low of Dilute Oil-in-Water Emulsions in Porous Media

by
Zuleyka Del Carmen Mendez, Ph.D.

University of Texas at Austin, 1999
Supervisor: Mukul M. Sharma

The flow of dilute oil-in-water emulsions is of critical concern in produced water reinjection. Oil droplets and solids suspended in produced water are often hard to remove and are, therefore, reinjected into subsurface formations. A rapid injectivity decline in such water injection wells is commonly encountered. Eventually, these wells may have to be operated above the fracture gradient. The flow of emulsions in porous media determines the performance and lifetime of such water injection wells. Flow of dilute oil-in-water emulsions in porous media may also be encountered during enhanced oil recovery and stimulation operations.

This dissertation is aimed at investigating the mechanisms of permeability impairment caused by flow of dilute oil-in-water emulsions in cores containing residual oil. The study has both experimental and modeling components.

The experimental program consisted of injecting well-characterized oil-in-water emulsions into cores containing residual oil saturation. The permeability of different sections of the core as well as the droplet concentration and size distribution were measured as a function of time and position. Two crude oils, one from Prudhoe Bay and another from the North Sea (Brent crude oil) were used in core tests. Berea sandstone and Aloxite cores were used as porous media.

Experimental results indicated that the presence of residual oil had a profound effect on the measured permeability decline. Droplets were generated from the residual oil present a pore throats after a critical capillary number is exceeded. It was found that high injection rates and low permeabilities enhanced droplet formation. The generation of droplets is a primary contributing factor to the permeability reduction observed in different sections in the core. The permeability of the core, the droplet concentrations, the concentration of emulsifier present, the flow rate, and the properties of the crude oil all play important roles in determining the extent and rate of permeability impairment. High pressure gradient, high flow rate, low permeabilities, high oil concentrations and large droplet sizes contribute to a more rapid decline in permeability. It was observed that the permeability decline occurs in two stages, one associated with the injected droplets followed by a second stage during which generation of droplets plays an important role. After the onset of droplet generation, permeability decline is faster and more severe. This stage is evidenced by a high droplet concentration, in excess of the injected droplet concentration.

Based on experimental data, a simplified mathematical model has been formulated to compute the permeability reduction and the droplet concentration in core flow tests. The model accounts for droplet trapping and generation phenomena. An empirical droplet-generation model is proposed based on the experimental observations. Reasonably good agreement is obtained between the model and the experimental results. The trends with flow rate and droplet concentration that are observed experimentally are also obtained from the model. The rates of droplet generation and capture need to be measured experimentally since they are a sensitive function of composition of the crude oil, brine and of the pore structure. Once these empirically determined constants are specified, the model can predict the permeability decline as well as the droplet concentration profile in a porous medium.

The study presented in this dissertation is the first systematic investigation of the flow of dilute oil-in-water emulsions in a porous medium with a residual oil saturation present. The results presented and their interpretation will provide new insights into the mechanisms responsible for injectivity decline in produced-water injection wells. The model proposed will form the basis of injectivity decline simulators for such water injection wells.

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