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Surface Casing Pressure As an Indicator of Well Integrity Loss and Stray Gas Migration in the Wattenberg Field, Colorado

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The risk of environmental contamination by oil and gas wells depends strongly on the frequency with which they lose integrity. Wells with compromised integrity typically exhibit pressure in their outermost annulus (surface casing pressure, SfCP) due to gas accumulation. SfCP is an easily measured but poorly documented gauge of well integrity. Here, we analyze SfCP data from the Colorado Oil and Gas Conservation Commission database to evaluate the frequency of well integrity loss in the Wattenberg Test Zone (WTZ), within the Wattenberg Field, Colorado. Deviated and horizontal wells were found to exhibit SfCP more frequently than vertical wells. We propose a physically meaningful well-specific critical SfCP criterion, which indicates the potential for a well to induce stray gas migration. We show that 270 of 3,923 wells tested for SfCP in the WTZ exceeded critical SfCP. Critical SfCP is strongly controlled by the depth of the surface casing. Newer horizontal wells, drilled during the unconventional drilling boom, exhibited critical SfCP less frequently than other wells because they were predominantly constructed with deeper surface casings. Thus, they pose a lower risk for inducing stray gas migration than legacy vertical or deviated wells with surface casings shorter than modern standards.
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Surface Casing Pressure As an Indicator of Well Integrity Loss and
Stray Gas Migration in the Wattenberg Field, Colorado
Greg Lackey,*
,
Harihar Rajaram,
Owen A. Sherwood,
Troy L. Burke,
and Joseph N. Ryan
Department of Civil, Environmental and Architectural Engineering, University of Colorado, Boulder, Colorado 80309, United States
Institute of Arctic and Alpine Research, University of Colorado, Boulder, Colorado 80309, United States
*
SSupporting Information
ABSTRACT: The risk of environmental contamination by oil
and gas wells depends strongly on the frequency with which they
lose integrity. Wells with compromised integrity typically exhibit
pressure in their outermost annulus (surface casing pressure,
SfCP) due to gas accumulation. SfCP is an easily measured but
poorly documented gauge of well integrity. Here, we analyze
SfCP data from the Colorado Oil and Gas Conservation
Commission database to evaluate the frequency of well integrity
loss in the Wattenberg Test Zone (WTZ), within the
Wattenberg Field, Colorado. Deviated and horizontal wells
were found to exhibit SfCP more frequently than vertical wells.
We propose a physically meaningful well-specic critical SfCP
criterion, which indicates the potential for a well to induce stray
gas migration. We show that 270 of 3923 wells tested for SfCP in the WTZ exceeded critical SfCP. Critical SfCP is strongly
controlled by the depth of the surface casing. Newer horizontal wells, drilled during the unconventional drilling boom, exhibited
critical SfCP less frequently than other wells because they were predominantly constructed with deeper surface casings. Thus,
they pose a lower risk for inducing stray gas migration than legacy vertical or deviated wells with surface casings shorter than
modern standards.
INTRODUCTION
Despite recent slowing of drilling activity in the United States,
concerns about the environmental impacts of hydraulic
fracturing associated with unconventional oil and gas develop-
ment persist.
1,2
Among these concerns is the potential for stray
gas originating from oil- and gas-bearing reservoirs to migrate
into shallow groundwater aquifers.
38
Current evidence
suggests that faulty oil and gas wells with compromised
integrity are the principal transport pathway for stray
gas.
57,915
Thus, understanding well integrity and quantifying
the rates at which wells lose integrity is critical for assessing
risks to groundwater quality.
1618
Wells with integrity issues
can also contribute to fugitive emissions of methane and other
volatile organic compounds to the atmosphere, thus contribu-
ting to greenhouse gas burdens and deterioration of regional air
quality.
13,1921
Oil and gas wells are built as a system of nested steel casings
(pipes) and cement. At a minimum, wells consist of a surface
casing set deep enough to protect shallow aquifers and a
production casing that connes hydrocarbons as they are
brought to the surface (Figure 1). Cement pumped into the
annular space outside the casings forms a seal and isolates
hydrocarbons and other uids in their respective forma-
tions.
11,22
In the U.S., regulations for surface casing depth
and production casing cement height vary by state. Of the 36
states with oil and gas regulations, only six states (AK, AZ, ID,
KY, MO, NC) require production casings to be cemented into
the base of the next larger diameter casing or to the ground
surface (Supporting Information (SI) Table S1). The remaining
states typically require the production casing to be cemented to
a specied height above the shallowest hydrocarbon bearing
formation; above this height, the production casing is left
uncemented with an open annulus extending to ground surface
(Figure 1). Oil and gas operators are free to install wells with
deeper surface casings and greater production casing cement
coverage than required by regulation. This practice has become
more common in recent years for modern unconventional wells
in Colorado (SI Table S3, Figure S2).
