Vertical Displacement Efficiency

Key Takeaways

• Mobility ratio effects on vertical displacement efficiency are one of the primary controls — favorable mobility ratios (M < 1, displacing fluid less mobile than displaced fluid) provide piston-like displacement that supports uniform vertical sweep; unfavorable mobility ratios (M > 1, displacing fluid more mobile than displaced fluid) create flow instabilities that cause the displacing fluid to finger through the reservoir, with the resulting irregular front leaving substantial unswept pay zones; for typical waterflooding (water displacing oil), the mobility ratio is approximately the ratio of oil viscosity to water viscosity, which is favorable for oil viscosities of 0.5-3 cP (close to or less than water viscosity) but increasingly unfavorable for heavier oils where viscous fingering becomes severe; mobility control through polymer addition to the displacing water (increasing water viscosity) or foam generation (reducing gas mobility through liquid lamellae) can dramatically improve vertical displacement efficiency in mobility-ratio-challenged systems.
• Reservoir heterogeneity effects on vertical displacement efficiency arise from non-uniform vertical permeability that causes preferential flow paths through the reservoir — high-permeability layers receive disproportionate share of injected fluid (a 10x permeability ratio between layers can result in 10x flow rate ratio), with the high-permeability layers reaching breakthrough quickly while low-permeability layers remain largely unswept; the resulting vertical displacement efficiency is much lower than would be achieved in a homogeneous reservoir of the same average permeability; reservoir characterization through detailed log and core analysis identifies the permeability variations that affect vertical sweep, supporting completion design that targets the layers most likely to contribute to commercial production while avoiding completion in the highest-permeability streaks that would cause early breakthrough.
• Gravity override in horizontal and inclined injection represents a specific case of vertical displacement efficiency reduction — when gas or low-density fluids are injected into a sloping reservoir with significant vertical permeability, the buoyancy force drives the injected fluid upward through the reservoir, with the result being preferential sweep of the upper portion of the pay zone and minimal sweep of the lower portion; gravity override is particularly severe in thick reservoirs (greater than 30 m thick) with high vertical-to-horizontal permeability ratios; mobility control techniques including foam injection and gas-water alternating injection (WAG) can mitigate gravity override by increasing the apparent viscosity of the gas phase or by mixing it with the heavier water phase to reduce buoyancy effects.
• Operational implications of vertical displacement efficiency for field development include completion design (selecting perforation intervals that balance contact with multiple reservoir layers vs avoiding the highest-permeability streaks), well spacing optimization (closer well spacing reduces the unswept volume between wells and supports better vertical sweep), and EOR planning (mobility control techniques are typically required for unfavorable mobility ratios or strong heterogeneity); reservoir simulation models predict vertical displacement efficiency for specific reservoir conditions and operational scenarios, supporting the field development decisions that maximize ultimate recovery from the reservoir.
• Improving vertical displacement efficiency through mobility control includes polymer flooding (adding polymer to injection water to increase viscosity, reducing the mobility ratio toward favorable values), foam injection (using surfactant to generate foam in-situ that reduces gas mobility), water-alternating-gas injection (WAG, alternating water and gas injection to take advantage of the relative permeability hysteresis that reduces gas mobility), and conformance treatments (placing chemical or mechanical barriers in high-permeability streaks to force injected fluid into the lower-permeability zones); each technique has specific applicability and economic considerations, with modern reservoir engineering integrating these techniques into the broader field development plan.