Global well inventory
As shale gas and oil exploitation has been carried out primarily onshore to date, the global well inventory in this study reports only the number of hydrocarbon wells drilled onshore, as this provides a more relevant historical context. Data in the public domain were used, sourced either from published reports or from online datasets populated by regulatory authorities. Several comprehensive review papers were also utilised, particularly those addressing the potential of CO2 to leak upwards through wells (e.g. Watson and Bachu, 2009).
A graph of wells drilled per year since the 1930s in Australia, Brazil,the Netherlands, Poland, the UK, and the USA shows that some countries, such as the UK, have very modest onshore drilling activity compared to others such as the USA (Fig. 5). Historical data are sparse, so it is difficult to estimate the total number of onshore hydrocarbon wells drilled globally, but in the USA alone, at least 2.6 million wells have been drilled since 1949 (EIA database). Former Soviet countries such as Azerbaijan, where many thousands of wells have been drilled, are not included in this study due to a lack of access to adequate data. Nonetheless, taking into consideration those drilled only in Australia, Austria, Bahrain, Brazil, Canada, the Netherlands, Poland, the UK and the USA, we estimate there are at least 4 million onshore hydrocarbon wells (Table 1).
Figure 5. Number of wells drilled annually since the 1930s in Australia, Brazil, Netherlands, Poland, the UK and the USA. Sources: DECC, 2013; Geoscience Australia; Geological Survey of the Netherlands; Brazil Database of Exploration and Production (BDEP); EIA, Polish Geological Institute.
Table 1. Number of hydrocarbon boreholes drilled onshore in selected nation states.
The online database collated by the Department of Environmental Protection (DEP) in the US state of Pennsylvania allows oil and gas well records to be searched by various criteria, such as well status, operator and drilling date. The unconventional hydrocarbon wells included in that database are those that were drilled to target the Marcellus Shale Formation. From these data, Vidic et al. (2013) derived a figure of 3.4% well barrier leakage for shale gas production sites in Pennsylvania (219 violations for 6466 wells) between 2008 and 2013.
Using the same database, Ingraffea (2012) argued that 211 (6.2%) of 3391 shale gas wells drilled in Pennsylvania in 2011 and 2012 had failed. More recently, Considine et al. (2013) identified 2.58% of 3533 individual wells as having some form of barrier or integrity failure. This consisted of 0.17% of wells having experienced blowouts (4 wells), venting or gas migration (2), and 2.41% having experienced casing or cementing failures. Measurable concentrations of gas were present at the surface for most wells with casing or cementing violations. Figure 6 shows a breakdown of the 1144 environmental violations issues for the 3533 wells.
Figure 6. Breakdown of 1144 notices of violations from 3533 wells in Pennsylvania from 2008 to 2011 (after Considine et al., 2013). Red font indicates those related to well barrier and integrity failure. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
In this study, the search criteria used to categorise leakage incidents in Pennsylvania followed the approach described by Ingraffea (2012) and are based on code violations reported during site inspections. Code violations that would constitute a well failure are those likely to result in a significantly increased risk of contaminants reaching either the surface or potable water sources. They include: (a) failure to case and cement the well properly; (b) excessive casing seat pressure; (c) failure to case and cement sufficiently to prevent migrations into fresh groundwater; and (d) insufficient cement and steel casings between the wellbore and the near-surface aquifer to prevent seepage of fluids.
Using the Pennsylvania state database, a well barrier or integrity failure rate of 6.3% is identified for the years 2005–2013. This includes failures noted in inspection reports that were not recorded as a violation, following the methodology of Ingraffea (2012). Without including these reports, the failure rate would be 5%. This is higher than the 3.4% well leakage figure reported by Vidic et al. (2013) for the period 2008–2013, and close to the well failure rate of 6.2% reported by Ingraffea (2012).
Gulf of Mexico, USA
Data from the US Minerals Management Service show that, of 15,500 producing, shut in and temporarily abandoned wells in the outer continental shelf of the Gulf of Mexico, 6692 (43%) have sustained casing pressure on at least one casing annulus (Brufatto et al., 2003). Of these incidents, 47.1% occurred in the production strings, 26.2% in the surface casing, 16.3% in the intermediate casing, and 10.4% in the conductor pipe.
Vignes and Aadnøy (2010) examined 406 wells at 12 Norwegian offshore facilities operated by 7 companies. Their dataset included producing and injection wells, but not plugged and abandoned wells. Of the 406 wells they examined, 75 (18%) had well barrier issues. There were 15 different types of barrier that failed, many of them mechanical (Fig. 7), including the annulus safety valve, casing, cement and wellhead. Issues with cement accounted for 11% of the failures, whilst issues with tubing accounted for 39% of failures.
