A framework for the assessment of ductile shales for wellbore sealing and abandonment

Publieke samenvatting / Public summary

There is increasing interest to use the ductile behavior of natural shales in order to seal and/or abandon boreholes. It is known that this ductile behavior may vary depending on various factors such as (clay) mineralogy, petrophysical and mechanical parameters. The major advantage of such materials is that they can remain stable for milions of years, are ductile and have very low permeabilities. Shales represent the most abundant sedimentary rock and their ductility behavior is extremely complex (compared to evaporites) as it depends on numerous properties. Therefore this proposal will develop a methodology to classify ductile shales in the subsurface and assess the potential of clayey formation for natural wellbore sealing with focus on NL.

The objective of the proposed project is to answer the following three questions
1. which shales show ductile behaviour and which do not?
2. which parallels exist for ductile shales between geomechanical properties and other petrophysical or mineralogical properties?
3. Can we establish an inventory of shales in NL and their properties with respect to ductility/usage for wellbore sealing?

Korte omschrijving
It is known that the ductile behaviour of shales depending on (clay) mineralogy, petrophysical and mechanical properties. Although variable information is available about the geomechanical properties of buried shales the relationship between ductility and petrophysical or mineralogical parameters is not well understood, except the role of smectite content which can be used as an indicator for ductility. However, to increase the understanding is crucial in order to identify potential natural sealing formation for wellbore closure in the subsurface. When limited geomechanical information is available, parallels may be found between geomechanical properties and other properties on mineralogy, geohydrology, etc. Such properties refer to clay type, permeability, burial depth, diagenetic features, etc. Insight is thus needed to couple the ductile properties to better known properties such as hydraulic conductivity, mineral composition or porosity, as this relation has not been investigated intensively up to now, to provide a methodology to identify ductile shales in the subsurface (of NL).

The foreseen project result is a report in which the following is addressed:
1. Identification of the geomechanical properties of shales that lead to ductile behaviour as opposite to stiff or brittle behaviour
2. Identification of correlations between geomechanical parameters and other geo-scientific parameters of shales with a focus on ductile shales
3. Selection criteria for the identification of ductile shales useful for natural sealing of wellbores

The results will enable the identification of shale rocks that are supposed to show ductile behaviour. The gained information will support the set-up of pilot test to investigate the potential of ductile shales in the field in order to work towards an implementation of this technology (in NL).