Understanding the rate of clean up for oil zones after a gel treatment

In our previous work, X-ray computed microtomography (XMT) was used to establish why pore-filling Cr(III)-acetate-HPAM gels reduced permeability to water much more than to oil. Our results suggest that permeability to water was reduced to low values because water must flow through gel itself, whereas oil pressing on the gel in a porous medium forced pathways by dehydration - leading to relatively high permeability to oil. Those studies involved obtaining 3D pore-level X-ray images at the saturation endpoints - for example, after forcing 20 pore volumes of oil or water through the core following gel placement. The dependence of oil permeability on oil throughput determines how long it takes for a production well to "clean up" or restore productivity after a gel treatment. Consequently, we were interested in how the gel dehydration process progresses as a function of oil throughput. This paper describes a new study where pore-scale XMT images were obtained at a variety of oil (hexadecane) throughput values after gel placement (involving a pore-filling Cr(III)-acetate-HPAM gel). For each pore in our image volume, we followed oil and water saturations as a function of oil throughput. These studies were performed both in water-wet Berea sandstone and in hydrophobic porous polyethylene. In hydrophobic porous polyethylene, oil saturations increased and gel was destroyed (presumably dehydrated) quite quickly in the smallest pores (10^-6 mm³). Also, oil saturations increased and gel was destroyed quickly in the largest pores (<0.005 mm³). In contrast, oil saturations rose much more gradually for the most common or intermediate-sized pores (around 10^-4 mm³, the peak in the pore size distribution). The minimum in oil saturation versus pore size may result from a balance between gel dehydration by oil film growth versus gel extrusion. Presumably, during oil injection after gel placement, an oil film forms with a thickness that is about the same on all polyethylene surfaces. However, because the ratio of film thickness to pore or throat size increases with decreased pore size, gel dehydration occurs faster and more effectively in the smallest polyethylene pores, causing oil saturation to increase with decreased pore size. This effect may lose its signicance for pore sizes above 10^-4 mm³. As pore size increases above 10^-4 mm³, gel extrusion from the pore becomes more likely - explaining why oil saturation increased with increased pore size for the largest pores. In contrast in water-wet Berea sandstone, increases in oil saturation occurred evenly over all pore sizes (10^-6 to 0.02 mm³) for all oil throughput values. Consistent with imbibition and drainage studies performed before gel placement, oil apparently had equal access to Berea pores of all sizes, and thus uniformly dehydrated gel in pores of all sizes. Gel extrusion did not appear to be significant in the Berea pores.