Figure 7. Causes of barrier failures for the 75 (of 406) production and injection wells surveyed in offshore Norway that showed evidence for such failures (from Vignes, 2011).
The PSA has also performed analyses of barrier failures and well integrity on the Norwegian continental shelf. Its analysis showed that, in 2008, 24% of 1677 wells were reported to have well barrier failures; in 2009, 24% of 1712 wells had well barrier failures; and in 2010, 26% of 1741 wells had well barrier failures. It is unclear whether the same wells were tested in successive years or whether surveys targeted different wells (Vignes, 2011). A study of 217 wells in 8 offshore fields was also carried out by SINTEF (see Vignes, 2011). Between 11% and 73% of wells had some form of barrier failure, with injectors 2 to 3 times more likely to fail than producers (Vignes, 2011).
At the 20th Drilling Conference in Kristiansand, Norway, in 2007, Statoil presented an internal company survey of offshore well integrity (Vignes, 2011). This analysis showed that 20% of 711 wells had integrity failures, issues, or uncertainties (Vignes, 2011). When subdivided into production and injection wells, the survey concluded that 17% of 526 production wells and 29% of 185 injection wells had well barrier failures.
The results of an inspection project carried out by the State Supervision of Mines Netherlands were also reported by Vignes (2011). Their inspections, carried out in 2008, included only 31 wells from a total of 1349 development wells from 10 operating companies. Of those wells, 13% (4 of 31) had well barrier problems; by well type, problems were identified in 4% of the production wells (1 of 26) and 60% of the injection wells (3 of 5).
Offshore and onshore UK
For offshore wells on the UKCS, Burton (2005) found that 10% of 6137 wells (operated by 18 companies) had been shut-in (valves at the well head closed) during the last five years as a result of ‘structural integrity issues’. The total number of wells drilled on the UKCS is 9196; exploration boreholes that did not make commercial discoveries were not included in the Burton (2005) study.
Onshore, 2152 hydrocarbon wells have been drilled in the UK between 1902 and 2013. Although the onshore sedimentary succession is not thought to be overpressured, hydrocarbons could still migrate upwards because of their buoyancy relative to pore water or the fluid in a borehole (e.g. the Hatfield blow-out near Doncaster, UK; Ward et al., 2003). Pollution incident data were reviewed for all incidents reported within 1 km of wells in England between 2001 and 2013 (the only time period for which data are available). These data were filtered for those indicating a release of crude oil to the environment.
These incidents were described as pipe failures above or below ground and could be related to the well or pipelines connected to the wells. To act as a control to this data, pollution incidents within a 5 km radius of the well were also examined to assess whether there was a broader issue of hydrocarbon pollution incidents that should be considered and taken into account.
The number of wells active prior to the period covered by the pollution records was also calculated. Based on data provided by DECC, 143 onshore oil and gas wells were producing at the start of the year 2000. Between 2000 and 2013, the Environment Agency records nine pollution incidents involving the release of crude oil within 1 km of an oil or gas well (Table 7, Fig. 8). The records are not clear as to whether the incidents were due to well integrity failure, problems with pipework linked to the well, or other non-well related issues.
In February 2014, therefore, the present-day operators of the wells at which the nine events occurred were contacted (Perenco, IGas, and Humbly Grove Energy Ltd.). The two pollution incidents at the Singleton Oil Field (now operated by IGas but operated by a different company when the incidents occurred) occurred in the early 1990s, and were caused by failure of cement behind the conductor and the 9 5/8-inch casing. This was identified as a result of five groundwater monitoring boreholes installed at the Singleton Oil Field in 1993.
The leak was from the well cellar (cement lined cavity in which the well head sits) via the pre-installed conductor and the 9 5/8-inch casing, both of which appear not to have been adequately cemented in-situ in at least one well. A thorough investigation commenced in 1997, including the drilling of a number (>11) of additional boreholes, and the carrying out of tracer tests and CCTV examination under the auspices of, and in consultation with, the UK Environment Agency. The leak paths, once identified and verified, were remediated. Monitoring has continued since that time and the observed pollution levels have remained below those set by the Environment Agency as requiring further action.
The other seven pollution incidents recorded by the Environment Agency between 2000 and 2013 were not caused by well integrity failure, but due to leaks from pipework linked to the well. No incidents were reported at the other well sites in the UK that were inactive or abandoned.
