Environmental Risk Arising From Well-Construction Failure
Authors: Carpenter Chris 
Published: February 2015

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 166142, ’Environmental Risk Arising From Well-Construction Failure: Difference Between Barrier and Well Failure, and Estimates of Failure Frequency Across Common Well Types, Locations, and Well Age,’ by George E. King, Apache, and Daniel E. King, WG Consulting, prepared for the 2013 SPE Annual Technical Conference and Exhibition, New Orleans, 30 September-2 October. The paper has been peer reviewed. Published: November 2013 SPE Production & Operations, page 323.
The great majority of wells do not pollute. With that fact in mind, 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, using published investigations and reviews from data sets taken from wells worldwide. Ultimately, it is clear that there is absolutely no one-size-fits-all well-failure frequency.
Introduction
For purposes of focus and brevity, this work is limited to the failure potential of the constructed barriers remaining in the producing well after drilling (e.g., casing, cement, packers, tubing, and wellheads) and of other downhole equipment that remains part of the producing well at the handover from drilling to production operations.
Well-Design Overview: Establishing Redundant Barriers
Barriers may be active, passive, or, in some cases, reactive. Active barriers such as valves can enable or prevent flow, while passive barriers are fixed structures such as casing and cement. When barriers are used in series (nested one inside the other), a multiple-barrier system is created, essentially a “defense-in-depth” barrier system. Reactive barriers are invisible or unobtrusive in normal operations, but they deploy a containment response when a pressure, flow-rate, or other behavior limit is exceeded. In the oil and gas industry, a reactive barrier may be a human or mechanical response to an activating or triggering event.
The difference between drilling and production-well barriers is that most production- well barriers are static (available continuously over an extended period of time, usually without requiring human observation or action), whereas most drilling and completion-activity barriers are dynamic (control is variable with time and activity). Production barriers require less-continuous monitoring compared with drilling and completion barriers that are dependent on correct human activity.
Important Concepts in Understanding Well-Construction Failure
Barriers and Well Integrity. Oil- and gas-producing wells are a nested collection of pipe, cement, seals, and valves that form multiple barriers between produced well fluids and the outside environment. Barriers are containment elements that can withstand a specific design load. These may consist of pipe that is effectively cemented, as well as seals, valves, and pressure-rated housings.
Multiple barriers are nested individual barriers designed and built to withstand a specific load without help from other barriers. If an inside (or outside) barrier fails, the next barrier will provide isolation so that a leak path will not form.