Micro formation mechanism of oil–water inversion in reservoirs based on water film theory—An example from the Dawangzhuang Oilfield in Bohai Bay Basin

Abstract

In oil fields, the inverted oil–water phenomenon of “water above oil”, which cannot be explained by the traditional principle of gravity differentiation. In hydrophilic reservoirs, water films easily form on rock particle surface. Water films reduce the effective pore throat radius and the space between pores and throats, generate disjoining pressure, and significantly reduces the seepage capacity of pore fluids, which leading to the phenomenon of oil water inversion. Based on laboratory experiments, a model of the relationship between the disjoining pressure -water film thickness was established, and the dynamic mechanism of oil–water inversion was studied using water-film theory. The main factor controlling oil–water inversion is the interaction between the dynamic (buoyancy) and resistance (capillary force and disjoining pressure) forces of oil–water migration and the configuration relationship with the average pore throat radius. The effects of the water film thickness and pore throat radius on the disjoining pressure and capillary resistance were quantitatively analysed, and the critical pore radius model for oil–water inversion was determined. Combining the capillary pressure curve and pore throat distribution, a practical and feasible theoretical technology and method for judging whether oil–water inversion occurs in a reservoir was established. 22 sets of experiments were conducted to verify the reliability and sensitivity of the disjoining pressure - water film thickness model. Taking the Dawangzhuang Oilfield, Raoyang Sag, Bohai Bay Basin, China as an example, through laboratory experiments, specific strata and reservoir data were obtained in the study area, and the disjoining pressure, capillary resistance, and buoyancy were calculated using the disjoining pressure - water film thickness model. By comparing the relationship between the buoyancy and the sum of the equilibrium disjoining pressure and capillary resistance, combined with the distribution of the pore throat radius, it is determined whether there is oil water inversion phenomenon. The practical application based on the theory and method of water film thickness has strong promotion potential for medium–low porosity and permeability reservoirs with the phenomenon of oil water inversion distribution of “water above oil ” whcih can effectively guide oil fields to further select favorable exploration and development targets.