23
Hydrocarbons that
enter the open annulus are naturally buoyant and migrate
upward. Migrated gases are either vented through the wellhead
valve located between the surface and production casings (this
would create surface-casing-vent ow, SCVF) or collect behind
this valve and create pressure that we refer to as surface casing
pressure (SfCP).
24,25
Regulations that determine whether or
not the wellhead valve is left open or sealed vary across the
U.S.; and Canada
13
(SI Table S2).
Received: November 30, 2016
Revised: February 15, 2017
Accepted: February 16, 2017
Published: February 16, 2017
Article
pubs.acs.org/est
© 2017 American Chemical Society 3567 DOI: 10.1021/acs.est.6b06071
Environ. Sci. Technol. 2017, 51, 35673574
SfCP and SCVF are valuable quantitative indicators of oil
and gas well integrity. Except in the case of thermally induced
SfCP during initial operation,
24
positive SfCP and SCVF only
occur when gases from a production casing leak, faulty cement
seal, or improperly isolated hydrocarbon-bearing formation
escape into the open annulus. Stray gas migration occurs when
gas traveling vertically along the annulus either circumvents the
surface casing (Figure 1) or builds SfCP sucient enough to
force gas out of the bottom of the surface casing.
9,10,13,26
Leaving the surface casing valve open helps prevent SfCP-
induced stray gas migration; however, SCVF constitutes a
greenhouse gas emission. SfCP and SCVF data are more
valuable than qualitative indicators of well integrity loss, such as
inspector notes, violation notices, and well remediation records,
because they provide insight into various levels of integrity loss,
some of which pose a greater risk of causing contamination.
Due to a lack of publicly available data
18
(SI Table S2), recent
studies of oil and gas well integrity have relied largely on the
aforementioned qualitative indicators.
1,16,18,23,27
Only one
study, in Alberta, Canada, has analyzed a quantitative indicator
of well integrity (SCVF) to investigate the frequency of
integrity loss among onshore oil and gas wells.
28
However, this
study focused on well integrity in the context of carbon dioxide
sequestration and did not connect well integrity issues to stray
gas migration, which is the primary purpose of this work.
In Colorado, the Colorado Oil and Gas Conservation
Commission (COGCC) has the authority to establish
designated SfCP testing regions.
29
SfCP data collected from
these designated SfCP testing regions are made publicly
available online.
30
The Wattenberg Test Zone (WTZ), a
subset of the Wattenberg Field, was designated for SfCP testing
by the COGCC in 2010 (SI Table S5).
31
The Wattenberg Field
is the most densely drilled and productive area of the Denver-
Julesburg Basin in northeastern Colorado (Figure 2). Conven-
tional reserves in the Wattenberg Field were discovered in the
1970s
32
and development of the unconventional reservoirs in
the eld, which began in the 1980s,
33
has made it the fourth-
most productive crude oil and ninth-most productive gas eld
in the U.S.
34
Three well congurations, classied by the
COGCC, have been installed in the WTZ: vertical, deviated
and horizontal (SI Table S3 and Figure S1). Deviated wells are
drilled at an angle from the vertical but are not fully horizontal
(e.g., S-shaped wells). Hydraulic fracturing has been used in the
Wattenberg Field for over 60 years, regardless of the well
conguration, to stimulate the low-permeability reservoirs in
the region.
32,33,35
Public attention was drawn to the potential
relationship between oil and gas development and stray gas
migration in the Wattenberg Field after a number of water well
Figure 1. Cross-sectional schematic (not to scale) of a vertical well
installed in the Wattenberg Field. Major well components are
identied along with geologic formations and their depths. Four gas
leakage scenarios are shown: (1) production casing leak, (2)
improperly isolated hydrocarbon bearing formation, (3) faulty cement
seal along the production casing, and (4) interval of faulty production
casing cement. Scenarios 1, 2, and 3 lead to the development of SfCP
and in Scenario 4 stray gas circumvents the open annulus and the
surface casing. Improperly isolated gas bearing formations (Scenario 2)
need to be considered in general, but are not relevant in the Pierre
Shale.
Figure 2. Map of the Wattenberg Field in Northeastern Colorado that
includes the Wattenberg Test Zone (study area) and surface casing
pressure (SfCP) hotspot. All oil and gas wells in the region are
displayed and wells with SfCP data are distinguished. Thermogenic
methane contamination incidents in groundwater and oil and gas wells
with SfCP critical are shown. Culprit oil and gas wells and wells with
both SfCP critical and short surface casings (SSC) are identied.
