Gravity override

Gravity override of low density steam leads to poor volumetric sweep efficiency and low oil recovery in steam floods. Nonchemical methods of improving steam volumetric sweep efficiency include completing the injection well so steam is only injected in the lower part of the oil-bearing zone (181), alternating the injection of water and steam (182), and horizontal steam injection wells (183,184). Surfactants frequently are used as steam mobihty control agents to reduce gravity override (185). Field-proven surfactants include alpha-olefin sulfonates (AOS), alkyltoluene sulfonates, and neutralized... 

Gravity override is the migration of the steam to the upper portion of the formation and is caused by the low steam density. This results in channeling of the steam through the upper portion of the reservoir and a low volumetric sweep efficiency. 

Surfactants have been used as steam mobility control agents in both laboratory and field tests to prevent this gravity override thereby increasing volumetric sweep efficiency. Surfactants that have been... 

Figure 1 shows schematically viscous fingering, which occurs because the injection fluid is less viscous, and gravity override, which occurs because the injection fluid is less dense. Chapter 2 of this book, by Chang and Slattery, describes a linear instability analysis for gas flooding and also introduces a nonlinear treatment of the flow instabilities. 

The mobility-control surfactant increased the apparent viscosity of CO2 sufficiently to prevent gravity override and viscous fingering. The bulk CO2 phase passed through the core in a piston-like manner. Oil and most of the brine were displaced from the core ahead of the bulk-phase CO2. Differential pressure measurement across the length of the core indicated an average gradient, 1.3 psi/ft, similar to that observed during the brine flood. 

As a result of an increased flow resistance, foam injection is often accompanied by an increase in the injection-well bottomhole pressure (BHP). Temperature surveys at injection, production, and observation wells indicate whether foam is successfully diverting steam. By minimizing gravity override, a successful foam application increases the temperature... 

Even in thin reservoirs, foam can be effective by reducing gravity override. In these reservoirs the growth of the foam zone is spherical. 

Allows operator changes of selected parameters(i.e. orifice size, run sizes, default value for BTU and Specific gravity, override setpoints, etc) through the console unit. 

The miscible process requirements are characterized by a low-viscosity crade oil and a thin reservoir. A low-viscosity oil will usually corrtain enough of the intermediate-range components for the multicontact miscible process to be established. The requiremerrt of a thin reservoir reduces the possibility that gravity override will occitr and yields a more even sweep efficiency. 

A large percentage (80%) of Canadian enhanced oil production comes from hydrocarbon miscible flooding [1], The low density and viscosity of the injected fluids cause hydrocarbon miscible EOR processes to suffer from poor sweep efficiency, due to viscous fingering and gravity override. Mobility control foams provide a means for improving the sweep efficiency and could significantly increase oil production from Canadian reservoirs. 

CO2 and some reservoir oils are nearly equal, thus minimizing CO2 gravity override. 

Foams have been tested as diverting agents in waterfloods, 39 as steam-diverting agents to reduce gravity override or steam channeling in steamfloods, and as mobility-control fluids in CO2 displacements. 4 ... 

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