Interpretation Of Wettability In Sandstones With Nmr Analysis

Abstract

Abstract: A systematic interpretation of wettability alteration by NMR analysis was carried out on three types of sandstones with varying shalyness. Two fluid systems were investigated. Soltrol 130 was used as the refined oil. A deep water Gulf of Mexico crude oil was used as the crude oil that is known to alter the wettability in restored state core analysis. Fluids that are in molecular contact with mineral surfaces have a relaxation time that is less than the bulk fluid relaxation time due to surface relaxation phenomena. Mixed wettability can result in lengthened NMR relaxation of brine compared to 100% brine saturation and shortened NMR relaxation of oil compared to bulk oil when oil becomes in contact with part of the rock surfaces. Thus, NMR T1 relaxation time distribution provides qualitative description of wettability alteration. Bentheim and Berea were water-wet with refined oil. When saturated with crude oil and brine at Swir they became mixed-wet after aging. After forced imbibition, apparently patches of crude oil remained on the rock surfaces because the residual oil had a shorter relaxation time compared to the bulk oil. North Burbank sandstone was mixed-wet when either saturated with refined oil at Swir or crude oil after aging. Micropores formed by chlorite flakes were filled with brine. Oil in the macropores was in contact with the tips of chlorite flakes. After forced imbibition, brine in the macropores was partially shielded from the pore walls by a film of oil spanning the tips of chlorite clays. The evidence for this was that macro-brine relaxed slower than 100% brine saturation but faster than bulk brine and oil relaxed faster than bulk oil. Thus North Burbank appeared mixed-wet with either refined oil or with crude oil. However, the extent of wetting change was greater with crude oil. This special wetting behavior of North Burbank came from its microporous structure formed by pore-lining chlorite flakes. The spontaneous imbibition and forced imbibition wettability test proposed by Amott provides a measurement of wettability. North Burbank sandstone had little spontaneous imbibition with either refined oil or crude oil. Bentheim and Berea sandstones had spontaneous imbibition with refined oil while only Berea had small amount of spontaneous imbibition with crude oil. Residual oil saturation estimate provides another quantitative indication of wettability alteration. For both refined oil and crude oil systems, North Burbank has significantly less residual oil saturation than that estimated from counter current imbibition. This further confirms that North Burbank was mixed-wet with Soltrol and this crude oil. Berea has the highest residual oil saturation and exhibits about 10 percent reduction of Sor from refined oil to crude oil.