- Oil and gas well barrier elements can fail.
- The percentage of wells with barrier element failure is between 1.9% and 75%.
- Pennsylvanian shale wells have well barrier and failures rates of 6.3% or less.
Data from around the world (Australia, Austria, Bahrain, Brazil, Canada, the Netherlands, Poland, the UK and the USA) show that more than four million onshore hydrocarbon wells have been drilled globally. Here we assess all the reliable datasets (25) on well barrier and integrity failure in the published literature and online. These datasets include production, injection, idle and abandoned wells, both onshore and offshore, exploiting both conventional and unconventional reservoirs.
Richard J. Daviesa, Sam Almonda, Robert S. Wardb, Robert B. Jacksonc,d, Charlotte Adamsa, Fred Worralla, Liam G. Herringshawa, Jon G. Gluyasa, Mark A. Whiteheade
aDurham Energy Institute, Department of Earth Sciences, Durham University, Science Labs, Durham DH1 3LE, UK. bGroundwater Science Directorate, British Geological Survey, Keyworth, Nottingham NG12 5GG, UK. cSchool of Earth Sciences, Woods Institute for the Environment, and Precourt Institute for Energy, Stanford University, Stanford, CA 94305, USA. dNicholas School of the Environment, Division of Earth and Ocean Sciences, Duke University, Box 90338, 124 Science Drive, Durham, NC 27708-0338, USA. eWard Hadaway, Sandgate House, 102 Quayside, Newcastle Upon Tyne NE13DX, UK
The datasets vary considerably in terms of the number of wells examined, their age and their designs. Therefore the percentage of wells that have had some form of well barrier or integrity failure is highly variable (1.9%–75%). Of the 8030 wells targeting the Marcellus shale inspected in Pennsylvania between 2005 and 2013, 6.3% of these have been reported to the authorities for infringements related to well barrier or integrity failure. In a separate study of 3533 Pennsylvanian wells monitored between 2008 and 2011, there were 85 examples of cement or casing failures, 4 blowouts and 2 examples of gas venting.
In the UK, 2152 hydrocarbon wells were drilled onshore between 1902 and 2013 mainly targeting conventional reservoirs. UK regulations, like those of other jurisdictions, include reclamation of the well site after well abandonment. As such, there is no visible evidence of 65.2% of these well sites on the land surface today and monitoring is not carried out. The ownership of up to 53% of wells in the UK is unclear; we estimate that between 50 and 100 are orphaned. Of 143 active UK wells that were producing at the end of 2000, one has evidence of a well integrity failure.
The rapid expansion of shale gas and shale oil exploration and exploitation using hydraulic fracturing techniques has created an energy boom in the USA but raised questions regarding the possible environmental risks, such as the potential for groundwater contamination (e.g. Jackson et al., 2013, Vidic et al., 2013) and fugitive emissions of hydrocarbons into the atmosphere (e.g. Miller et al., 2013).
Boreholes drilled to explore for and extract hydrocarbons must penetrate shallower strata before reaching the target horizons. Some of the shallower strata may contain groundwater used for human consumption or which supports surface water flows and wetland ecosystems. Although it has been routine practice to seal wells passing through such layers, they remain a potential source of fluid mixing in the subsurface and potential contamination (King and King, 2013). This can occur for many reasons, including poor well completion practices, the corrosion of steel casing, and the deterioration of cement during production or after well abandonment.
Boreholes can then become high-permeability potential conduits for both natural and man-made fluids (e.g. Watson and Bachu, 2009), and vertical pressure gradients in the subsurface can drive movement of fluids along these flow paths. The potential importance of wellbore integrity to the protection of shallow groundwater has recently been highlighted in research papers and reports (e.g. Osborn et al., 2011, The Royal Society and The Royal Academy of Engineering, 2012); Jackson et al., 2013, King and King, 2013).
In addition to protecting ground and surface waters, effective well sealing prevents leakage of methane and other gases into the atmosphere. This is important as methane is 86 times more effective than CO2 at trapping heat in the atmosphere over a 20-year period and 34 times more effective over a century (IPCC, 2013). Well barrier and integrity failures can occur during drilling, production, or after abandonment; in rare examples, including in the USA, well leakage has led to explosions at the Earth’s surface (e.g. Miyazaki, 2009).
