Surface Casing Pressure As an Indicator of Well ... - ACS Publications

Feb 16, 2017 - We propose a physically meaningful well-specific critical SfCP criterion, which indicates the potential for a well to induce stray gas ...
0 downloads 0 Views 2MB Size
Download PDF
Article pubs.acs.org/est

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

‡

S Supporting 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-specific 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 development 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.3−8 Current evidence suggests that faulty oil and gas wells with compromised integrity are the principal transport pathway for stray gas.5−7,9−15 Thus, understanding well integrity and quantifying the rates at which wells lose integrity is critical for assessing risks to groundwater quality.16−18 Wells with integrity issues can also contribute to fugitive emissions of methane and other volatile organic compounds to the atmosphere, thus contributing to greenhouse gas burdens and deterioration of regional air quality.13,19−21 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 confines 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 fluids in their respective formations.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, © 2017 American Chemical Society

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 specified 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 flow, 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 Canada13(SI Table S2). Received: Revised: Accepted: Published: 3567

November 30, 2016 February 15, 2017 February 16, 2017 February 16, 2017 DOI: 10.1021/acs.est.6b06071 Environ. Sci. Technol. 2017, 51, 3567−3574

Article

Environmental Science & Technology

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 DenverJulesburg Basin in northeastern Colorado (Figure 2). Conven-

Figure 1. Cross-sectional schematic (not to scale) of a vertical well installed in the Wattenberg Field. Major well components are identified 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 identified. 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.

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 sufficient enough to force gas out of the bottom of the surface casing.9,10,13,26 Leaving the surface casing valve open helps prevent SfCPinduced 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 data18 (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.

tional reserves in the Wattenberg Field were discovered in the 1970s32 and development of the unconventional reservoirs in the field, which began in the 1980s,33 has made it the fourthmost productive crude oil and ninth-most productive gas field in the U.S.34 Three well configurations, classified 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 configuration, 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 3568

DOI: 10.1021/acs.est.6b06071 Environ. Sci. Technol. 2017, 51, 3567−3574

Article

Environmental Science & Technology contamination incidents in the early 1980s resulted in fires 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 “culprit” oil and gas well with compromised integrity was identified (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 defined 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-specific 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 development in the WTZ and determine the impact of the recent expansion of unconventional drilling on oil and gas well integrity and stray gas migration.

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 infiltration 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 S14−S16) 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 gasproducing formations in the Wattenberg Field indicates that annular gas originated from all four of the principal hydrocarbon reservoirs (Sussex/Shannon, Niobrara, Codell, and JSand) (SI Figure S15). Thus, SfCP in the WTZ is not the result of production from or improper isolation of a specific formation and there is no evidence that SfCP has been caused by gas infiltration 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 fluids 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 justified. 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 influence 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 identification of a SfCP hotspot (Figure 2).40 Any section with a hotspot confidence ≥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

■

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 defined in the COGCC “Wattenberg Bradenhead Testing Policy”.31 All oil and gas wells were filtered 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 identified. Ninetynine 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. 3569

DOI: 10.1021/acs.est.6b06071 Environ. Sci. Technol. 2017, 51, 3567−3574

Article

Environmental Science & Technology

surrounding formation. Thus, exceedance of critical SfCP is a necessary (but not sufficient) 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 dw for each aquifer and assigned a value to each oil and gas well based on location and surface casing depth. The potentiometric head Hw in the surface casing of each oil and gas well was determined by subtracting the associated dw from the surface casing depth. Wells with surface casings installed below the Laramie-Fox Hills aquifer were given the dw of the Laramie-Fox Hills. The formation fluid pressure at the bottom of the surface casing was calculated using the estimated Hw and by assuming ρw = 1000 kg m−3 for water (see SI). Logistic Regression. We performed 32 logistic regressions (SI Tables S12−S19) 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

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.

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 confined 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 Patm + ρm gHm + SfCP = Patm + ρw gH w

■

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 different 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 flow of hydrocarbons into the uncemented annulus.11 Unfortunately, we did not have the bottom hole temperature data needed to confirm this

(1)

where Patm is atmospheric pressure, ρm is the annular liquid density, ρw is the density of water, g is the acceleration due to gravity, Hm is the height of the liquid column above the bottom of the surface casing in the annulus, and Hw is the height of the potentiometric surface above the bottom of the surface casing (SI Figure S19). As SfCP increases, Hm decreases (SI Figure S19), that is, annular liquid is displaced to maintain equilibrium with the formation fluid pressure. Here, we propose a wellspecific “critical” SfCP as the SfCP required to push annular liquids below the bottom of the surface casing of a well. Critical SfCP is reached when Hm is zero, thus critical SfCP is equal to the fluid 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