The locations of northeast-trending wrench faults, the High Plains
aquifer (HP), Dakota-Cheyenne aquifer (DC), and the Denver Basin
Aquifers (Denver (De), Arapahoe (Ar), Laramie (La), Laramie-Fox
Hills (LFh)) are also shown.
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contamination incidents in the early 1980s resulted in res and
explosions.
36,37
COGCC documents and investigative reports
indicate that 42 water wells in the region were contaminated
with thermogenic methane (see SI) between 1988 and 2014
(Figure 2). Thirty-two separate cases were opened by the
COGCC to investigate the contamination of these water wells
and in 11 cases a culpritoil and gas well with compromised
integrity was identied (SI Table S6); the remaining cases were
either settled privately or are currently unresolved.
7
The 2010 COGCC policy that established the WTZ
mandated SfCP testing for legacy wells with short surface
casings. Any surface casing that did not extend below the depth
of the deepest principal aquifer in the Denver Basin, the
Laramie-Fox Hills aquifer, was dened as a short surface casing
(Figure 2). After the SfCP testing requirement lapsed in late
2010, a misunderstanding of the regulation by operators
resulted in the submission of thousands of additional SfCP tests
from various types of wells with and without short surface
casing (SI Figure S3), which serendipitously produced a
detailed data set in the region. In this study, we analyze SfCP
and well construction data collected by the COGCC prior to
2015, to assess the integrity of oil and gas wells in the WTZ.
We identify a region of higher SfCP (SfCP hotspot) occurrence
within the WTZ. We present the concept of critical surface
casing pressure, a well-specic physically based SfCP limit that
when exceeded indicates the potential for a well to induce stray
gas migration. Potential relationships between well construction
factors and the development of SfCP are evaluated through
logistic regression. We estimate overall rates of SfCP develop-
ment in the WTZ and determine the impact of the recent
expansion of unconventional drilling on oil and gas well
integrity and stray gas migration.
MATERIALS AND METHODS
Data Collection and Quality Control. Using custom
computer scripts, well construction details were downloaded
from the COGCC online facility database.
30
SfCP test reports
for wells in the WTZ were downloaded from the COGCC
online document database.
30
Only data from SfCP test reports
submitted in text-based portable document format were
electronically read (SI Figure S4). Water well locations,
screened interval depths, and depth to water were downloaded
from the Colorado Division of Water Resources (DWR)
database.
38
All data were limited to the WTZ, whose extent (T 1S-4N, R
64W-68W) was dened in the COGCC Wattenberg Braden-
head Testing Policy.
31
All oil and gas wells were ltered by
current status to include only wells that are currently, or have
once been, active. We considered only wells installed and SfCP
tests performed before 2015. To ensure the quality of the well
construction data, we discarded wells with erroneous
construction data (see SI). In the WTZ, 10 365 of the 15 463
active oil and gas wells in the WTZ passed our quality control
(QC) (SI Table S10). To assess the validity of the custom
computer scripts used in this study, 100 random SfCP tests,
100 random DWR aquifer depth reports and well construction
data from 50 random oil and gas wells were manually compared
with their source information, no errors were identied. Ninety-
nine SfCP test reports with SfCP > the 98th percentile were
also manually inspected, no errors were found. Water wells,
used to determine the static depth to water in the WTZ, with
depths to water > the 98th percentile were considered outliers
and removed from the data set.
Well Integrity Loss. In general, oil and gas wells are
considered to have lost integrity if they exhibit sustained SfCP,
a phenomenon that only occurs when there is a constant source
of gas entering the outermost annulus of the well (Figure 1).
Wells can also exhibit unsustained SfCP which can be thermally
induced after initial well construction or caused by the
inltration of small quantities of gas into the open annulus
from the shallow subsurface.
24
For these reasons, investigations
of well integrity based on SfCP distinguish between wells with
sustained and unsustained SfCP.
24
Unfortunately, SfCP testing
in Colorado only requires pressure to be recorded as gas is bled
from the annulus and the majority of wells in the WTZ have
been tested for SfCP only once (see SI). Thus, we had to
develop other criteria for distinguishing sustained and
unsustained SfCP.
