Fault seal behaviour in Permian Rotliegend reservoir sequences: case studies from the Dutch Southern North Sea

Abstract

Permian Rotliegend reservoir rocks are generally characterized by high net/gross (N/G) ratios, and faults in such sand-dominated lithologies are typically not considered likely to seal. Nevertheless, many examples of membrane sealing are present in Rotliegend gas fields in the Southern Permian Basin. This manuscript reviews examples of membrane sealing in the Dutch Rotliegend; it presents an extensive dataset of petrophysical properties of Rotliegend fault rocks and analyses two case studies using commonly used workflows. Fault (membrane) seal studies have been carried out on two Rotliegend fields to test the level of confidence and uncertainty of prediction of 'across fault pressure differences' (AFPD) based on existing SGR-based algorithms. From the field studies it is concluded that observable small AFPDs are present and that these are likely pre-production AFPDs due to exploration-time scale trapping and retention of hydrocarbons. Two shale gouge ratio (SGR)-based empirical algorithms have been used here to estimate AFPDs in lower N/G reservoir intervals with the aim of predicting membrane seal behaviour, and these results are compared to field data. It is concluded the selected SGR-based tools predict AFPD for Upper Rotliegend lower N/G reservoir rocks with reasonable results. Nonetheless, the core sample datasets show a much wider range of permeability and capillary entry pressure than predicted by the selected SGR transforms. This highlights the potential to modify existing workflows for application to faults in high N/G lithologies. Data sharing and collaboration between industry and academics is encouraged, so that in the long run workflows can be developed specifically for faults in high N/G lithologies.

Structural traps and seals for expanding CO2 storage in the northern Horda platform, North Sea

“…However, Bretan and others (2003), with similar results to Harris et al (2002), suggested that AFPD between two saline aquifers can represent a hydraulic resistance (i.e., hydrodynamic) seal (i.e., Heum, 1996) in the presence of fine-grained fault rock with low permeabilities. In an attempt to consider grain size and predict fault rock membrane seal presence, we utilize the shale gouge method (SGR; Yielding et al, 1997;Freeman et al, 1998), which has been presented in many studies to explain or predict instances of perceived subsurface fault rock seal due to fine-grained fault rock for hydrocarbon (e.g., Lyon et al, 2005;van Ojik et al, 2020), groundwater (e.g., Bense and Van Balen, 2004), and CO2 storage systems (e.g., Bretan et al, 2011;Karolytė et al, 2020). Yielding (2002) provided empirical evidence from the North Sea suggesting that SGR values >0.15-0.2 correlated with areas along faults known to seal hydrocarbons, and 0.15 is used herein as a minimum threshold value for indicating areas of potential fault rock membrane seal.…”

EXTREME CAPILLARY HETEROGENEITIES AND IN SITU FLUID COMPARTMENTALIZATION DUE TO CLUSTERS OF DEFORMATION BANDS IN SANDSTONES

“…There are numerous examples of the impact of geologic structures on fluid compartmentalization in the subsurface (Gainski et al, 2010;Gill et al, 2010;Hansen et al, 2013;Milkov et al, 2007;Richards et al, 2010;Scott et al, 2010;Van Hulten, 2010;Rotevatn et al, 2017). Deformation bands have impacted production in several hydrocarbon reservoirs (for example, Antonellini et al, 1999;Leveille et al, 1997;Lewis and Couples, 1993;van Ojik et al, 2020). One of the clearest examples is reported in a recent study of Wilkins et al, 2019.…”