Environmental Risk Arising From Well-Construction Failure—Differences Between Barrier and Well Failure, and Estimates of Failure Frequency Across Common Well Types, Locations, and Well Age

Abstract

Summary Do oil and gas wells leak to the environment? This paper will show the great majority of wells do not pollute. The purpose of this paper is to explain basic concepts of well construction and illustrate differences between single-barrier failure in multiple-barrier well design and outright well-integrity failure that could lead to pollution by use of published investigations and reviews from data sets of more than 600,000 wells worldwide. For US wells, while individual-barrier failures (containment maintained and no pollution indicated) in a specific well group may range from very low to several percent (depending on geographical area, operator, era, well type, and maintenance quality), actual well-integrity failures are very rare. Well-integrity failure occurs when all barriers fail and a leak is possible. True well-integrity-failure rates are two to three orders of magnitude lower than single-barrier-failure rates. When one of these rare total-well-integrity failures occurs, gas is the most common fluid lost. Common final-barrier leak points are failed gaskets or valves at the surface and are easily and quickly repaired. If the failure is in the subsurface, an outward leak is uncommon because of a lower pressure gradient in the well than in outside formations. Subsurface leaks in oil and gas wells are rare, and routinely comprise exterior-formation salt water leaking into the well toward the lower pressure in the well. Failure frequencies are estimated for wells in several specific sets of environmental conditions (i.e., location, geologic strata, produced-fluid composition, and soils). Estimate accuracy depends on a sufficient database of wells with documented failures, divided into (1) barrier failures in a multiple-barrier system that did not create pollution, and (2) well-integrity failures that created a leak path, whether or not pollution was created. Estimated failure-frequency comparisons are valid only for a specific set of wells operating under the same conditions with similar design and construction quality. Well age and construction era are important variables. There is absolutely no universal definition for well-failure frequency.

Groundwater methane in relation to oil and gas development and shallow coal seams in the Denver-Julesburg Basin of Colorado

“…The majority (89%) of horizontal wells also have production casings that are either fully cemented or cemented above the shallowest hydrocarbonbearing formation (20). This evidence supports the growing consensus that wellbore barrier failure, not the process of high-volume hydraulic fracturing itself, is the main thermogenic stray gas migration pathway (1,3,9,10,20,27,29,30).…”

“…These rates are 1.6-4 times lower than the fraction of shale gas wells in Pennsylvania that have led to methane migration into groundwater (0.24%) (9). The contrast in rates highlights differences in geology, regulations, operators, and construction practices among different petroleum-producing basins (29)(30)(31). It should also be noted that the occurrence of thermogenic stray gas is likely underrepresented because of the spatial density of sampled water, willingness of landowners to have their water tested, and the possibility that stray hydrocarbons are rapidly oxidized under certain redox conditions (38).…”

Numerical investigation of methane and formation fluid leakage along the casing of a decommissioned shale gas well

“…Well integrity to prevent leaks of gas and other fluids to groundwater or to ground surface is a cornerstone of environmental protection in all oil and gas drilling operations. Well integrity can be compromised as a result of poor completion or abandonment, formation damage around the wellbore, geomechanical effects, geochemical degradation of well cements, casing and tubing corrosion, and casing failure as a result of thermal or mechanical stresses (see King and King [2013] for a review). Poor well construction is a critical issue throughout the oil and gas industry and is not unique to any specific region, type of well, drilling period or sector of activity.…”

Arsenic and Selenium in Formation Waters: Environmental Implications for Carbon Dioxide Storage in Geological Media

“…Oil and gas wells are constructed with multiple engineered barriers against fluid flow leakage into other aquifers, protected groundwater, and/or atmosphere [19]. Local loss of well integrity may occur when there is debonding of cement from casing or from the rock wall, when the well cement has wormholes or fractures, when the well cement is not properly set in the well annulus, or when the casing fails due to stress [10].…”

“…In all these cases, leakage of reservoir or formation fluids, inside or outside well casing, will be stopped by the next barrier (e.g., the next cement plug or the next well section with good quality cement). However, if a continuous leakage pathway is formed, then the well loses its integrity and fails as a whole [19]. In a study of more than 300,000 wells in Alberta, Bachu & Watson [20], have found that approximately 6.5% of the oil and gas wells in the province have lost their integrity, leading to surface casing vent flow or gas migration.…”