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Summary Well abandonment is one of the biggest challenges in the oil and gas industry, both in terms of cost and effort as well as the technical hurdles associated with wellbore sealing for an indefinite term. A mechanism that may be exploited to simplify well abandonments is using natural shale or salt formations for the creation of annular barriers. Currently, uncemented annuli often require casing cutting and pulling or milling before abandonment plugs can be set, which necessitates the use of a drilling rig. This is an expensive, time- and labor-intensive process, particularly offshore. However, shale or salt creep may naturally form a barrier behind uncemented casing sections. With a qualified annular shale barrier in place, the well may only require the setting of abandonment plugs within the existing casing string(s), a task that can often be done rigless and with significantly less effort.
The work described in this paper presents the results of a rock mechanical investigation into the creep behavior of North Sea shales and their ability to form effective annular barriers. Field core from the Lark shale, a member of the Hordaland Group, was used to conduct dedicated, customized experiments that simulated the behavior of shale confined under downhole effective stress, pressure, and temperature conditions to fill in an annular space behind a simulated casing string. Full-scale triaxial rock mechanics equipment was used for testing cylindrical shale samples obtained from a well-preserved field core in a setup that mimicked an uncemented casing section of a well. The deformation behavior of the shale was monitored for days to weeks, and the formation of the annular barrier was characterized using dedicated strain measurements and pressure pulse decay probing of the annular space.
The large-scale laboratory results clearly show that the Lark shale will form competent low permeability annular barriers when left uncemented, as confirmed using pressure-pulse decay measurements. They also show that experimental conditions influence the rate of barrier formation; higher effective stress, higher temperature, and beneficial manipulation of the annular fluid chemistry all have a significant effect. This then opens up the possibility of activating shale formations that do not naturally create barriers by themselves into forming them (e.g., by exposing them to low annular pressure, elevated temperature, different annular fluid chemistry, or a combination of these). The results are in very good agreement with field observations reported earlier by several North Sea operators.
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