Gravity Override

Key Takeaways

• The gravity override mechanism creates a characteristic injected-fluid tongue that thickens upward toward the top of the reservoir as the fluid migrates updip while advancing laterally, eventually breaking through to the producing wells preferentially through the top of the formation while the lower portions remain unswept with original oil saturation; the breakthrough of gas or steam at a producing well without corresponding oil production in the lower portion of the interval is the surface manifestation of gravity override, and the time from injection start to gas or steam breakthrough is a measure of how severely override has occurred; in a reservoir with uniform permeability and strong gravity override, the gas breaks through at the top perforations of the producer while the lower perforations continue to produce oil, and the producing gas-to-oil ratio increases rapidly after breakthrough as more of the flow comes from the overriding gas tongue rather than the displaced oil bank; this override-driven early breakthrough is distinct from viscous fingering breakthrough that occurs in unstable miscible floods but has similar economic consequences of reducing injection efficiency and forcing production of a high-gas or high-steam stream that must be processed at the surface.
• Density ratio and mobility ratio jointly determine the flooding stability of a gas or steam injection project, with gravity override occurring when the density contrast exceeds the viscous forces that would otherwise push the injected fluid forward uniformly; the gravity number G (ratio of gravitational to viscous forces) is defined as G = (rho_displaced - rho_injected) x g x k x A / (mu x q), where rho represents fluid densities, g is gravitational acceleration, k is permeability, A is cross-sectional area, mu is viscosity, and q is volumetric flow rate; a gravity number much greater than 1 indicates gravity-dominated flow with severe override, while G much less than 1 indicates viscosity-dominated flow where the override effect is minimal; at intermediate gravity numbers, the flood front tilts (higher injection fluid saturation near the top, lower near the bottom) but complete override has not occurred; reducing the injection rate (which reduces the denominator q while keeping the gravity terms constant) can be counter-productive because it increases G and worsens override, while increasing the injection rate improves the viscous-to-gravity force ratio but may exceed formation parting pressure and cause fracturing.
• Gravity-stable displacement is achieved when the injected fluid is placed below the oil-water contact (for gas flooding with water below oil) or when horizontal injection wells are positioned near the base of the pay zone to inject the lighter fluid at the bottom and rely on gravity to drain oil downward toward horizontal producer wells — a design that intentionally exploits density differences to improve sweep rather than fighting them; SAGD (steam-assisted gravity drainage) in oil sands reservoirs is the most commercially significant example of a gravity-stable thermal process, in which steam injected through an upper horizontal well creates a steam chamber that heats bitumen, reducing its viscosity by orders of magnitude, and the heated bitumen drains by gravity to a lower producer well below — gravity override is not fought but is the actual production mechanism; for conventional gas flooding in moderately dipping reservoirs, injecting gas downdip (at the structurally low end of the reservoir) and producing updip allows the lighter gas to sweep vertically upward through the reservoir while oil drains down and is produced at the updip wells, achieving much higher volumetric sweep than horizontal injection into the same reservoir would achieve.
• CO2 flooding presents a particularly complex gravity situation because the density of CO2 relative to reservoir oil depends strongly on reservoir pressure and temperature: in high-pressure, moderate-temperature reservoirs (typical of many U.S. Permian Basin and Gulf Coast depleted fields), CO2 at reservoir conditions is in a dense supercritical phase with density of 0.6-0.8 g/cc, potentially denser than some light oils (density 0.7-0.8 g/cc at reservoir conditions) and lighter than heavier oils; in low-pressure reservoirs, CO2 is less dense than most oils and override is severe; the density relationship also changes as CO2 mixes with and swells the oil near the miscibility front, temporarily changing the effective density of the mixed fluid; practical CO2 flood design accounts for the density uncertainty by combining CO2 injection with water injection (WAG — water alternating gas — injection) that periodically pushes the CO2 slug deeper into the reservoir with a denser water bank, reducing the effective gravity override while maintaining mobility control over the CO2 that might otherwise finger severely into high-permeability streaks.
• Vertical conformance measurements from production logging (spinner surveys, temperature logs, noise logs) in injection and production wells are the primary diagnostic tool for detecting and quantifying gravity override in operating floods: a temperature log in an injection well after extended CO2 or gas injection shows the coolest temperatures at the intervals taking the most injection (because gas entering the formation cools due to the Joule-Thomson effect), allowing identification of the vertical distribution of injection; in a reservoir with override, the upper perforations show the coolest temperatures (most gas injection) while the lower perforations show warmer temperatures (less gas injection or no injection at all); production logs in producing wells show the vertical distribution of inflow, with gas or steam breakthrough visible as anomalously high flow rates in the upper perforations; these measurements guide perforation optimization decisions (cementing off upper perforations that are taking overriding gas and reperforating lower intervals to access bypassed oil) and injection rate adjustments to improve sweep efficiency.