Orphan wells are wells with no current owner. Typically,
they are very old wells, but newer wells may be orphaned, too. The issue with
orphan wells for groundwater is that they may provide a path for contaminants
to enter groundwater aquifers. Both hydrocarbons and brine water are
contaminant sources. Existing water quality data will often miss local
contamination from orphan wells. The USGS scientists used geospatial and
statistical methods to highlight areas of concern for potential groundwater
pollution from orphan wells. Three factors were used: 1) number and density of
orphan wells in an area; 2) factors that can threaten well integrity and
contribute to contaminant transport; and 3) factors related to groundwater
withdrawal rates and the affected populations/communities in the event of water
quality disturbances. About half of the identified orphan wells can threaten
principal aquifers, and the other half can threaten secondary aquifers. Three
areas were defined as having the highest susceptibility to groundwater
contamination from orphan wells:
“1) The Appalachian Basin (including the Pennsylvanian
Aquifer System), 2) The Gulf Coast Aquifers (including the Coastal Lowlands
Aquifer system) and 3) The California Aquifers (including the California
Coastal Basin Aquifer system).”
Well age is a very good proxy
for well integrity since old wells degrade more as time passes. Standards for
casing wells were not developed until the 1950s, so wells older than that are
particularly susceptible to loss of integrity. Well integrity loss can take a
few different forms. Corrosive fluids can cause deterioration of casing,
tubing, and the cement around the casing. If brine water migrates up from
below, it can enter freshwater aquifers. The freshwater groundwater can also be
corrosive, and this is a big factor in well integrity.
“We further identified three eras to classify wells on a
national scale: pre-1935 wells, which predate the Interstate Oil Compact
Commission (IOCC) (which would go on to become the Interstate Oil and Gas
Commission, IOGCC); 1935–1972 wells, which predate the Safe Drinking Water Act
(Safe Drinking Water Act, 1974) and many regulations focused on protecting
aquifer systems; and post-1972 wells, where regulatory frameworks typically
include both environmental and oil and gas conservation concepts on a national scale
(Nickolaus et al., 2009).”
The location of orphan wells
relative to coal seams was identified as an important factor. Mine subsidence,
acid mine drainage, and coalbed methane releases can all affect well integrity.
Aquifer properties can also
affect contaminant transport. Aquifers with significant fracture porosity and
those in limestone karst voids can transport contaminants faster and farther,
and so are more susceptible to contamination.
As noted, corrosivity is an
important factor for potential contamination. They explain how they arrived at
corrosivity susceptibility:
“To derive this dataset, single point Langelier
Saturation Indices (LSI), a metric for corrosivity, were compiled for
groundwater samples from 20,962 locations in the United States from (Belitz et
al., 2016a) and joined to each principal or secondary aquifer. The mean of
these points within each AS were then computed and compared to each other.”
Another factor is groundwater
withdrawal rates. California has among the highest withdrawal rates due to
population and agricultural use. Different aquifers have different sustainable
yields, so withdrawal rates need to be normalized for the different yields of
different aquifers.
Along with corrosivity, the
groundwater interacts chemically with casing metals and cement, which can make
it more corrosive. Corrosion rates generally increase with lower pH and higher
total dissolved solids (TDS) conditions.
Aquifers that intersect
surface water bodies and wetlands are of particular concern for aquifer
contamination from orphan wells. Sensitive ecological systems can be damaged.
Below are some land use categories for orphan well areas. Many old wells were
drilled on land but are now underwater. Those wells are particularly
challenging to plug, but they also have a higher potential for damaging
contamination.
The researchers combined
susceptibility factors and withdrawal metrics to arrive at the areas most
susceptible to contamination from orphan wells, as shown below.
The researchers also
acknowledged that information about orphan wells is still very limited, and
this is problematic. They also think there could be other, as of yet,
unidentified factors that could influence groundwater contamination from orphan
wells.
The AAPG webinar, presented
by three authors of the paper, adds some additional information. They note that
hydrocarbons in groundwater may be gaseous or dissolved in water. Gas will
likely bubble out at atmospheric pressures but can present an explosion hazard.
Leaks can kill surrounding vegetation. Hydrocarbons in groundwater can feed
bacteria and lead to metals mobilization, such as iron and H2S formation from
sulfate reduction. This can lead to more corrosion in the form of pipe scale or
cement degradation. They also note that hydrocarbons and brines can occur
naturally in groundwater, which can complicate analysis. Carbon and hydrogen
isotope analysis can determine if gas in groundwater has a biogenic source or a
deeper thermogenic source, which is associated with oil & gas development.
The USGS released a dataset
in 2024 of water quality within 1 mile of orphan wells. Parameters such as TDS,
TSS, dissolved hydrocarbons, pH, temp, etc., are considered. The dataset is
targeted at industry, policymakers, and researchers. Safety, well plug
integrity, plug design vs. water quality parameters are all considered. Sulfate
analysis is important since sulfate can attack well cement. Thus, cement
recipes can be tweaked to help prevent corrosion in areas with known corrosive
groundwater. The researchers also utilize a produced water geochemistry
database to determine if brine is contaminating aquifers. They plan to release
a paper on wellbore deterioration soon.
In some legacy wells in the
Los Angeles Basin, there is groundwater entering the orphan wells. Cones of
depression in those wells have likely been detected.
Tools to assess wellbore
integrity include downhole acoustics, which the USGS utilizes. Baseline water
quality data, or pre-drill water quality data, is very important for linking
contamination to oil & gas operations, even for orphan wells. Even very
old, historical water quality data can be considered as a proxy for baseline
data before the drilling of some orphan wells. Trying to understand why plugs
fail or assessing plug integrity needs more study, since previously plugged
wells can cause problems. The giant Texas sinkhole in the Permian region is an
example where pressurized groundwater due to wastewater disposal in shallower
zones, along with loss of well integrity, can lead to serious issues. The well
around which the sinkhole occurs was plugged in the 1970s, but apparently, the
plugs are not preventing groundwater from invading the well. New cementing
recipes with clays, resins, and/or bio-cement can help improve plug
integrity.
References:
A
geospatial analysis of water-quality threats from orphan wells in principal and
secondary aquifers of the United States. USGS. May 10, 2025. A
geospatial analysis of water-quality threats from orphan wells in principal and
secondary aquifers of the United States | U.S. Geological Survey
A
geospatial analysis of water-quality threats from orphan wells in principal and
secondary aquifers of the United States, Joshua Woda, Karl B. Haase, Nicholas
J. Gianoutsos, Kalle Jahn, and Kristina Gutchess. Science of The Total
Environment. Volume 976, 10 May 2025, 179246. A
geospatial analysis of water-quality threats from orphan wells in principal and
secondary aquifers of the United States - ScienceDirect
USGS
Geospatial Analysis of Water-quality Threats from Orphan Wells in Principal and
Secondary Aquifers of the United States. AAPG Academy Webinar. May 22, 2025.
No comments:
Post a Comment