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Cataclastic deformation bands in porous sandstones present a potentially significant barrier or baffle to fluid flow because the bands form in complex networks of laterally extensive, tabular zones with permeability up to several orders of magnitude less than that of the sandstone host rock. Previous work suggests that in some geologic settings cataclastic deformation bands become systematically jointed in the subsurface. Calculations presented here demonstrate that under most realistic circumstances, volumetric flow rate across a jointed deformation band can equal or exceed discharge through an equivalent volume of host sandstone. Results indicate that jointed deformation bands are not significant barriers or baffles to fluid movement. [ABSTRACT FROM AUTHOR]

Abstract: Understanding the impact of faults on fluid flow in the subsurface is important for the extraction of oil, gas and groundwater as well as the geological storage of waste products. We address two problems present in current industry-standard workflows for fault seal analysis that may lead to fault rocks not being represented adequately in computational fluid flow models. Firstly, fluid flow properties of fault rocks are often measured only for small-scale faults with throws not exceeding a few centimetres. Large seismic-scale faults (throws >20 m) are likely to act as baffles or conduits to flow but they are seldom recovered from subsurface cores and consequently fault rock data for them is sparse. Secondly, experimental two-phase fluid flow data is lacking for fault rocks and, consequently, uncertainties exist when modelling flow across faults in the presence of two or more immiscible phases. We present a data set encompassing both single- and two-phase fluid flow properties of fault and host rocks from the 90-Fathom fault and its damage zone at Cullercoats Bay, NE England. Measurements were made on low-throw single and zones of deformation bands as well as on slip-surface cataclasites present along the ∼120 m throw main fault. Samples were analysed using SEM and X-ray tomography prior to petrophysical measurements. We show that single deformation bands, deformation band zones and slip-surface cataclasites exhibit dissimilar single- and two-phase fluid flow properties. This is due to grain-size reduction being more pronounced in slip-surface cataclasites and changes in microstructure being fault-parallel for deformation bands but mostly fault-perpendicular for slip-surface cataclasites. A trend of fault rocks with low absolute permeabilities exhibiting lower relative permeabilities than more permeable rocks at the same capillary pressure is evident. [Copyright &y& Elsevier]

Abstract: It is becoming increasingly common practice to model the impact of faults on fluid flow within petroleum reservoirs by applying transmissibility multipliers, calculated from the single-phase permeability of fault rocks, to the grid-blocks adjacent to faults in production simulations. The multi-phase flow properties (e.g. relative permeability and capillary pressure) of fault rocks are not considered because special core analysis has never previously been conducted on fault rock samples. Here, we partially fill this knowledge gap by presenting data from the first experiments that have measured the gas relative permeability (krg ) of cataclastic fault rocks. The cataclastic faults were collected from an outcrop of Permo-Triassic sandstone in the Moray Firth, Scotland; the fault rocks are similar to those found within Rotliegend gas reservoirs in the UK southern North Sea. The relative permeability measurements were made using a gas pulse-decay technique on samples whose water saturation was varied using vapour chambers. The measurements indicate that if the same fault rocks were present in gas reservoirs from the southern Permian Basin they would have krg values of <0.02. Failure to take into account relative permeability effects could therefore lead to an overestimation of the transmissibility of faults within gas reservoirs by several orders of magnitude. Incorporation of these new results into a simplified production simulation model can explain the pressure evolution from a compartmentalised Rotliegend gas reservoir from the southern North Sea, offshore Netherlands, which could not easily be explained using only single-phase permeability data from fault rocks. [Copyright &y& Elsevier]