Formation as Barrier for Plug and Abandonment of Wells

Abstract

To permanently plug and abandon (P&A) of drilled wells, permanent barrier(s) should be established. Cement is the primer material used for zonal isolation and permanent P&A as barrier material. However, it is recognized that cement may not be a suitable material. Other barrier materials are being developed and tested. The high costs of establishing barriers and durability of materials persuaded engineers to check the usability of naturally established barriers, such as creeping formations. The concept of formation as barrier (FAB) is to use earth itself as barrier material. It is desirable to exploit the displaced formation surrounding casing considering it is cost efficient, saves time and makes operation performance carried out in a safe manner. Bonded and impermeable in-situ formation (e.g. shale, salt) is known to have sufficient formation integrity and is accepted as an annulus well barrier element. The present work reviews the fundamental concept of FAB, creep process, the properties of creeping formation(s), impacts causing creep, self-healing and self-sealing capability of formations, and description of empirical and rheological models and methods that need to be utilized to find creeping formations. Due to the large deformations needed to establish a barrier through creep process, it appears that best candidates are shales with a low threshold for plastic flow and a high ability to sustain large plastic deformations. The findings show the mechanisms that may cause the gap closure process.