Effects Of Heterogeneity on Sweep Efficiency and Ultimate Recovery of Waterfloods in Sandstone Reservoirs Copyright 2010, Society of Petroleum Engineers Inc.
The objective of this study is to investigate the effects of heterogeneity in the form of shale discontinuities on sweep efficiency and recovery in sandstone reservoirs. In sandstone dominated successions of sheet-like turbidites, erosion of thin shale horizons during deposition of the overlaying turbidite may lead locally to holes of different sizes between the sandstone beds, resulting in discontinuous thin fine-grained beds or, in extreme case, thoroughly amalgamated sandstone. The shales cannot be correlated between wells and their likely location within the reservoir must be predicted using stochastic techniques conditioned to core and outcrop data. This study uses a two-D slice of a three-D model to try to understand the effects of the shales on sweep efficiency and recovery. We find that as the amount of shale present is removed, the oil recovery is increased and the sweep becomes more gravity dominated. We also find that as the size of diameter of the holes in the shale layers is increased at constant amount of shale removed, the recovery also increases.
Reservoir heterogeneity is the degree of variation in reservoir properties as a function of space and scale, starting from pore up to field level. It is largely dependent on the depositional environment and subsequent events including the nature of the sediments which make up the reservoir. This variation in rock properties is due to the differing time sequences of the depositional environment. In sandstone reservoirs, these rock properties (permeability and porosity) are dependent on the depositional environment, the nature of the sediments and subsequent compaction and/or cementation (Stephen et al 1999). Total production from hydrocarbon reservoirs by water-flooding can vary, depending on the structure of the reservoir, the nature of the fluids and the management strategy (Stephen et al 1999). Vertical permeability barriers and baffles within genetically related turbidite sandstone units results from argillaceous beds, abandoned channel fills, mud-clast conglomerate layers, pebbly mudstones and digenetic concretions (kleverlaan & Cossey 1993). The argillaceous beds may be deposited from the low density tail of turbidity currents, low-density muddy turbidities, or pelagic and hemipelagic settling of fine-grained material between sandy turbidity current events. Successive turbidity current events may result in the partial or complete removal of thin, fine-grained beds producing compound, amalgamated sandstone units (fig.1) (Stephen et al 1999). This study aims to demonstrate the effects of these vertical barriers or baffles to flow in the form of transmissibility multipliers on sweep efficiency of a waterflood and to also investigate the effects of these barriers on ultimate recovery in sandstone reservoirs.
Model description & Methodology
Most clastic turbidite deposits, such as those that form reservoir units, contain laterally extensive sandstones with discontinuous shales which act as barriers or baffles to flow. The shale layers may be removed over time by erosion leaving layers with holes in them. Stephan studied the effects of these shales in 2-D but this study extends it to 3-D. A 3-D, 2 phase flow model was set up using Eclipse 100 to investigate and analyse the effect of the reservoir vertical heterogeneity in the form of transmissibility barriers/multipliers on the waterflood sweep efficiency and recovery efficiency of sandstone reservoirs. A total of 45 different scenarios were simulated with multiple realizations done on some scenario to be able to better observe the effects of these barriers on sweep efficiency and recovery efficiency. Because such...