Magnitude and Detailed Structure of Residual Oil Saturation

Experimental results are presented that demonstrate the effect on residual oil, under water-wet conditions, of particle size, particle-size distribution, macroscopic particle size, particle-size distribution, macroscopic and microscopic heterogeneities, microscopic dimensions such as ratio of pore-body to pore-throat size, and pore-to-pore coordination number. Experiments were pore-to-pore coordination number. Experiments were performed in random packs of equal spheres, heterogeneous performed in random packs of equal spheres, heterogeneous packs of spheres with microscopic and macroscopic packs of spheres with microscopic and macroscopic heterogeneities, two-dimensional (2D) capillary networks having various pore geometries, and Berea sandstone. Detailed information on residual oil structure is presented, including blob-size distributions of residual presented, including blob-size distributions of residual oil. Major conclusions areresidual saturations are independent of absolute pore size, per se, in systems of similar pore geometry;well-mixed two-component aggregates of spheres gave virtually the same residual saturations as random packings of equal spheres;clusters of large pores accessible through small pores will retain oil;high aspect ratios tend to cause entrapment of oil as a large number of relatively small blobs, each held in single pores; andthe role of pore-to-pore coordination number is generally secondary; pore-to-pore coordination number is generally secondary; hence, correlations that have been proposed between residual oil and coordination number are unreliable. Introduction In recent years, there has been increased interest in the factors that determine the magnitude of residual oil and its microscopic distribution. Residual oil remaining in the swept zone of a waterflood is often taken as the target oil for enhanced recovery processes. Oil saturations remaining in these zones typically can occupy 15 to 35% of the pore space, but values outside this range are often measured. For the reservoir, it can be expected that the pore structure, the initial water content, and the superimposed effects of wettability determine recovery behavior and residual oil distribution under normal waterflood conditions. Salathiel has presented examples of the manner in which pore geometry, wettability, and volume throughput of floodwater can interact to affect oil recovery characteristics and final oil saturation. The likely complexity of trapping phenomena is indicated by the work of Wardlaw and Cassan, who investigated possible correlations between residual oil and 27 petrophysical parameters. Rocks with similar macroscopic properties often differed markedly in their residual oil saturations, and no significant correlation was observed between displacement efficiency and permeability. A tendency for residual nonwetting-phase permeability. A tendency for residual nonwetting-phase saturations to increase as porosity decreased was noted. This was related to a strong relationship between trapping and aspect ratio (ratio of pore-body to pore-throat size). A theory of residual oil trapping has been proposed by Larson et al. that provides an alternative explanation of the relationship between residual oil and porosity. It was reasoned that the trapped nonwetting-phase saturation will correspond reasonably well to the percolation threshold i.e., to the oil saturation at which oil continuity through the pore space is lost. SPEJ p. 311