Isotopic and compositional analyses of gases collected from
the surface casing annuli of 48 oil and gas wells in the
Wattenberg Field (SI Table S11, Figures S14S16) indicated
that thermogenic gas was predominantly the cause of SfCP in
the region. Only two surface casing gas samples had mixed
microbial-thermogenic methane (SI Figure S15). Comparison
of surface casing gas with gas from known oil- and gas-
producing formations in the Wattenberg Field indicates that
annular gas originated from all four of the principal hydro-
carbon reservoirs (Sussex/Shannon, Niobrara, Codell, and J-
Sand) (SI Figure S15). Thus, SfCP in the WTZ is not the result
of production from or improper isolation of a specic formation
and there is no evidence that SfCP has been caused by gas
inltration from the widely distributed shallow coal seams in
the Denver Basin aquifers, which have a distinctly microbial
signature.
7
Additionally, the presence of thermogenic surface
casing gas demonstrates that SfCP in the sampled wells was not
thermally induced, because thermally induced SfCP would be
caused by vapors from the heated uids in the annulus and not
thermogenic methane. In this study, we set a relatively high
SfCP limit of 1034 kPag (150 psig) as an indicator of well
integrity loss. This limit has a history of use in Colorado as it
has been previously used by the COGCC as a limit of
concern.
39
We acknowledge that the percentage of wells that
we identify with compromised integrity is sensitive to the
selected SfCP limit and lower limits of SfCP could be justied.
However, the trends of well integrity loss that we examine in
this study remain the same regardless of the chosen SfCP limit
and we show the full distribution of the frequency with which
wells in the WTZ exceed various rates of SfCP in Figure 3.
Surface Casing Pressure Hotspot. The properties of the
geologic formations in which wells are drilled inuence their
initial construction and their ability to maintain their integrity.
11
Thus, the incidence of SfCP may vary spatially within a basin.
We divided the WTZ into Public Land Survey System sections
(1 mile2) and calculated the percentage of oil and gas wells in
each section with SfCP 1034 kPag. We then employed the
ArcGIS hotspot analysis tool that tests for spatial clustering
(SfCP 1034 kPag) based on the Getis-Ord Gi*statistic,
which resulted in the identication of a SfCP hotspot (Figure
2).
40
Any section with a hotspot condence 90% was
considered to be a part of the SfCP hotspot (SI Figure S17).
In this study, we juxtapose SfCP occurrence rates inside and
outside the hotspot to illustrate the varying degrees of well
integrity loss that can occur within a basin.
Calculation of Critical Surface Casing Pressure. While
SfCP can indicate that the integrity of a well has been
compromised, its occurrence does not always correspond with
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stray gas migration. Stray gas that enters the uncemented
annulus of an oil and gas well travels vertically until it is trapped
in the headspace above the liquid in the annulus and conned
in the wellhead. Hydrostatic equilibrium between the annular
liquid and the water in the formation at the bottom of the
surface casing is maintained because the portion of the
outermost annulus below the surface casing is open to the
surrounding formation (Figure 1). This relationship is
described by
ρρ++=+
P
gH P gHSfCP
atm mmatm
w
w
(1)
where Patm is atmospheric pressure, ρmis the annular liquid
density, ρwis the density of water, gis the acceleration due to
gravity, Hmis the height of the liquid column above the bottom
of the surface casing in the annulus, and Hwis the height of the
potentiometric surface above the bottom of the surface casing
(SI Figure S19). As SfCP increases, Hmdecreases (SI Figure
S19), that is, annular liquid is displaced to maintain equilibrium
with the formation uid pressure. Here, we propose a well-
speciccriticalSfCP as the SfCP required to push annular
liquids below the bottom of the surface casing of a well. Critical
SfCP is reached when Hmis zero, thus critical SfCP is equal to
the uid pressure in the formation at the bottom of the surface
casing (ρwgHw). When critical SfCP is exceeded, there is no
longer a barrier between the gas in the annulus and the
surrounding formation. Thus, exceedance of critical SfCP is a
necessary (but not sucient) condition for inducing stray gas
migration. Stray gas migration will be induced if SfCP also
exceeds the entry pressure of the formation or fractures within
the formation.
13
If SfCP becomes larger than lithostatic
pressure at the bottom of the surface casing, the surrounding
formation will fracture, which further exacerbates gas
migration.
13
To determine Hw, we downloaded static depth to water data
(dw) for 10 715 water wells (SI Figure S20) installed in the
WTZ from the Colorado DWR database.
38
From these water
wells, we interpolated a dwfor each aquifer and assigned a value
to each oil and gas well based on location and surface casing
depth. The potentiometric head Hwin the surface casing of
each oil and gas well was determined by subtracting the
associated dwfrom the surface casing depth. Wells with surface
casings installed below the Laramie-Fox Hills aquifer were given
the dwof the Laramie-Fox Hills. The formation uid pressure at
the bottom of the surface casing was calculated using the
estimated Hwand by assuming ρw= 1000 kg m3for water (see
SI).