For context, it should be noted that there are natural, high permeability geological pathways for the migration of buoyant fluids, which are typically associated with structural features such as faults and folds (Selley, 1992). Gas and oil are naturally mobile in the UK subsurface: around 200 natural hydrocarbon seeps, mainly of oil, are known from the onshore UK and some have been used to initiate localised exploitation (Selley, 1992, Selley, 2012). A small number of natural gas seeps from shales were recorded by Selley (2012), with notable occurrences in the Weald Basin of south-east England (Selley, 2012, Fig. 5).
Summary of well barrier and integrity failure
For the countries listed (Table 1), publicly available data were tabulated on well type, well location, completion date, well status, number of wells drilled and whether well barriers and integrity failures had occurred (Table 2). Tabulation of all published and online data on well barrier and integrity failure (Table 3, Fig. 9) shows substantial variability in the number of wells that have experienced both categories of failure. This probably relates to the fact that the sizes of the datasets are variable; the included wells were drilled over a period of more than a century, using different well designs and technology; were targeting unconventional and conventional hydrocarbons; and were drilled in diverse geological settings. The most recent dataset from the Marcellus Shale (Pennsylvania, USA), which includes several thousand wells, has some of the lowest well barrier and failure rates (Fig. 9). In Table 3 we have been careful to provide the exact wording from the published source as to the nature of the failure, and to discriminate between well barrier and well integrity failures.
Table 2. Sources of data reporting well barrier and well integrity failures.
Table 3. Compilation of published statistics on well barrier and well integrity failure, including information on well age, number of wells included in study, well location, and terminology used to describe nature of well barrier or integrity failures.
Figure 8. (a) UK map showing locations of wells active in 1999 and crude oil discharges (b) Coincidence of pollution reports with well pads in the Wytch Farm area, southern England.
Figure 9. Graph of percentage of well barrier and integrity failures reported in 25 different studies around the world, with drilling dates and number of wells in each study.
Orphaned, abandoned or idle wells
The terms ‘abandoned’, ‘idle’ and ‘orphaned’ are used to describe the state of a well that did not locate economic hydrocarbons or a well at the end of its production lifecycle. The USA has the most established and comprehensive definitions of such terms, although their meaning can vary at state and federal levels.
A review of the various state regulatory practices regarding idle wells in the USA was conducted by Thomas (2001) and defined idle wells as those not currently being used for production or injection, but which have not yet been plugged and abandoned. In California, Hesson and Glinzak (2000) and Evans et al. (2003) defined idle wells as those that have been non-producing and non-injecting for six consecutive months.
In the USA, the definition of an orphaned well depends largely on the state regulatory body. Thomas (2001) defined orphaned wells as those in which the operator has gone out of business or is insolvent, such that the company that operated the well is no longer responsible for it. Based on Californian practices, Hesson (2013) defined orphaned wells as those where the operator is defunct, or where the state regulatory body has determined, based on certain criteria, that a well is orphaned. Such criteria include a well having been idle for 25 years or more, without being in compliance with idle well requirements.
In Texas, the oil and gas regulatory body – the RRC – defines orphaned wells as those which have, without permit, been inactive for a year or more. In Pennsylvania, a 1992 amendment to the 1984 Oil and Gas Act defined an orphaned well as one which was abandoned prior to April 1985, which has not been operated by the present owner, and for which the present owner has received no economic benefit. For the UK data in this study, we follow the definition of Thomas (2001) and use ‘orphaned’ to describe wells where the operator is no longer solvent.
Thirty-two US states have reported data on orphaned oil and gas wells (IOGCC, 2013). Fifteen of these states account for around 320,000 orphaned wells in total, with ∼53,000 of these wells targeted for plugging (Table 4). The states vary greatly in how they treat wells for which they have no data. Two decades ago, the US EPA estimated that there were at least 1.2 million abandoned oil and gas wells in the United States (EPA, 1987); more than 200,000 of these wells appear to be unplugged (EPA, 1987).
Table 4. Estimated numbers of orphaned oil and gas wells for each U.S. state reporting at least 1000 orphaned wells (IOGCC, 2008). Thirty-two of 50 states reported data on orphaned wells.
As the first state to produce oil commercially in the USA, Pennsylvania illustrates the difficulty in characterizing abandoned and orphaned wells. The state has seen around 325,000 to 400,000 oil and gas wells drilled since 1859. As of 2010, the Pennsylvania Department of Environmental Protection (DEP) reported 8823 oil and gas wells targeted for plugging (IOGCC 2013). The PA DEP also reported more than 100,000 orphaned wells, but the precise location and depth of most of these was unidentified.
