Extended abstract of a paper presented at Microscopy and Microanalysis 2010 in Portland, Oregon, USA, August 1 – August 5, 2010.

A series of laboratory tests was conducted to investigate the in-situ steamdrive process and its effects on fluid displacement in porous media. The experiments show that in-situ hot-water flash may recover up to 60% of moveable oil and that the residual oil saturation after flash increases with lower initial water saturations. Conventional simulation with external-drive relative permeabilities led to under-prediction of oil recovery. IntroductionCyclic steaming through hydraulic or natural fractures is often used for oil recovery from low-permeability reservoirs (e.g., diatomite). The efficiency of the process is controlled by fluid and heat interactions between fracture and matrix. During injection and soak, steam condenses in the fracture and hot water imbibes into the matrix. 1 During production, water pressure in the hot matrix often drops to less than the vapor pressure, causing in-situ boiling. In-situ boiling of hot water also occurs in "flash-driven" steamfloods in nonfractured reservoirs. 2 The displacement process associated with waterto-steam phase change in porous media is called "in-situ steamdrive." The in-situ process is different than the external steamdrive process in which the displacement occurs by steam injection.The physics of water vaporization in porous media has been the subject of a number of papers for application in geothermal reservoirs, nuclear waste disposal and thermal oil recovery. Thermodynamic properties of vapor and liquid in porous media are different than "flat-surface" conditions. The degree of superheat and vaporpressure lowering is greater for smaller pore radii. 3 The water retained by surface adsorption when the pore is filling (for pore radii in the range of 2-35Å) and by capillary condensation (up to 120Å) is known to be the main reason for vapor-pressure lowering in porous media. 4-8 Recent work on adsorption/desorption of water with network models shows that access of the vapor phase to large pores is often blocked by small pores during the desorption process. 9 Comparison of water-vapor desorption and conventional capillary pressure measurements at low-wetting-phase saturation shows that adsorbed films can be a significant part of the liquid saturation. [10][11] Currently, cyclic-steam performance in low-permeability, heavyoil reservoirs is modeled with conventional thermal compositional simulators with flat-surface properties. Relative permeabilities are commonly measured by methods where gas is injected "externally." This may result in poor prediction if the relative permeabilities representing the flow process are different from those in conventional steamdrive. Subsequent errors in recovery prediction of cyclicsteam processes in fractured low-permeability reservoirs have not been quantified in the literature before.Because the source of water vapor in the in-situ steamdrive process is the formation water, the residual oil saturation after the flash process may be affected by the initial water saturation, its distribution, and the rate of pressure decline. Unlike ...