulus may become pressurized, and a hydraulic gradient may be created between the potential contaminants in the annulus (e.g., brine and/or natural gas) and the overlying fresh‐water aquifers. If a permeability pathway exists between the pressurized annulus and an overlying fresh‐water aquifer, contamination of the aquifer will result. The risk of contaminating fresh ground water with the contents of a gas‐ or oil‐well annulus could be greatly reduced by filling the annulus with cement. An alternative precaution would be to operate the well in a manner that does not allow the annulus pressure to exceed the normal pressure of the formations exposed to the annulus.

Well water contamination in a rural community in southwestern Pennsylvania near unconventional shale gas extraction

“…[5][6][7][8][9][10] Freshwater aquifers may be contaminated by brines or hydrocarbons from underlying formations, chemicals used in the drilling or fracturing processes, waste water, and the natural gas itself through a number of pathways. [11][12][13][14][15][16][17][18] These include but are not limited to surface contamination from leaky impoundments and spills, poor well construction resulting in failed cement and improper casing, a pressurized annulus, as well as pre-existing faults and legacy issues from previous mining and drilling operations. [11,12,19] Well construction issues involving casing and cementing failures of the gas wells account for most contamination incidents.…”

“…[11][12][13][14][15][16][17][18] These include but are not limited to surface contamination from leaky impoundments and spills, poor well construction resulting in failed cement and improper casing, a pressurized annulus, as well as pre-existing faults and legacy issues from previous mining and drilling operations. [11,12,19] Well construction issues involving casing and cementing failures of the gas wells account for most contamination incidents. [19] They result in the creation of vertical migration pathways for fluids to migrate from the surface downward or from deep formations under extreme pressure upwards.…”

Current perspectives on unconventional shale gas extraction in the Appalachian Basin

“…[97][98][99][100] In what now seems prescient, a hydrogeologist from Allegheny College, Samuel S. Harrison published two papers in the mid 1980s that discussed the groundwater contamination potential of hydraulic fracturing in the Appalachian Basin. [101,102] Among the routes of contamination considered were leaking slush pits (impoundments), surface discharge during fracturing or servicing, and road application of brine; from the subsurface through pre-existing natural fractures, a faulty bottom seal above the production zone as well as brine and gas from strata above the production zone; leaking from a pressurized annulus; and from casing of insufficient depth or improper construction. [101,102] Flow of fluids towards the surface through advective transport and natural fractures may be accelerated by the hydraulic fracturing process.…”

“…[101,102] Among the routes of contamination considered were leaking slush pits (impoundments), surface discharge during fracturing or servicing, and road application of brine; from the subsurface through pre-existing natural fractures, a faulty bottom seal above the production zone as well as brine and gas from strata above the production zone; leaking from a pressurized annulus; and from casing of insufficient depth or improper construction. [101,102] Flow of fluids towards the surface through advective transport and natural fractures may be accelerated by the hydraulic fracturing process. [97] A recent study bears out the concern for proper casing as they documented that over the past thirteen years, unconventional wells had a sixfold higher number of casing and/or cement issues reported.…”

Pennsylvania Legacy Well Integrity and Emissions Study

“…al. (2014), and historically in other publications (Harrison, 1983;Harrison, 1985 The state's plugging program, which is almost exclusively subsidized by surcharges on drilling permits that range from $150 (oil wells) to $250 (gas wells), has been unable to make substantial progress in its efforts to properly decommission wells that have no associated responsible party outside of the years when external grant monies were available ( Figure 1). In comparison, federal and other funding sources aimed at addressing legacy mining and other industrial impacts dwarf the funding allocated for managing abandoned wells in the state.…”