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Horizontal Floods, Vertical Floods, Sweep Efficiency
Reservoir heterogeneities assume different significance in horizontal versus vertical floods.
The effect of permeability stratification on vertical sweep efficiency in horizontal floods was first quantified by Dykstra and Parson (1950), and is designated by their coefficient of permeability variation, VDP. VDP represents the amount of stratification in the reservoir where a value of 0.0 represents a homogeneous reservoir and a value of 1.0 signifies a reservoir that is completely heterogeneous. Oil recovery at breakthrough during waterflooding is reduced drastically as permeability variation coefficient increases.
The Dykstra-Parsons coefficient of permeability variation does not take into account the manner in which permeability varies spatially. Vertical permeability stratification may sometimes actually help in a horizontal displacement scenario by limiting gravity override (or underride). Belgrave and Win (1993) showed that for any given VDP, oil recovery at breakthrough during gas injection depended on depositional environment, gas injection rate, and well completion strategy.
In vertical miscible floods discontinuous permeability barriers would hinder the downward movement of the solvent and cause excessive loss of the solvent by entrapment above permeability barriers.
Thermal versus Non-Thermal Processes
Unlike thermal recovery methods where the injected fluid need not contact the reservoir oil to displace it (the heat from injected steam significantly assists displacement), miscible methods rely on fluid-oil contact for efficient displacement. Heterogeneities in the reservoir such as lenses, permeability pinchouts, etc. reduce the contacted volume and hence, productivity. This is true for all non-thermal injection processes.