Logistic Regression. We performed 32 logistic regressions
(SI Tables S12S19) to investigate potential relationships
between predictor variables in the COGCC online database
and the occurrence of SfCP 1034 kPag and SfCP critical.
Logistic regressions were calculated using the logit function in
the Statsmodels 0.6.1 package for the Python programming
language.
41,42
RESULTS AND DISCUSSION
Surface Casing Pressure and Integrity Loss. Nonzero
SfCP was recorded in 3046 of the 3923 oil and gas wells with
SfCP tests. Reported SfCP values range from 0 to 7943 kPag
(SI Figure S5). The entire frequency distribution of SfCP for
wells in the WTZ is shown in Figure 3 (and SI Figure S6),
which facilitates evaluation of dierent degrees of well integrity
loss. In the WTZ, 541 wells exhibited SfCP 1034 kPag, our
assumed SfCP limit indicating well integrity loss (Table 1).
Within the SfCP hotspot (Figure 2) 2306 of the 5040 wells
investigated had SfCP tests (SI Table S7). Of the tested wells,
2,045 had nonzero SfCP and 435 exhibited SfCP 1034 kPag
(Table 1). The SfCP hotspot overlies the portion of the
Wattenberg Field with the highest formation pressures and
temperatures (SI Figure S18).
43
A possible explanation for the
SfCP hotspot is that temperatures and pressures in the targeted
formation make cement installation more challenging and
increase the pressure gradient across production casing
cements, which drives upward ow of hydrocarbons into the
uncemented annulus.
11
Unfortunately, we did not have the
bottom hole temperature data needed to conrm this
Figure 3. SfCP exceedance frequency distribution in the WTZ,
colored by well type. Percentages were calculated as a fraction of wells
with readable SfCP tests.
Table 1. Summary of the Number and Percentages of Wells with Readable Tests of Each Conguration with Surface Casing
Pressure (SfCP) 1034 kPag and SfCP Critical
a
well SfCP 1034 kPag SfCP critical
conguration entire WTZ inside hotspot outside hotspot entire WTZ inside hotspot outside hotspot
all 541 (13.79%) 435 (18.87%) 106 (6.55%) 270 (6.88%) 215 (9.33%) 55 (3.40%)
vertical 155 (7.41%) 129 (10.85%) 26 (2.88%) 109 (5.21%) 86 (7.23%) 23 (2.55%)
deviated 316 (21.88%) 257 (28.43%) 59 (10.93%) 156 (10.80%) 126 (13.94%) 30 (5.56%)
horizontal 70 (18.04%) 49 (23.11%) 21 (11.93%) 5 (1.29%) 3 (1.42%) 2 (1.14%)
a
Numbers are provided for the entire Wattenberg Test Zone (WTZ) and inside and outside of the SfCP hotspot in the the WTZ. Percentages in
each category were calculated as a fraction of the number of wells with readable SfCP tests.
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hypothesis. Regions with elevated rates of integrity issues,
analogous to the SfCP hotspot, have also been identied in
Pennsylvania and Alberta, Canada.
16,28
We also compare the frequency of integrity loss between the
three dierent well congurations identied by the COGCC:
vertical, deviated and horizontal. In the WTZ, deviated and
horizontal wells exhibited SfCP 1034 kPag more frequently
than vertical wells inside and outside the SfCP hotspot (Tables
1,SI S7,S8, Figure S7). The higher frequency of SfCP 1034
kPag occurrence among deviated and horizontal wells indicates
that directionally drilled wells in the WTZ lose integrity more
frequently than vertical wells. This agrees with the previous
studies of Watson and Bachu 2009 and Ingraea et al., 2014,
but disagrees with Fleckenstein et al., 2015.
16,23,28
Watson and
Bachu 2009 reported that deviated wells exhibited SCVF at a
higher rate than vertical wells in Alberta, Canada; and Ingraea
et al., 2014 used inspector notes and violation notices to show
that unconventional horizontal wells have lost integrity at a
higher rate than conventional vertical wells in Pennsylvania.
16,28
Fleckenstein et al., 2015 concluded that there was no evidence
of integrity loss in horizontal wells installed in the Wattenberg
Field; however, their criterion of remedial cement below the
surface casing bottom as an indicator of integrity loss
automatically excluded a majority (73.5%) of horizontal wells
in the region that were originally constructed with production
casings cemented into the bottom of the surface casing.