The number of orphaned wells in Pennsylvania is probably closer to 180,000, being the difference between the conservative estimate of ∼325,000 wells drilled in the state and the ∼140,000 wells listed in the PA DEP database. These wells are mostly a legacy of the first 75–100 years of oil and gas drilling, before record keeping was commonplace. In fact, the earliest regulations on well plugging were designed to stop water entering hydrocarbon wells, particularly during floods, rather than to isolate oil and gas from the environment.
Lost wells represent a different classification to abandoned or orphaned wells. States in the USA report that somewhere between 828,000 and 1,060,000 oil and gas wells were drilled prior to a formal regulatory system, most of which have no information available in state databases (IOGCC, 2008). A New York state report in 1994 estimated that, of the 61,000 oil and gas wells drilled to that date, no records existed for 30,000 of them; Bishop (2013) referred to these as ‘forgotten’ rather than abandoned or orphaned wells.
The growing number of unplugged wells in New York State illustrates the difficulty of keeping remediation levels commensurate with the number of wells being drilled and abandoned (Bishop, 2013). Up to 2010, a total of ∼75,000 oil and gas wells had been drilled in the state. Eleven thousand wells were still active at that time, leaving 64,000 ‘abandoned’ wells (after Bishop, 2013). Of these, 15,900 had been plugged but 48,000 remained unplugged; thus only 25% of the abandoned wells in 2010 had been plugged, down from 27% in 1994. More importantly, the number of unplugged wells had grown by 13,000 since 1994, when 35,000 such wells existed (Bishop, 2013). This demonstrates that, in at least some regions, the plugging of abandoned wells is not keeping pace with the rate at which wells are being abandoned.
Some states have aggressive programmes for plugging abandoned oil and gas wells. Texas has one of the most ambitious, having plugged 41,000 wells between 1991 and 2009 at a cost of ∼$80 million (IOGCC, 2008). Overall, US states spent ∼$319 million in recent decades to plug and remediate ∼72,000 oil and gas wells, at an average cost of ∼$4500 per well. Based on that unit cost, plugging 150,000 more wells would require $668 million, and plugging all 320,000 wells estimated in Table 4 would cost $1.43 billion. In 2009, the combined balance available in all US state funds for plugging wells was ∼$2.8 million, many orders of magnitude less than that required to finish the job (IOGCC, 2008).
In the UK a total of 2152 hydrocarbon wells were drilled onshore between 1902 and 2013, with a peak in drilling activity during World War II (Fig. 10). Approximately 1000 were drilled by companies that still exist. Approximately 1050 were drilled by companies that were subject to takeovers or mergers. For example, 543 wells were drilled by the D’Arcy company, mainly between 1941 and 1961 and D’Arcy is no longer operating.
Figure 10. Graph showing number of hydrocarbon wells drilled in UK per year.
We estimate that between 50 and 100 of the 2152 wells were drilled by companies that no longer exist and were not bought or merged. In the USA such wells are termed orphaned wells. Where the company that drilled the well no longer exists, or has been taken over or merged (up to 53% of UK wells), liability for any well integrity failures that lead to pollution is unclear; in some cases it may be that of the landowner. Even if a chain of ownership through acquisition of prior licensees can be identified, the position is likely to be more complex as the legal mechanism used for the acquisition may not be known. In some instances, it is possible that a company was purchased for its assets and the liabilities were left with the original entity.
As a case study, one of the 2152 wells listed by DECC was examined (Fig. 11). Drilled in Sunderland in 2002, the well targeted coal mine gas. In February 2014 the company that drilled the well was contacted to confirm the status of the well as either abandoned or temporarily abandoned (suspended). No gas had been produced due to elevated water levels and the well was temporarily abandoned (suspended) in 2002, pending transfer of ownership to the Coal Authority, for water level monitoring or abandonment. The surrounding land has since been acquired by developers and is currently (February 2014) the site of a new residential housing estate. As of February 2014, the well is now being abandoned (DECC, pers. comm.).
Figure 11. Case study of gas exploration well abandonment in Sunderland, UK: (a) Map of the UK; (b) location of Sunderland; (c) location of new housing estate; (d) photograph of temporarily abandoned (suspended) mine gas exploration borehole on building site of new housing estate (Grid Ref. 438260 557420). Well was completed in 2002 to a depth of 465 m.