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Experimental Investigation on the Effect of Wettability on Rock-Electricity Response in Sandstone Reservoirs

Abstract

Experimental investigation on the effect of wettability on rock-electricity response in sandstone reservoirs

Abstract: Wettability plays an important role in petroleum exploration and oil recovery. In recent years, the preferentially oil-wet reservoirs encountered are increasing with the exploration of unconventional reservoirs and near-source reservoirs, which strongly restricts the accuracy of saturation calculation and oil layers identification. Therefore, it is imperative to clarify the impact mechanism of wettability on the rock-electricity relationship. To fundamentally study the effect of wettability on sandstone conductivity, a series of experiments, including rock-electricity, Nuclear magnetic resonance (NMR) and wettability tests, were conducted systematically on high- and low-permeability sandstone samples. The results show that the change of wettability is one of the most important factors that influence the electrical response of rocks. Compared with the water-wet samples, the resistivity of the oil-wet rocks increased obviously, and it exhibited a significantly nonlinear variation with the decrease of oil saturation. Moreover, wettability had different influence on the conductivity laws of high- and low-permeability sandstones, which was mainly caused by the complex combination of pore and pore-throat. The conductivity laws of oil-wet high-permeability sandstones, which mainly developed large pores and coarse throats with small water-wet bound space, were mainly followed the oil-wet conduction law. However, the preferentially oil-wet low-permeability sandstones primarily developed middle-to-small pores and micro throats with large water-wet bound space. Hence a large amount of bound space followed the water-wet conduction law; while the relatively large pore, which is oil-wet, was dominated by oil-wet conduction law. Consequently, the conductivity of oil-wet sandstones with low-permeability was co-controlled by the wettability and pore structure. The study can result in a quite different saturation under the same resistivity compared with Archie’s law, which is meaningful for

establishing oil-saturation calculation model, guiding oil-reservoir identification and enhancing recovery in oil-wet reservoirs.

Keywords: Wettability; Rock-electricity relationships; Pore structure; Nuclear magnetic resonance; Sandstone reservoir.