23
Critical Surface Casing Pressure and Stray Gas
Migration. In the WTZ, 270 oil and gas wells had SfCP that
matched or exceeded their calculated critical SfCP (Figure 4).
Deviated wells exceeded critical SfCP more frequently than
vertical and horizontal wells both inside and outside the SfCP
hotspot (Table 1,SI Table S8). A majority (79.6%) of the wells
with SfCP critical SfCP were located within the SfCP hotspot
(Figure 2).
The 2000 m thick Pierre Shale, an extremely low
permeability formation (1 ×1018 m2), lies beneath the
Denver Basin aquifers in the WTZ.
44
A well that has SfCP
critical is less likely to leak stray gas into the overlying aquifers
if its surface casing extends into the Pierre Shale. The
eectiveness of the Pierre Shale as a barrier may be undermined
in some locations by the presence of fractures that have lower
entry pressure or an interbedded unit of high permeability.
35,37
Gas migration could also occur in the absence of SfCP or SCVF
if stray gas circumvents the surface casing altogether (Figure
1).
28
However, circumventing gas would need to migrate
thousands of meters upward through the Pierre Shale to reach
overlying aquifers, which is relatively improbable. Thus, in the
geological setting of the WTZ, legacy wells with short surface
casings installed above the Pierre Shale that develop SfCP
critical pose the greatest risk of contaminating potable
groundwater with stray gas.
Ten of the 11 culprit wells identied by the COGCC as
sources of thermogenic stray gas in their investigations of water
well contamination are in the WTZ and three of these are
within the SfCP hotspot (Figure 2). Although all of the culprit
wells had SfCP tests, only seven wells had tests performed
around the time of their stray gas release and before the well
was remediated (SI Table S6). SfCP was recorded in all seven
wells with appropriately timed tests. Six of these culprit wells
had SfCP that exceeded their critical SfCP (Figure 4) and one
had SfCP greater than lithostatic pressure (SI Figure S21). All
of the 11 culprit wells were legacy vertical wells with short
surface casings that did not protect the contaminated aquifer.
While few in number, these culprit wells serve to illustrate well
construction practices and SfCP levels that have caused stray
gas migration and they also conrm the validity of the critical
SfCP as an index of gas migration risk.
Of the 1531 legacy wells with short surface casings in the
WTZ, 916 had readable SfCP tests and 46 exhibited SfCP
critical (SI Table S9,Figure 4,SI Figure S6). The majority of
the 46 wells with short surface casings that had SfCP critical
were vertical (41) and the remainder were deviated (5). Three
horizontal wells have been installed with a short surface casing
in the WTZ, but none of them have undergone a SfCP test.
Only eight wells, six vertical and two deviated, have exhibited
SfCP greater than the lithostatic pressure at the base of the
surface casing (SI Table S7, Figure S21). Additional cement has
been added to 709 legacy wells in the WTZ indicating that they
were targeted for remediation, 28 of these remediated wells
exhibited SfCP critical at some point in their lifetime (SI
Table S4). We could not electronically identify remediation
dates, so the ecacy of well remediation was not evaluated.
Factors Contributing to Surface Casing Pressure.
Previous studies have investigated the inuence of a variety
of factors on well integrity loss.
28
We employed logistic
regression to identify potential relationships between all the
well information present in the COGCC database that could
logically inuence well integrity and the development of SfCP
1034 kPag and SfCP critical (see SI). Because of the
controversy surrounding the process of hydraulic fracturing, we
specically investigated whether the number of fracture
treatments and the volume of fracturing uid used inuence
SfCP (SI Tables S16S19, Figure S22). Of the 13 factors
investigated, only wellbore deviation (odds ratio (OR) = 1.8, p-
Figure 4. Formation uid pressure at surface casing bottom plotted
against surface casing pressure for all wells installed in the WTZ,
colored by well type. Wells with short surface casing (SSC) are
distinguished from wells installed with a surface casing that meets
current regulations. SfCP tests performed on culprit wells, identied
by the COGCC as a source of stray gas contamination, before the well
was remediated are also identied. Wells in the gray region have SfCP
critical SfCP (1:1 line) and pose a higher risk of inducing stray gas
migration.
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value =1.6×105) and location in the high SfCP hotspot (OR
= 1.6, p-value =1.8 ×105) were found to be associated with
SfCP 1034 kPag. Well construction after 2010 (OR = 0.36, p-
value = 2.9 ×1010) was the only factor found to be negatively
associated (OR < 1) with SfCP 1034 kPag (SI Tables S12,
S13). Similar relationships were found between wellbore
deviation, location in the SfCP hotspot, and well installation
era and the development of critical SfCP (SI Tables S14, S15).
