Water/Oil Displacement Characteristics in Crossbedded Reservoir Zones

Kortekaas, T.F.M., SPE, Shell Research B.V. Abstract Festoon crossbedding is a typical sedimentary structure in sandstone reservoirs. It is especially common in fluvial deposits. The important elements are the foreset laminae, which vary in permeability, and the bottomsets of lower permeability. To understand the complex, direction-dependent displacement characteristics of a crossbedded reservoir zone, we first conducted numerical simulations on a centimeter scale in a small part of a water-wet crossbedded reservoir zone. The simulations indicate that, during water/oil displacement, considerable amounts of movable oil initially are left behind in the higher-permeability foreset laminae with fluid flow perpendicular to the foreset laminae, while with flow parallel to the foreset laminae the displacement efficiency is good. To describe the displacement characteristics on a reservoir scale, we developed a procedure for calculating direction-dependent pseudo relative-permeability and capillary-pressure curves to be used as input for the simulations of water/oil displacement in a crossbedded reservoir zone. On a reservoir scale, the displacement characteristics in a water-wet crossbedded reservoir zone are slightly more favorable with the main fluid flow perpendicular to the foreset laminae. perpendicular to the foreset laminae. In addition, the sensitivity of the displacement characteristics to moderate reductions in interfacial tensions (IFT's) and to increases in water viscosity was investigated, both on a centimeter scale and on a reservoir scale. The simulations indicate the potential for substantial improvement in recovery from crossbedded reservoir zones if diluted surfactant or polymer is added to the drive water. Introduction Detailed studies of the effect of reservoir heterogeneities on water/oil displacement characteristics have been conducted on a well-to-well (layering) scale and on a pore scale, but few studies on an intermediate scale have been done. Therefore, we embarked on a study of the effect of centimeter-scale heterogeneities on water/oil displacement characteristics. We studied festoon crossbedding, one of the typical sedimentary structures in sandstone reservoirs, particularly common in fluvial deposits. A schematic particularly common in fluvial deposits. A schematic representation of a small part of a crossbedded reservoir zone is given in Fig. 1A. The important elements are the foreset laminae, which vary in permeability, and the bottom-sets, which are of lower permeability. The width of the foreset laminae is exaggerated in Fig. 1A; typically it is a few centimeters. First, we will discuss a mathematical simulation study in a very limited area of a water-wet crossbedded reservoir zone (1.97 × 26.2 × 0.66 ft [0.6 × 8 × O.2 m]). After a brief discussion of the water/oil displacement characteristics near a single permeability transition, we present the water/oil displacement characteristics in some cross sections of a simplified model (Fig. 1B) of a small part of a crossbedded reservoir zone. In addition, their sensitivity to moderate reductions in IFT's and increases in water viscosity are discussed. Second, we describe the effect of crossbedding on water/oil displacement characteristics on a reservoir scale, discuss a procedure for calculating dynamic, direction-dependent pseudo relative-permeability and capillary-pressure curves, and present the results of a reservoir-scale mathematical simulation study, including the pseudo-properties. Also, the sensitivity of the results to changes pseudo-properties. Also, the sensitivity of the results to changes in IFT and water viscosity is discussed. One-Dimensional Water/Oil Displacement Characteristics Near an Abrupt Permeability Transition Permeability Transition suppose we have a one-dimensional (1D) system consisting of two zones with different absolute, but identical relative, permeabilities. Furthermore, the system is horizontal and contains oil and connate water. The Buckley-Leverett first-order partial differential equation describes the water/oil displacement in each zone.In the absence of capillary and gravitational forces, the water fractional flow Fwo) is given byEq. 1, together with Eq. 2, usually leads to a sharp shock front: at each location, water saturation will instantaneously jump from connate water to shock-front saturation when the water arrives. SPEJ p. 917