The Effects of Small-Scale Reservoir Architecture on Hydrocarbon Flow Illustrated Using an Outcrop in Fife, Scotland

Abstract

We have used a coastal outcrop of Lower Carboniferous deltaic and shallow-marine sediments to illustrate reservoir flow process. We show how the sediment architecture (rippled and cross-bedded lamination and intraformational faulting) influences the effective permeability at various scales, and derive tensor permeabilities for the different architecture types. Pervasive cross-lamination can produce significant cross flow, the degree of which is determined by the internal permeability contrasts and external boundary conditions (such as the presence of shale above and below). When two immiscible phases are flowing, the sediment architecture can generate additional anisotropy in the relative permeability functions of the two phases. This results in varying amounts of trapping of the nonwetting phase, depending on the flow direction with respect to the sediment architecture or fracture fabric. The waterflood behavior of the formations observed in the cliff (10 m x 100 m ) is modeled, using a data set based on probe permeameter measurements taken from two vertical transects representing boreholes. We contrast the degree of oil recovery from the formation when different assumptions are made: (a) using averaged borehole data and no geological structure, (b) using traditional geostatistical methods to extrapolate from the borehole data, and (c) modeling the sediment architecturemore » of the interwell volume using mixed stochastic/deterministic methods. We find that, in most cases, the sediment architecture has an important effect on flow performance. Differences in predicted oil recovery of up to 20% can occur when these small-scale effects are correctly modeled. Traditional reservoir engineering methods, using average permeability values, only prove acceptable in high-permeability/low-heterogeneity zones.« less