AAPG Geoscience Technology Workshop

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Efficient Inclusion of Faults and Fault Flow Properties in Reservoir Models. The Example of Karish Field, Offshore Israel

Abstract

Faults in siliciclastic reservoirs are normally conceived as planar surfaces in 3D seismic data, and are incorporated as single surfaces in reservoir simulation models due to geometrical and numerical limitations. However, examination of reservoir-analogue faults at both outcrop and laboratory scales reveals that the former often comprise structurally complex three-dimensional regions with highly variable distributions of single and two-phase fault-rock flow properties. This volume and property segmentation may have a remarkable effect on cross-fault flow paths, and eventually on trapped volumes and hydrocarbon recovery. Understanding fault sealing processes and variations is pivotal in evaluating seal integrity, reservoir compartmentalization and migration pathways, hence, should not be discounted in either exploration or development studies. How do faults seal? The major mechanisms that direct the sealing character of faults are either the juxtaposition of reservoir units against sealing lithologies (juxtaposition seals) or fault-rock seals dominated by the flow properties of the faulted interfaces. Industry established workflows exist for reservoir-scale fault property prediction and quantification. In the general run of things, such an assessment demands undertaking a thorough structural analysis of the available 3D seismic data and identification and mapping of faults. Subsequently, construction of fault surface juxtaposition maps (Allan diagrams) spotlight at once both potential juxtaposition seals and cross-fault flow corridors. The most important fault property for calculating the sealing strength of a fault zone is the fault-rock capillary entry (or threshold) pressure as a function of the clay composition of the fault gouge. Fault clay distribution is universally represented using applied proxy-properties (e.g., Shale Gouge Ratio – SGR, Effective Shale Gouge Ratio - ESGR) and capillary entry pressure is mapped onto the fault planes using industry-standard formulas. The calculated threshold pressure can eventually be used to predict the maximum height of hydrocarbon column that a particular fault can support. In production schemes, reservoir performance indicators signify the potentially impairing across-fault fluid flow behaviour, be it poor productivity/injectivity, rapid decline rate, early water break-through, fault seal breakdown, and/or unexpected 4D seismic response. The first step in determining the influence of faults on reservoir performance is the flow characterisation of faults and fault-related products and their representation in reservoir simulation models. Faults alter the reservoir model transmissibilities by introducing new cell connections and new contact areas between the juxtaposed cells adjacent to the fault interfaces. Likewise, natural variability in fault zone permeabilities and thicknesses affect the effective transmissibility of the faulted cells. Contemporary reservoir simulation studies conventionally treat faults as either sealing or open to flow by assigning transmissibility multipliers of 0 or 1 respectively. These multipliers modify transmissibilities between faulted grid-blocks, yet they do not account for the inherent geometrical and fault-property heterogeneity. To capture this effect, geologically sound methods for estimating fault transmissibility multipliers are utilised. These methods calculate fault-rock properties for every faulted cell connection as a function of fault shale content, grid-block permeabilities and grid geometries. Eventually, these techniques allow fault-rock properties to be implicitly incorporated to full-field reservoir simulation models. The present study applies the workflows described above in the Karish field, offshore Israel. The fault seal analysis results were appropriately calibrated to allow coherent conclusions to be drawn in terms of reservoir behaviour. Effective Shale Gouge Ratio (ESGR) was calibrated against RCI pressure data across the field. By collating the calibrated ESGR with across-fault pressure difference from global datasets, fault-seal failure envelopes were drawn suggesting maximum column heights that can be sustained for particular fault-clay compositions. In addition the influence of the predicted fault-rock properties on the production figures was investigated. Rigorous and geologically credible fault transmissibility models were constructed, and the impact of faults on likely cross-fault fluid flow and final recoverable volumes was evaluated. The upcoming field development will allow for further calibration of fault analysis results by integrating with history-matching and 4D (time lapse) seismic monitoring. Repeating the exercise will further refine and constrain any current uncertainties associated with column height estimates and likely fault-controlled reservoir compartmentalisation.