This paper has four aims: 1) to estimate the number of onshore hydrocarbon wells globally; 2) to explain how onshore wells are categorised (e.g. producing, abandoned, idle, orphaned) and what statistical data are available on the numbers of wells in these groups; 3) to document the number of wells that are known to have had some form of well barrier and/or integrity failure, placing these numbers in the context of other extractive industries; and 4) to analyse how many onshore wells in the UK can be easily accessed to assess for barrier and integrity failure.
For well barrier and integrity failure our approach has been to include all the reliable datasets that are available, rather than de-select any data. This inclusive approach has the draw-back that the data we present include wells of different age, of different designs and drilled into different geology. Unsurprisingly there is a significant spread in the statistics on the percentage of wells that have well barrier or integrity failure.
The review is largely focused on North America, as it has a long history of onshore hydrocarbon drilling (including wells drilled for shale gas and shale oil) and the UK, which contrasts in having a mature offshore drilling industry, but relatively little onshore drilling. It mainly, but not exclusively, covers static well failure (i.e. after drilling operations are completed), and summarises currently available data for regulators, non-government organisations, the public, and the oil and gas industry.
Barriers are containment mechanisms within a well or at the well head that are designed to withstand the corrosion, pressures, temperatures and exposure times associated with the phases of drilling, production and well abandonment. The types of barriers used to prevent contamination of groundwater, surface water, soils, rock layers and the atmosphere depend on whether the well is for exploration or production, but generally include cement, casing, valves and seals (Fig. 1). Barriers can be nested, so that a well has several in place. They can be dynamic (e.g. a valve) or static (e.g. cement), and may or may not be easily accessible for assessment or monitoring (see King and King, 2013).
Figure 1. Schematic diagram of typical well design, showing (A): structure of an exploration well; and (B): a production well. Depths to which different casings are used vary according to geology and pressure regime of drill site. Well diameter exaggerated to show sections more clearly.
Drilling a well for exploration or production is a multistage process during which the upper parts of a borehole, once drilled, are sealed with steel casing and cemented into place. Cement was introduced to the petroleum industry as early as 1903, when Frank Hill of Union Oil Co. poured 50 sacks of Portland cement into a well to seal off water-bearing strata (Smith, 1976). Cementing is now typically carried out by pumping water-cement slurries down the casing to the bottom of the hole, displacing drilling fluids from the casing-rock and other annuli, leaving a sheath of cement to set and harden (Fig. 1).
The integrity of these seals is pressure-tested before the next stage of drilling occurs. Only if the well passes these pressure tests will drilling continue. If the well fails the test, the casing is re-cemented before drilling continues. The sizes and lengths of casing, and the depths at which different casings are used depend upon the geology, the importance or sensitivity of the groundwater that the well penetrates, and the purpose of the well (Fig. 1). Well completion should follow statutory regulations and/or industry best practice. When a well is abandoned, cement is normally pumped into the production tubing to form a cement plug to seal it. Commonly (e.g. in the UK), the top of the well is welded shut.
The terms ‘well barrier failure’ and ‘well integrity failure’ were differentiated by King and King (2013). They used ‘well integrity failure’ for cases where all well barriers fail, establishing a pathway that enables leakage into the surrounding environment (e.g. groundwater, surface water, underground rock layers, soil, atmosphere). ‘Well barrier failure’ was used to refer to the failure of individual or multiple well barriers (e.g. production tubing, casing, cement) that has not resulted in a detectable leak into the surrounding environment. The same terminology is used in this paper: ‘well integrity failure’ includes cases when gas or fluids are reported to have leaked into soils, rock strata or the atmosphere, and ‘well barrier failure’ includes cases where a barrier failure has occurred but there is no information that indicates that fluids have leaked out of the well.
Routes and driving mechanisms
For a well to leak, there must be a source of fluid (Fig. 2), a breakdown of one or more well barriers, and a driving force for fluid movement, which could be fluid buoyancy or excess pore pressure due to subsurface geology (e.g. Watson and Bachu, 2009). There are seven subsurface pathways by which leakage typically occurs (Figs. 3, 4). These pathways include the development of channels in the cement, poor removal of the mud cake that forms during drilling, shrinkage of cement, and the potential for relatively high cement permeability (e.g. Dusseault et al., 2000).
There are other mechanisms that can operate in specific geological settings. Reservoir compaction during production, for example, can cause shear failure in the rocks and casing above the producing reservoir (Marshall and Strahan, 2012; route 7 marked on Fig. 3). Leaking wells can also connect with pre-existing geological faults, enabling leakage to reach the surface (Chillingar and Endres, 2005). A range of fluids can leak, for instance formation fluids, water, oil and gas, and they can move through or out of the well bore by advective or diffusive processes (e.g. Dusseault et al., 2000). Overpressure may be the driving force for fluid flow (e.g. the Hatfield blow-out near Doncaster, UK; Ward et al., 2003), but hydrostatically pressured successions can also feed leaking wells, with fluids migrating due to buoyancy and diffusion.
