Investigation of scale-dependent dispersivity and its impact on upscaling misicble displacements

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Title: Investigation of scale-dependent dispersivity and its impact on upscaling misicble displacements
Author: Garmeh, Gholamreza
Abstract: Mixing of miscible gas with oil in a reservoir decreases the effective strength of the gas , which can adversely affect miscibility and recovery efficiency . The mixing that occurs in a reservoir , however , is widely debated and often ignored in reservoir simulation , where very large grid blocks are used . Large grid blocks create artificially large mixing that can cause errors in predicted oil recovery . Reservoir mixing , or dispersion , is caused by diffusion of particles across streamlines of varying velocities . Mixing is enhanced by any mechanism that increases the area of contact between the gas and the oil , thereby allowing the effects of diffusion to be magnified . This is , in essence , the cause of scale -dependent dispersion . The contact area grows primarily because of variations in streamlines and their velocities around grains and through layers of various permeabilities (heterogeneity ) . Mixing can also be enhanced by crossflow , such as that caused by gravity and by the effects of other neighboring wells . This dissertation focuses on estimation of the level of effective local mixing at the field scale and its impact on oil recovery from miscible gas floods . Pore -level simulation was performed using the Navier -Stokes and convection -diffusion equations to examine the origin of scale dependent dispersion . We then estimated dispersivity at the macro scale as a function of key scaling groups in heterogeneous reservoirs . Lastly , we upscaled grid blocks to match the level of mixing at the pattern scale . Once the contact area ceases to grow with distance traveled , dispersion has reached its asymptotic limit . This generally occurs when the fluids are well mixed in transverse direction . We investigated a variety of pore -scale models to understand the nature of scale dependency . From the pore -scale study , we found that reservoir mixing or dispersion is caused by diffusion of particles across streamlines . Diffusion can be significantly enhanced if the surface area of contact between the reservoir and injected fluid are increased as fluids propagate through the reservoir . Echo and transmission dispersivities are scale dependent . They may or may not reach an asymptotic limit depending on the scale of heterogeneities encountered . The scale dependence results from an increase in the contact area between solute (gas ) and resident fluid (oil ) as heterogeneities are encountered , either at the pore or pattern -scale . The key scaling groups for first -contact miscible (FCM ) flow are derived and their impact on mixing is analyzed . We examine only local mixing , not apparent mixing caused by variations in streamline path lengths (convective spreading ) . Local mixing is important because it affects the strength of the injected fluid , and can cause an otherwise multicontact miscible (MCM ) flood to become immiscible . We then showed how to upscale miscible floods considering reservoir mixing . The sum of numerical dispersion and physical dispersion associated with the reservoir heterogeneities , geometry and fluid properties must be equal at both the fine - and large -scales . The maximum grid -block size allowed in both the x - and z -directions is determined from the scaling groups . Small grid -blocks must be used for reservoirs with uncorrelated permeabilities , while larger grid blocks can be used for more layered reservoirs . The predicted level of mixing for first -contact miscible floods can be extended with good accuracy to multicontact miscible (MCM ) gas floods .
URI: http : / /hdl .handle .net /2152 /ETD -UT -2010 -05 -738
Date: 2010-09-03

Citation

Investigation of scale-dependent dispersivity and its impact on upscaling misicble displacements. Doctoral dissertation, The University of Texas at Austin. Available electronically from http : / /hdl .handle .net /2152 /ETD -UT -2010 -05 -738 .

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