Activating Shale to Form Well Barriers: Theory and Field Examples

Shale is a general term used for argillaceous (clay-rich) rocks which are the most abundant sediment on the earth. It is believed that clay rich rocks comprise more than 50-75% of the geologic column. Shale has very varying petrophysical and mechanical properties. Shale is in the most cases acting as a trap or seal for hydrocarbon migration, but has also in more recent years been targeted as a reservoir target in some basins. In some wells it has been observed on cement bond logs that shales in uncemented intervals have moved in and closed the annulus. Pressure communication testing has been performed on these sections and the sections has been qualified as well barrier elements (Williams et al., 2009) for plug and abandonment (P&A) purposes. The main mechanism behind the deformation process is believed to be shale creep. In this paper we will discuss shale creep and other shale deformation mechanisms and how an understanding of these can be used to activate shale that has not contacted the casing yet to form a well barrier. We have developed a numerical model based on first order principles to better understand the mechanical deformation process. We are also supporting the modeling results with laboratory experiments, before we discuss a couple of field cases where shale intervals have been activated and verified to have formed a well barrier as part of the well construction process in new wells.