Table 1. Summary of the Number and Percentages of Wells with Readable Tests of Each Configuration with Surface Casing Pressure (SfCP) ≥ 1034 kPag and SfCP ≥ Criticala SfCP ≥ 1034 kPag

well

SfCP ≥ critical

configuration

entire WTZ

inside hotspot

outside hotspot

entire WTZ

inside hotspot

outside hotspot

all vertical deviated horizontal

541 (13.79%) 155 (7.41%) 316 (21.88%) 70 (18.04%)

435 (18.87%) 129 (10.85%) 257 (28.43%) 49 (23.11%)

106 (6.55%) 26 (2.88%) 59 (10.93%) 21 (11.93%)

270 (6.88%) 109 (5.21%) 156 (10.80%) 5 (1.29%)

215 (9.33%) 86 (7.23%) 126 (13.94%) 3 (1.42%)

55 (3.40%) 23 (2.55%) 30 (5.56%) 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. 3570

DOI: 10.1021/acs.est.6b06071 Environ. Sci. Technol. 2017, 51, 3567−3574

Article

Environmental Science & Technology

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 × 10−18 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 effectiveness 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 identified 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 confirm 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 efficacy of well remediation was not evaluated. Factors Contributing to Surface Casing Pressure. Previous studies have investigated the influence 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 influence 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 specifically investigated whether the number of fracture treatments and the volume of fracturing fluid used influence SfCP (SI Tables S16−S19, Figure S22). Of the 13 factors investigated, only wellbore deviation (odds ratio (OR) = 1.8, p-

hypothesis. Regions with elevated rates of integrity issues, analogous to the SfCP hotspot, have also been identified in Pennsylvania and Alberta, Canada.16,28 We also compare the frequency of integrity loss between the three different well configurations identified 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 Ingraffea 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 Ingraffea 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

Figure 4. Formation fluid 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, identified by the COGCC as a source of stray gas contamination, before the well was remediated are also identified. Wells in the gray region have SfCP ≥ critical SfCP (1:1 line) and pose a higher risk of inducing stray gas migration. 3571

DOI: 10.1021/acs.est.6b06071 Environ. Sci. Technol. 2017, 51, 3567−3574

Article

Environmental Science & Technology value =1.6× 10−5) and location in the high SfCP hotspot (OR = 1.6, p-value =1.8 × 10−5) were found to be associated with SfCP ≥ 1034 kPag. Well construction after 2010 (OR = 0.36, pvalue = 2.9 × 10−10) 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 significant relationship between the number of hydraulic fracturing treatments or the volume of hydraulic fracturing fluid 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 development.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 specific 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 Occurrence. 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 specific year corresponding to a shift in technology. Unlike other regions of the US, there is a longer history of unconventional drilling in the Wattenberg field. In the early 2000s, oil and gas development in the WTZ began to expand (Figures 5, SI S8−S13). 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 5 and SI Figures S8−S13). The reasons for the reduced occurrence of SfCP ≥ 1034 kPag among wells drilled after 2009

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).

are not clear and it is difficult 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 configuration. 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 S8−S13). 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. Specifically, 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 effectively prevents SfCP-induced stray gas migration. Consequently, 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 difficult 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 3572

DOI: 10.1021/acs.est.6b06071 Environ. Sci. Technol. 2017, 51, 3567−3574

Environmental Science & Technology

■

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 S8−S13). This may explain the findings 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 find 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 significant (270 (6.88%)) and while 107 (39.63%) of these wells have been remediated, efforts to identify and fix faulty wells should be continued and expanded. Our findings 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 fields. 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).

■

Article

AUTHOR INFORMATION

Corresponding Author

*Phone: 303-492-6604; fax: 303-492-7317; e-mail: gregory. [email protected]. ORCID

Greg Lackey: 0000-0003-2538-3485 Notes

The authors declare no competing financial 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.

