Introduction

During miscible flooding of oil reservoirs by light hydrocarbon gases, deposition of asphaltenes inside reservoirs, and injection and production wells, can cause severe problems thereby adversely affecting the economics of oil recovery. In spite of this, very limited information regarding the asphaltene adsorption/deposition within porous media during gas miscible flooding, is available.

In the present work, the flow of oil-solvent mixtures through porous media during a gas miscible flood was mechanistically studied in the laboratory under simulated reservoir conditions. The tests mimic the phenomena occurring within a small pore volume encroached by increasing amounts of solvent, in the course of a miscible flood.

Experimental

Two Canadian crude oils, known to be prone to asphaltene deposition in the field, were selected for this study: one from the Pembina Nisku pool in west-central Alberta and the other from Rainbow Keg River field in north-western Alberta.

Both these oils have an asphaltene content of about 2%.

For studying asphaltene related effects inside porous media during miscible flooding, a high pressure coreflood equipment was constructed (Figure 1). Berea cores were used as porous media during this investigation. The lengths of the cores were over 20 cm and the diameters, 1.9 cm. The porosity was 16-17%. The effective permeability to oil was in the range of 36 to 116 mD; these values were measured at a residual water saturation of 27–28% and were used as reference values for the subsequent flow of oil-solvent mixtures. Reservoir brine was used to initially saturate the core; a subsequent oil flood reduced the brine saturation to the residual brine saturation.

Results

Variations in effective permeability ratio (Kef/Kefo) upon flow of Rainbow Keg River oil-propane mixture are presented in Figure 2; here Kefo is the original value of effective permeability (for oil), while Kef is the current value for a given propane concentration in the mixture. All the concentrations are expressed in vol. %. The reduction of Kef begins as soon as propane is injected, that is, at a concentration of 15% propane, and it continues until propane concentration, reaches 100%. As a matter of fact, when injecting pure propane, the Kef is reduced to almost zero, implying a total blockage of the porous medium. There are two rates of permeability decrease with increasing propane concentrations, corresponding to two distinct ranges of propane concentrations. The first range is up to 51% for which the decrease is very steep. For propane concentration higher than 51%, the decrease is less severe. At the end of the first range, the Kef had decreased to 26% of its original value.

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