The logistic regression did not reveal a statistically signicant
relationship between the number of hydraulic fracturing
treatments or the volume of hydraulic fracturing uid used
and the occurrence of either SfCP 1034 kPag or SfCP
critical.
Regional Rates of Well Integrity Loss. Estimating
regional rates of well integrity loss has been the primary
focus of many recent studies, which have also attempted to
characterize the overall environmental risks posed by the
expansion of unconventional oil and gas develop-
ment.
1,16,18,23,27
It would be inaccurate to extrapolate a
frequency of integrity loss for all the oil and gas wells in the
entire Wattenberg Field from SfCP data in the WTZ because
the SfCP data we analyzed in this study was acquired through
nonuniform and inherently biased sampling techniques (see SI)
and our estimated frequency of integrity loss is tied to the
chosen SfCP limit. However, we can determine a low-end
estimate of the frequency with which wells in the WTZ exhibit
specic levels of SfCP by calculating the percentage of wells
that exhibit SfCP as a fraction of all the QC wells installed in
the region (10,365). Note that we assume zero SfCP in QC
screened wells that did not have readable SFCP tests, to derive
the low-end estimates. Using this approach, we estimate that at
least 29.4% (3047), 5.2% (541) and 2.6% (270) of oil and gas
wells in the WTZ have exhibited nonzero SfCP, SfCP 1034
kPag, and SfCP critical, respectively. Wells with short surface
casings and SfCP critical that pose the greatest risk of
releasing stray gas comprise at least 0.4% (46) of wells in the
WTZ. For comparison, previously estimated rates of integrity
loss are 4.6% in Alberta, Canada,
28
between 2.0% and 6.6% in
Pennsylvania,
1,16,18,27
and 2.3% in the Wattenberg Field.
23
While our low-end estimates of the percentage of wells in the
WTZ that have exhibited SfCP 1034 kPag and SfCP
critical are within the range of integrity loss frequencies
estimated elsewhere, our data suggests a higher percentage of
wells with nonzero SfCP, and a lower percentage of wells that
pose a high risk for inducing stray gas migration.
Temporal Trends in Surface Casing Pressure Occur-
rence. Assessing the impact of the unconventional drilling
boom on well integrity in the WTZ is more complex than
estimating the frequencies of integrity loss before and after a
specic year corresponding to a shift in technology. Unlike
other regions of the US, there is a longer history of
unconventional drilling in the Wattenberg eld. In the early
2000s, oil and gas development in the WTZ began to expand
(Figures 5,SI S8S13). Deviated drilling grew between 2003
and 2009 but quickly gave way to horizontal drilling,
introduced in 2010 (SI Figure S1). The fraction of wells
installed each year in the WTZ that developed SfCP 1034
kPag increased with the expansion of deviated drilling. Since
2009, the percentage of new wells installed in the WTZ
exhibiting SfCP 1034 kPag decreased annually, but 2012
levels still remained above those observed before 2003 (Figure
5and SI Figures S8S13). The reasons for the reduced
occurrence of SfCP 1034 kPag among wells drilled after 2009
are not clear and it is dicult to determine whether they are
due to better well construction practices or lag in the COGCC
database. Testing of horizontal wells drilled in 2012 and 2013
show that horizontal wells exhibited SfCP 1034 kPag at
statistically similar frequencies as deviated wells (SI Table S8).
Thus, if future drilling in the WTZ continues to involve
predominantly horizontal wells, frequencies of well integrity
loss (SfCP 1034 kPag) may exceed frequencies observed in
years when vertical wells were the principal well conguration.
The percentage of oil and gas wells installed each year that
developed SfCP critical also increased with deviated drilling
between 2003 and 2009 (Figure 5 and SI Figures S8S13).
However, unlike trends for SfCP 1034 kPag, the percentage
of wells installed after 2009 that developed SfCP critical was
similar to pre-2003 percentages. This reduced occurrence of
SfCP critical is attributed to improved well construction
practices for horizontal wells. Specically, horizontal wells in
the WTZ are constructed with surface casings 78 m deeper on
average than other wells (SI Table S3) and the majority
(73.5%) of horizontal wells have their production casings
cemented into the bottom of their surface casing which
eectively prevents SfCP-induced stray gas migration. Con-
sequently, SfCP data in the WTZ provides no evidence that
horizontal drilling has increased the risk of stray gas
contamination and related drinking water contamination in
the WTZ.
