Using Shale Gouge Ratio (SGR) to model faults as transmissibility barriers in reservoirs: an example from the Strathspey Field, North Sea

Abstract

Since the onset of production in 1993, extraction and injection in Strathspey has focused on the central region of the field, around the ‘Central Fault’. Analysis of RFT measurements from intermittent, but regular drilling on either side of this fault has allowed reconstruction of footwall/hanging wall pore-pressure fields for particular time instances. Comparison of these similar age pressure fields shows the Central Fault capable of maintaining up to 1300 psi pressure differentials. The results of the pressure analysis were used to calibrate a fault seal attribute, termed Shale Gouge Ratio (SGR) which is an estimate of clay concentration within fault gouge, mapped across the surface of the Central Fault. The calibration showed that as SGR increases, so does the measured pressure differential across the fault. This positive relationship between SGR and pressure differential suggests SGR is a guide to potential fluid-flow resistance exerted by faults. We therefore suggest that SGR can potentially be used as a guide to defining differences in permeability within and between faults in a given field. This ‘scaleability’ of SGR as an indicator of fault permeability within a field could provide hitherto unachievable flexibility in the systematic modelling of the hydraulic behaviour of faults during fluid flow simulations.