Molecular Adsorption And Flow Molecular Simulation Of Dense Carbonate Reservoirs

Tight oil reservoirs are low-permeability and low-porosity reservoirs. Sizes of rock pores in tight oil reservoirs are in nano-scale or micro-size scale.Many problems are not solved during the exploitation and production of tight oil reservoirs, for example:the production efficiency is low by injection of water/gases, oil production cost of the tight oil reservoirs is huge and oil production rate is slow, etc. Accurate measurement the flux of crude oil in rock samples is difficult and too much time is spent on the core test in the laboratory experiments. In order to grasp the distribution and motion regular of molecules in micro-size scale rock pores in tight oil reservoirs and the flow mechanism of fluid in the reservoirs from micro-scale simulation, we have performed the analysis of effects of flux of fluid. The work is supported by grants from the National Natural Science Foundation of project:"Considering the influence of capillary pressure and adsorption, CO2 and crude oil non-equilibrium diffusion theory and molecular dynamics study and transverse project:"Molecular dynamics(MD) simulation of formation of threshold pressure gradient and its influencing factors study".Based on the petro-physical property of tight oil reservoirs in Sichuan, we have constructed a series of nano-scale carbonate pores. The composition of the frames are calcite, montmorillonite and chlorite. The range of pore size is 2nm to 50nm. The shape of these pores are crack, tube, pore-throat and bending tube. A total of 14 pores were constructed. Alkanes adopted in the context include methane, ethane, n-hexane and n-pentadecane. Monte Carlo and molecular dynamics simulation of adsorption and flow of alkanes in crude oil are performed in the article.26 groups of flow simulation and 12 groups of adsorption simulation were carried out based on the combination of the above mentioned fluid and pore structure.The adsorption amount of alkanes at different pressures, density distribution of alkanes in these pores, velocity distribution of alkanes and curves of alkane flux and pressure gradient are obtained during the simulation. Analysis of effects of the composition of pore frames, shape of pores and pressure gradient on alkane flux and effects of the composition of pore frames on alkane adsorption amount is investigated.The research shows that peak point of alkane molecular density distribution appears near inner surface of the pore and boundary layer of adsorption phase of alkane appears near the wall. The boundary layer makes the flow velocity of alkanes in the vicinity of the pore wall lower than the speed in the middle area. In the micro-nano pores of tight carbonate reservoir, chlorite in the clay minerals lowers flux of alkanes. The flow of alkane molecules increases linearly with the increase of the external driving force.With the increase of molecular chain length and molecular mass, the flow rate of alkane decreases. The obtained rules may provide some theory guidance to the exploitation and production of tight oil reservoirs.