Abstract - Simple streamline simulation models are used to illustrate some of the properties of relays in a production flow context. The prevailing view of an unbreached relay is essentially of a hole in a fault providing an uninterrupted lateral flow path across it. Although in many circumstances this may be the most important flow path provided by a relay, the view represents a gross oversimplification of the complex juxtapositions produced. In addition to these uninterrupted flow paths in each layer, an unbreached, i.e. intact, relay ramp provides across-fault juxtaposition between every layer of the sequence displaced by the parent fault. For example, relays contained within a multi-layered sequence of permeable and impermeable layers, can represent extremely efficient conduits for vertical pressure communication. Potential flow-paths are formed between every pair of permeable layers from the lowermost hangingwall to the uppermost footwall layers, provided only that the ramp-bounding faults are not impermeable, and that the vertical separation between layers is less than the displacement of the parent fault. The most important characteristics of relays is the thoroughly 3D connectivity they provide a reservoir. The Kv/Kh ratio of the unfaulted sequence is, however, a fundamental determinant on the impact of relays on vertical flow. In the example discussed above, the relay provides vertical communication across impermeable layers i.e. although the sequence Kv/Kh ratio is zero, the effective Kv/Kh of the faulted sequence is not zero. If these layers were not impermeable, the relay would be less influential on flow as vertical flow need not rely on the relay alone. We present some simple models highlighting vertical and across-fault flow associated with relays contained within sequences with different Kv/Kh ratios. Though a recurring feature of all plausible geological models is enhancement of vertical flow, the nature of flow paths is highly sensitive to the faulted sequence, the fault and relay geometry, and the scale at which flow is considered. In principle, any displacement changes along a fault will enhance the vertical flow within a reservoir, though relays represent the most efficient means of changing displacement, representing as they do singularities along an otherwise continuous fault. These rapid displacement changes dictate that, within a given layer, a relay will represent either a source of, or sink for, flow. Flow models of relays highlight the thoroughly 3D connectivity they provide a reservoir and the enhanced vertical communication they give rise to. It is possible for example, that production from the uppermost footwall units of a reservoir can deplete the lowermost hangingwall units, a scenario for which some form of ‘fault leakage’ or ’valving’ might, erroneously, be concluded. The ubiquity of relays and their potential impact on 3-D flow within a reservoir, makes a strong case for the definition and development of a robust methodology for their routine inclusion in flow simulations.
Abstract of talk given to:
Fault and Top Seals, EAGE Conference, Montpellier, September 2003