Activating Shale to Form Well Barriers: Theory and Field Examples

Abstract

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.

Disposal of High-Level Nuclear Waste in Deep Horizontal Drillholes

“…Effective plugs may be installed by reaming the drillhole over a relatively short section, which is preferably located adjacent to a clay, shale or salt rock layer. The reamed drillhole section will be filled with a suitable backfill material, which will be compacted by the overburden and plastic deformation of the surrounding rock [50,51], effectively isolating the disposal section from the surface and at the same time sealing the excavation disturbed zone. Similar plugs may be installed throughout the disposal section to compartmentalize the disposal section, reducing the risk of axial radionuclide transport to transmissive zones that may intersect the drillhole.…”

Acid Treatment as a Way to Reduce Shale Rock Mechanical Strength and to Create a Material Prone to the Formation of Permanent Well Barrier

“…Figure 1 represents schematically how the P&A is performed currently (Figures 1A,B) and the proposed "shale as a barrier" concept (Figure 1C). Creep, i.e., time dependent plastic deformation, has been identified as the main shale deformation mechanism [5]. It has been suggested that this phenomenon can potentially be useful in P&A [4,5].…”

“…Creep, i.e., time dependent plastic deformation, has been identified as the main shale deformation mechanism [5]. It has been suggested that this phenomenon can potentially be useful in P&A [4,5]. Indeed, it has been confirmed in downscaled laboratory experiments that some shales like e.g., Lark-Horda can efficiently seal off annulus [6,7].…”

“…The Lark-Horda shale shown to undergo the chemical acceleration in Van Oort experiment [6] is a weak shale with a Young's modulus of 827.4 MPa and large clay content of 70-73%, thus it is not surprising that by modulating clay swelling shale creep abilities can be engineered. Creep, i.e., time dependent plastic deformation, has been identified as the main shale deformation mechanism [5]. It has been suggested that this phenomenon can potentially be useful in P&A [4,5].…”

Disposal of High-Level Nuclear Waste in Deep Horizontal Drillholes

“…Effective plugs may be installed by reaming the drillhole over a relatively short section that is preferably located adjacent to a clay, shale or salt rock layer. The reamed drillhole section will be filled with a suitable backfill material, which will be compacted by the overburden and plastic deformation of the surrounding rock[Kristiansen et al, 2018;Vrålstad et al 2019], effectively isolating the disposal section from the surface and at the same time sealing the excavation disturbed zone.…”