Empirical measures of wettability in porous media and the relationship between them derived from pore-scale modelling

The wettability of a crude oil/brine/rock system is of central importance i n determining the oil recovery efficiency of water displacement processes i n oil reservoirs. Wettability of a rock sample has traditionally been measu red using one of two experimental techniques, viz. the United States Bureau of Mines and Amott tests. The former gives the USBM index, I-USBM, and the latter yields the Amott-Harvey index, I-AH. As there is no well-establishe d theoretical basis for either test, any relationship between the two indic es remains unclear. Analytical relationships between I-AH and I-USBM for mixed-wet and fraction ally-wet media have been based on a number of simplifying assumptions relat ing to the underlying pore-scale displacement mechanisms. This simple appro ach provides some guidelines regarding the influence of the distribution of oil-wet surfaces within the porous medium on I-AH and I-USBM. More detaile d insight into the relationship between I-AH and I-USBM is provided by mode lling the pore-scale displacement processes in a network of interconnected pores. The effects of pore size distribution, interconnectivity, displaceme nt mechanisms, distribution of volume and of oil-wet pores within the pore space have all been investigated by means of the network model. The results of these analytical calculations and network simulations show t hat I-AH and I-USBM need not be identical. Moreover, the calculated indices and the relationship between them suggest explanations for some of the tre nds that appear in experimental data when both I-USBM and I-AH have been re ported in the literature for tests with comparable fluids and solids. Such calculations should help with the design of more informative wettability te sts in the future.