■

REFERENCES

(1) Vidic, R.; Brantley, S. L.; Vandenbossche, J. M.; Yoxtheimer, D.; Abad, J. D. Impact of shale gas development on regional water quality. Science 2013, 340, 1235009−1−1253009−9. (2) Vengosh, A.; Jackson, R. B.; Warner, N.; Darrah, T. H.; Kondash, A. A critical review of the risks to water resources from unconventional shale gas development and hydraulic fracturing in the United States. Environ. Sci. Technol. 2014, 48, 8334−8348. (3) Osborn, S. G.; Vengosh, A.; Warner, N. R.; Jackson, R. B. Methane contamination of drinking water accompanying gas-well drilling and hydraulic fracturing. Proc. Natl. Acad. Sci. U. S. A. 2011, 108, 8172−8176. (4) Jackson, R. B.; Vengosh, A.; Darrah, T. H.; Warner, N. R.; Down, A.; Poreda, R. J.; Osborn, S. G.; Zhao, K.; Karr, J. D. Increased stray gas abundance in a subset of drinking water wells near Marcellus shale gas extraction. Proc. Natl. Acad. Sci. U. S. A. 2013, 110, 11250−11255. (5) Darrah, T. H.; Vengosh, A.; Jackson, R. B.; Warner, N. R.; Poreda, R. J. Noble gases identify the mechanisms of fugitive gas contamination in drinking-water wells overlying the Marcellus and Barnett Shales. Proc. Natl. Acad. Sci. U. S. A. 2014, 111, 14076−14081. (6) Llewellyn, G. T.; Dorman, F.; Westland, J. L.; Yoxtheimer, D.; Grieve, P.; Sowers, T.; Humston-Fulmer, E.; Brantley, S. L. Evaluating a groundwater supply contamination incident attributed to Marcellus Shale gas development. Proc. Natl. Acad. Sci. U. S. A. 2015, 112, 6325− 6330. (7) Sherwood, O. A.; Rogers, J. D.; Lackey, G.; Burke, T. L.; Osborn, S. G.; Ryan, J. N. Groundwater methane in relation to oil and gas development and shallow coal seams in the Denver-Julesburg Basin of Colorado. Proc. Natl. Acad. Sci. U. S. A. 2016, 113, 8391−8396. (8) DiGiulio, D. C.; Jackson, R. B. Impacts to underground sources of drinking water and domestic wells from production well stimuatlion and completion practices in the Pavillion, Wyoming Field. Environ. Sci. Technol. 2016, 50, 4524−4536. (9) Harrison, S. S. Evaluating system for groundwater contamination hazards due to gas-well drilling on the glaciated Appalachian Plateau. Groundwater 1983, 21, 689−700. (10) Harrison, S. S. Contamination of aquifers by overpressuring the annulus of oil and gas wells. Groundwater 1985, 23, 317−324. (11) Brufatto, C.; et al. From mud to cement - building gas wells. Oilfield Review 2003, 15, 62−76. (12) Brantley, S. L.; Yoxtheimer, D.; Arjmand, S.; Grieve, P.; Vidic, R.; Pollak, J.; Llewellyn, G. T.; Abad, J.; Simon, C. Water resource impacts during unconventional shale gas development: The Pennsylvania experience. Int. J. Coal Geol. 2014, 126, 140−156. (13) Dusseault, M. B.; Jackson, R. E. Seepage pathway assessment for natural gas to shallow groundwater during well stimulation, in production, and after abandonment. Environ. Geosci. 2014, 21, 107− 126. (14) Nowamooz, A.; Lemieux, J. M.; Molson, J.; Therrien, R. Numerical investigation of methane and formation fluid leakage along

ASSOCIATED CONTENT

S Supporting 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 regression (PDF) 3573