Figure 2. Schematic diagram of typical sources of fluid that can leak through a hydrocarbon well. 1 – gas-rich formation such as coal; 2 – non-producing, gas- or oil- bearing permeable formation; 3 – biogenic or thermogenic gas in shallow aquifer; and 4 – oil or gas from an oil or gas reservoir.
Figure 3. Routes for fluid leak in a cemented wellbore. 1 – between cement and surrounding rock formations, 2 – between casing and surrounding cement, 3 – between cement plug and casing or production tubing, 4 – through cement plug, 5 – through the cement between casing and rock formation, 6 – across the cement outside the casing and then between this cement and the casing, 7 – along a sheared wellbore. After Celia et al. (2005) and this paper.
Figure 4. Photographic examples of leak pathways: (a) Corrosion of tubing (Torbergsen et al., 2012); (b) Cracks in cement (Crook et al., 2003); (c) Corrosion of casing (Xu et al., 2006).
A leak can be catastrophic, as seen in cases such as the recent blowout of a Whiting Petroleum Corp oil well (Cherry State 31-16H) in North Dakota (North Dakota Department of Health (2014)) and rare examples of explosions in urban areas (Chillingar and Endres, 2005), or be at sufficiently low rates to be barely detectable. The fluid sources can be hydrocarbon reservoirs (e.g. Macondo, Gulf of Mexico); non-producing permeable formations (e.g. Marshall and Strahan, 2012); coal seams (e.g. Beckstrom and Boyer, 1993, Cheung et al., 2010); and biogenic or thermogenic gases from shallow rock formations (e.g. Traynor and Sladen, 1997, Jackson et al., 2013).
Oil or gas emissions can seep to the surface, though leaking methane can be oxidised by processes such as bacterial sulphate reduction (e.g. Van Stempvoort et al., 2005). Well failures can potentially occur in any type of hydrocarbon borehole, whether it is being drilled, producing hydrocarbons, injecting fluid into a reservoir, or has been abandoned.
Wells can be tested at the surface for well barrier failure and well integrity failure by determining whether or not there is pressure in the casing at the surface. This is referred to as sustained casing pressure (e.g. Watson and Bachu, 2009), but does not necessarily prove which barrier has failed or its location. Channels in cement, which are potential leakage pathways, can be detected by running detection equipment down the borehole.
Migration of fluids outside the well is established by inserting a probe into the soil immediately surrounding the well bore, or by sampling groundwater nearby, hydraulically down-gradient of the well. Poor cement barriers can be identified by a number of methods (e.g. ultrasonic frequency detection; Johns et al., 2011) and can be repaired in some cases, using cement or pressure-activated sealants (e.g. Chivvis et al., 2009).
This paper draws on a variety of datasets, mostly published, but in some instances sourced from online repositories or national databases, and follows the approach of Davies et al. (2013). In that study, the risk of induced seismicity due to hydraulic fracturing was reviewed, and intentionally included all datasets in the public domain that were considered to be reliable, rather than de-selecting any data (Davies et al., 2013). This inclusive approach has a drawback because well barrier and well integrity failure frequencies are probably specific to the geology, age of wells, and era of well construction (King and King, 2013).
A wide range of failure statistics is therefore reported, and although they are presented on a single graph to show the spread of results (Fig. 9), this is not intended to imply that direct comparisons between very different datasets (i.e. size, age of wells, geology) can be made.
The sources we used do not report their findings consistently and it is unclear in some cases whether well barrier failures have led to leaks into groundwater, rock layers, soil or the atmosphere, producing a true well integrity failure. To be as clear as possible, well barrier and well integrity failure are distinguished in Table 3, quoting directly from the sources used and, where possible, providing additional information on the age of the well and when the monitoring was carried out.
To locate hydrocarbon wells drilled onshore in the UK since 1902 (the age of the earliest well recorded by DECC), the United Kingdom Onshore Geophysical Library (UKOGL) map of well locations was used (UKOGL, 2013), coupled with satellite imagery from Google Earth. A visual inspection and categorisation of the locations was carried out to assess whether the wells have a physical presence at the surface. Pollution incident data were provided by the Environment Agency (England); these data were used to identify incidents that occurred in close proximity to known well sites.