Well Integrity Inspection in Unconventional Gas Wells

The inspection of multi-casing/multi-cement-annuli in hydrocarbon producing wells is of paramount importance for the protection of the environment and to guarantee long term integrity of production wells. However, state-of-the-art imaging technologies used for monitoring the integrity of hydrocarbon-producing well bores are typically limited to inspection of the innermost metal casing and its surrounding cement bond interface. The scope of this work is the development of a multi-casing/multi-annuli inspection system for providing information about the flaw structure and topology of unconventional gas wells. The proposed well integrity inspection system utilizes high energy gamma-rays for deep penetration into metal casings and cement structures. The high-energy imaging modality is complemented by conventional imaging techniques such as ultrasound or electromagnetics.

Monte-Carlo N-Particle (MCNP) simulations and laboratory prototype testing suggest that it is possible to detect air voids and water defects well past the innermost casing. Numerical simulations clearly distinguish air voids with diameters less than ⅛’’ for several inches into the well bore structure. Preliminary data from our fist gamma-ray prototype clearly show evidence of a ¼’’ diameter air void in the first metal casing and a ½’’ diameter air void in the second metal casing. Both metal casings are separated by a cement annulus with a thickness of 1’’. Design improvements of this preliminary gamma-ray prototype are ongoing and further enhancement of the detection limit is expected. The high energy gamma-ray modality can be used to complement other more conventional imaging modalities such as ultrasound or electromagnetics. Current development of an electromagnetic-based imaging tool shows promising performance in a laboratory environment. An array of electromagnetic transducer coils is used to measure the material loss in the first and second casing as well as to detect the eccentricity between the two casing strings. However, the electromagnetic imaging tool cannot detect defects in non-conductive materials such as the cement annulus structure. By combining imaging modalities (e.g., gamma-ray and electromagnetics) into a single inspection system and by fusion of data from each sensing modality, we hope to further improve the detection limit of well integrity monitoring systems. Current efforts investigate benefits of data fusion between high energy modalities and conventional imaging techniques. It is expected that independently acquired data will provide enhanced detection limits and better accuracy compared to the sum of individually acquired data. A multi-modality inspection system with imaging capabilities for metal and concrete defects well past the first casing should be greatly beneficial for well integrity inspection of multi-casing wells at intermediate-to-surface depths along major ground water zones. We will present a tradeoff analysis of performance parameters such as minimum defect size, sector angle resolution, depth resolution, penetration depths, and logging speed. Benefits and drawbacks of the proposed technology will be discussed.

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