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Assessing the possibility of gas leakage through drilling induced fractures in shallow formations

What is the hypothesis? In recent literature it is proposed that drilling always creates fractures around the wellbore (drilling induced fractures) that serve as migration pathway for gas in shallow (< 1000 m depth) sediments. This hypothesis has been proposed in recent publications for the Nordland Group sediments underneath the North Sea in Norway but is also proposed for the British equivalent of the North Sea. TNO has investigated this hypothesis by means of a literature study that focuses on the mechanical properties of shallow sediments, their deformation characteristics and the drilling conditions that occur in the North Sea wells at the depths of interest.
The shallow part of the North Sea consists of unconsolidated (loose sediment, not solid rock) to weakly consolidated sand and shales layers. This study has specifically focused on drilling induced damage2 (including drilling induced fractures) of shale layers. After all, the shale layers are expected to act as seals that prevent upward migration of gasses (such as methane). This in contrast to the (highly) permeable sand intervals. Therefore, only shale deformation mechanisms and seal integrity are of interest when it comes to potential leakage mechanisms.

TNO’s literature review shows that the hypothesis is incorrect and that there is no evidence that drilling induced fractures serve as migration paths for (shallow) methane. The main conclusions from this study are:

1. Shallow sediments deform in a ductile manner. Therefore, it is highly unlikely that fractures form.
2. Even when rocks are sufficiently consolidated (brittle), fractures only occur when the drilling fluid (mud weight) is too heavy during drilling. However, these drilling conditions are not typically applied at shallow intervals.
3. To our knowledge, there are no published cases of gas migration through drilling induced fractures.