Water film thickness of tight reservoir in Fuyu oil layer of Cretaceous Quantou Formation in Songliao Basin and its influence on the lower limit of seepage

The water film thickness in tight sandstone reservoir is critical to determine the lower limit of both flow pore-throat radius and the physical property of the reservoir. In order to study the effect of water film thickness on fluid flow capacity, firstly, through molecular dynamics simulations, the adsorption behavior of different minerals on water molecules was simulated, and the adsorption characteristics of different minerals on water molecules were clarified. Secondly, a new method for calculating the water film thickness of bound water was established by comprehensively using three experimental methods: high-pressure mercury injection experiment (HPMI), cryogenic nitrogen adsorption experiment, and centrifugal-nuclear magnetic resonance (NMR). The lower limit of the flow pore-throat radius and the physical property of the tight reservoir were determined more accurately by improving the previous calculation methods. The results demonstrated that the adsorption of minerals to water molecules mainly formed a binding force through microscopic forces such as electrostatic force and Van Der Waals Force, in which Van Der Waals Force repels water molecules while electrostatic force absorbs water molecules. Different minerals have different adsorption capacities for water molecules. According to the established formula, the bound water film thickness of different samples is different. Then, the water film thickness of the Fuyu oil layer in the Songliao Basin can be calculated to be approximately 19.651 nm. By improving the formula used by the predecessors to determine the lower limit of physical property, it could be accurately determined that the lower limit of flow pore-throat radius was 26.751 nm, and the lower limit of permeability and porosity was 0.047mD and 9.87%, respectively. This method has both a strong theoretical basis and experimental data support. Thus, it can be generalized to a certain extent.