While our data indicate that the occurrence of SfCP critical
in the WTZ increased between 2003 and 2009 because of
deviated drilling, it is dicult to determine if the threat of
drinking water contamination also rose during that time. The
risks posed by deviated wells with SfCP critical are unclear
because the majority (95.1%) of deviated wells in the WTZ
Figure 5. Well installation and SfCP occurrence in the WTZ between
1972 and 2014. (a.) Bar chart showing the number of wells in the
WTZ installed each year, wells with readable SfCP tests, SfCP 1034
kPag, and SfCP critical. (b.) Percentage of wells with SfCP 1034
kPag and SfCP critical plotted against installation year (calculated as
a fraction of wells installed in each year with readable SfCP tests).
Environmental Science & Technology Article
DOI: 10.1021/acs.est.6b06071
Environ. Sci. Technol. 2017, 51, 35673574
3572
meet current regulations and have surface casings installed into
the Pierre Shale. As noted above, a surface casing installed in
the Pierre Shale reduces the likelihood that a well will
contaminate drinking water with stray gas but does not make
it impossible. Currently, we only have evidence of wells with
short surface casings, the majority of which are older vertical
wells, causing drinking water contamination in the Wattenberg
Field. The overall percentage of vertical wells drilled each year
in the WTZ that exhibit SfCP critical has remained relatively
constant over time (SI Figures S8S13). This may explain the
ndings of Sherwood et al., 2016, who found a steady rate of
thermogenic stray gas occurrence in water wells drilled in the
Wattenberg Field between 2001 and 2014.
7
Extensive SfCP data in the WTZ has allowed us to not only
identify oil and gas wells with integrity issues but also
distinguish wells that pose a higher risk of inducing stray gas
migration. With this data set we were able to make the nuanced
inference that although deviated and horizontal wells lose
integrity more frequently than vertical wells, improved well
construction practices put them at lower risk for contaminating
drinking water with stray gas. Thus, we nd that while the
expansion of deviated and horizontal well drilling has resulted
in an overall rise in the frequency of well integrity loss in the
WTZ since 2003, older legacy wells with short surface casing
still pose the greatest risk for contaminating drinking water
aquifers with stray gas in the region. Regardless of the overall
trends, the number of wells in the WTZ with critical SfCP is
signicant (270 (6.88%)) and while 107 (39.63%) of these
wells have been remediated, eorts to identify and x faulty
wells should be continued and expanded.
Our ndings illustrate the value of a regional SfCP
monitoring program maintained and made publicly accessible
by a petroleum industry regulator. Considering the ease with
which SfCP tests are administered and the valuable information
they provide, we suggest that SfCP testing in Colorado be
expanded beyond problematic regions to entire oil and gas
elds. SfCP tests could be improved to better identify wells
with integrity issues if they required SfCP buildup to be
recorded in addition to bleed down. Data accessibility could
also be improved by aggregating SfCP records and providing
them as a bulk download, as has been done for a number of
wells in the Piceance Basin. Despite these shortcomings, SfCP
testing and regulations pertaining to SfCP monitoring in
Colorado should serve as a model for other regulatory agencies
and states. Currently, only 12 US states (AK, AZ, CA, CO, IL,
MI, NE, ND, OH, PA, TX, WY) have regulations that include
SfCP or SCVF monitoring, many of which only require it
during hydraulic fracturing, and Colorado is the only state we
were able to identify that makes records of SfCP publicly
available (SI Table S2).
ASSOCIATED CONTENT
*
SSupporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.est.6b06071.
A detailed description of US oil and gas regulations, well
construction, QC methods, SfCP testing, isotope
analyses, SfCP hotspot deerivation, and logistic regres-
sion (PDF)
AUTHOR INFORMATION
Corresponding Author
*Phone: 303-492-6604; fax: 303-492-7317; e-mail: gregory.
lackey@colorado.edu.
ORCID
Greg Lackey: 0000-0003-2538-3485
Notes
The authors declare no competing nancial interest.
ACKNOWLEDGMENTS
This work was supported by a grant from the National Science
Foundation Sustainability Research Network Program (Grant
CBET-1240584). Additional thanks to the COGCC for
answering questions and providing data and to Elizabeth
Lackey, Devansh Chauhan, Adam Peltz, and Pete Penoyer.
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... Migration through the geologic strata surrounding the well tends to follow existing fault and fracture systems, particularly in areas with structural complexity (e.g. Dusseault and Jackson, 2014;Lackey et al., 2017;Llewellyn, 2014;Moortgat et al., 2018;Reese et al., 2014;Ryan et al., 2015;Woda et al., 2018). If pathways to the shallow subsurface are present, whether natural or anthropogenic, gas can enter into groundwater aquifers used for water supply, potentially impacting the beneficial use of the water resource. ...
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