DOI: 10.1021/acs.est.6b06071 Environ. Sci. Technol. 2017, 51, 3567−3574

Article

Environmental Science & Technology the casing of a decommissioned shale gas well. Water Resour. Res. 2015, 51, 4592−4622. (15) Roy, N.; Molson, J.; Lemieux, J.; Stempvoort, D. V.; Nowamooz, A. Three-dimensional numerical simulations of methane gas migration from decommissioned hydrocarbon production wells into shallow aquifers. Water Resour. Res. 2016, 52, 5598−5618. (16) Ingraffea, A. R.; Wells, M. T.; Santoro, R. L.; Shonkoff, S. B. C. Assessment and risk analysis of casing and cement impairment in oil and gas wells in Pennsylvania, 2000−2012. Proc. Natl. Acad. Sci. U. S. A. 2014, 111, 10955−10960. (17) Jackson, R. B. The integrity of oil and gas wells. Proc. Natl. Acad. Sci. U. S. A. 2014, 111, 10902−10903. (18) Davies, R. J.; Almond, S.; Ward, R. S.; Jackson, R. B.; Adams, C.; Worrall, F.; Herringshaw, L. G.; Gluyas, J. G.; Whitehead, M. A. Oil and gas wells and their integrity: Implications for shale and unconventional resource exploitation. Mar. Pet. Geol. 2014, 56, 239− 254. (19) Petron, G.; et al. Hydrocarbon emissions characterization in the Colorado Front Range: A pilot study. J. Geophys Res. 2012, 117, 1−19. (20) Kang, M.; Kanno, C. M.; Reid, M. C.; Zhang, X.; Mauzerall, D. L.; Celia, M. A.; Chen, Y.; Onstott, T. C. Direct measurements of methane emissions from abandoned oil and gas wells in Pennsylvania. Proc. Natl. Acad. Sci. U. S. A. 2014, 111, 18173−18177. (21) Brand, A.; Heath, G.; E.A. Kort, F. O.; Petron, G.; Jordaan, S.; Tans, P.; Wilcox, J.; Gopstein, A.; Arent, D.; Wofsy, S.; Brown, N.; Bradley, R.; Stucky, G.; Eardley, D.; Harriss, R. Methane Leaks from North American Natural Gas Systems. Science 2014, 343, 733−735. (22) U.S. Environmental Protection Agency. Review of Well Operator Files for Hydraulically Fractured Oil and Gas Production Wells: Well Design and Construction, 2015. (23) Fleckenstein, W. W.; Eustes, A. W.; C.H. Stone, P. H. An assessment of risk of migration of hydrocarbons or fracturing fluids to fresh water aquifers: Wattenberg Field, CO. 2015. (24) Bourgoyne, A. T.; Scott, S. L.; Manowski, W. A Review of Sustained Casing Pressure(SCP) Ocurring on the OCS, Final Report to MMS, 2000. (25) King, G. E.; King, D. E. Environmental risk arising from wellconstruction failure-Differences between barrier and well failure, and estimates of failure frequency across common well types, locations and well ages. SPE Production and Operations 2016, 28, 323−344. (26) Gorody, A. W. Factors affecting the variability of stray gas concentration and composition in groundwater. Environ. Geosci. 2012, 19, 17−31. (27) Considine, T.; Watson, R.; Considine, N.; Martin, J. Environmental Impacts during Marcellus Shale Gas Drilling: Causes, Impacts, And Remedies; Shale Resources and Society Institute, 2012. (28) Watson, T. L.; Bachu, S. Evaluation of the potential for gas and CO2 leakage along wellbores. Society of Petroleum Engineers, Drilling & Completion 2009, 24, 115−126. (29) Colorado Oil and Gas Conservation Commission, Rules and regulations . (accessed November, 1, 2016); http://cogcc.state.co.us/ reg.html#/rules. (30) Colorado Oil and Gas Conservation Commission, Colorado oil and gas information system (accessed September, 15, 2015); http:// cogcc.state.co.us/data.html#/cogis. (31) Colorado Oil and Gas Conservation Commission, Colorado oil and gas conservation commission approved Wattenberg bradenhead testing and staff policy (accessed October, 10, 2016); http://cogcc. state.co.us/documents/reg/Policies/ WattenbergBradenheadTestingPolicy.pdf. (32) Matuszczak, R. A. Wattenberg Field, Denver Basin, Coloradol. Mountain Geologist 1974, 10, 99−107. (33) Sonnenberg, S. A. New reserves in an old field, the Niobrara Resource Play in the Wattenberg Field, Denver Basin, Colorado. SPE Unconventional Resources Technology Conference, Denver, CO, 2013,. (34) U.S. Energy Information Administration, Top 100 U.S. Oil and Gas Fields; U.S. Department of Energy, Washington, DC, 2015.

(35) Higley, D. K., Cox, D. O. Oil and gas exploration and development along the Front Range in the Denver Basin of Colorado, Nebraska and Wyoming; U.S.Geological Survey, 2007; pp ch 2, 41 p. (36) Denver Post Editorial Board, Fiddling while water burns. Denver Post April 6, 1982, Section B, 2. (37) International Engineering Company, Inc., and Amuedo and Ivey, Inc., Natural Gas Hazard Potential; International Engineering Company, Inc., Denver, 1984; pp 1−57. (38) Colorado Division of Water Resources, HydroBase Bulk Data Exporter. (accessed September 15, 2015); http://water.state.co.us/ DataMaps/DataSearch/Pages/DataSearch.aspx#bulkdata. (39) Colorado Oil and Gas Conservation Commission, Notice to Operators Drilling Mesaverde Group of Deeper Wells in the Mamm Creek Field Area in Garfield County Well Cementing and Reporting Requirements. (accessed September, 15, 2015); http://cogcc.state.co. us/documents/reg/Policies/Mamm%20Creek%20Drilling.pdf. (40) Ord, J. K.; Getis, A. Local spatial autocorrelation statistics: distributional issues and an application. Geogr Anal 1995, 27, 286− 306. (41) Seabold, J.; Perktold, J. Statsmodels: Econometric and Statistical Modeling with Python. In Proceedings of the 9th Python in Science Conference, 2010. (42) Python Reference Manual. http://www.python.org (accessed December 10, 2015). (43) Higley, D. K.; Cox, D. O.; Weimer, R. J. Petroleum system and production characteristics of the Muddy (J) Sandstone (Lower Cretaceous) Wattenberg continuous gas field, Denver basin, Colorado. AAPG Bulletin 2003, 87, 15−37. (44) Neuzil, C. How permeable are clays and shales? Water Resour. Res. 1994, 30, 145−150.

3574

DOI: 10.1021/acs.est.6b06071 Environ. Sci. Technol. 2017, 51, 3567−3574