Determination of Mass Transfer Coefficient of Methane in Heavy Oil-Saturated Unconsolidated Porous Media Using Constant-Pressure Technique

Abstract

Measuring the solubility and diffusivity of methane in porous media saturated with oil is critical for analyzing the process of solvent-based recovery of heavy oil reservoir. In this study, a modified pressure-pulse decay (PPD) method is presented to measure the mass transfer coefficients of methane in porous media saturated with heavy oil as well as the solubility and diffusivity of methane in bulk oil under the same experimental conditions as those used in a comparison. The experiments are conducted under constant boundary pressure in a PVT cell, and the pressure is controlled using an auto pump, which continuously compresses the volume of an intermediate container. The accumulated volume change instead of the pressure change is recorded with time. The nonequilibrium boundary condition (BC) model is used to analyze the experimental process. The general solution of the mathematical model is derived using the Laplace transform and the approximate analytical solution of the accumulated dissolved gas is further presented for post processing of the experimental data. The mass transfer coefficients are estimated through using a multilevel single-linkage (MLSL) method to match the approximate solution with the recorded experimental data. The estimation of the parameters shows that the mass transfer coefficients are pressure dependent and that a high boundary pressure contributes to the mass transfer of methane in heavy oil due to a reduction in viscosity or density. The interface mass transfer coefficient (k int ) and interface saturated concentration (c int ) in porous media saturated with oil and bulk oil are almost the same under the same experimental conditions. The sensitivity analysis shows that the increase of the effective diffusion coefficient (D eff ) and k int contributes to enhancing the rate of mass transfer in the oil phase and that Henry's law constant (H) has no effect on the equilibrium time but only affects the initial saturated concentration at the interface or the total dissolved gas. The modified PPD method is robust, efficient, and easy to use in the laboratory.

An analytical method of estimating diffusion coefficients of gases in liquids from pressure decay tests

“…It is also assumed that the relationship between concentration at the gas–liquid interface and gas pressure can be described by the Henry's law. This has been confirmed in previous for CO 2 /Heavy oil and CH 4 /Heavy oil systems at pressures up to 10 MPa . The fourth assumption is that gas compressibility ( Z ) is considered negligible during the pressure decay test and a constant Z at average pressure is used.…”

“…The equilibrium model considers a constant pressure at the gas–liquid interface, which is equal to the final equilibrium pressure of the system. However, in a typical pressure decay test, the boundary pressure varies with time unless the dissolved gas in liquid is balanced with the supply gas . We have shown earlier that the equilibrium model is a special case of the quasi‐equilibrium model with large values of λ.…”

“…When the gas (CO 2 /CH 4 ) dissolves into bitumen, the density of which will decrease and the density at the bottom of the cell is always greater than that near the interface . For the same reason, it is a common practice to assume that there is no density induced convection taking place in CO 2 /Heavy oil or CH 4 /Heavy oil system . Meanwhile, even the error of convection is considered in CO 2 /Heavy oil or CH 4 /Heavy oil system, previous work has shown that it is less than 0.5% .…”

“…Assumptions are generally required to simplify modeling of diffusion of a gas into a liquid during the pressure decay test. We follow previous studies to simplify the mathematical model and analysis of the test results . We assume that the temperature remains constant during the test, the liquid is nonvolatile and diffusion coefficient of gas in liquid is constant.…”

“…Therefore, the indirect methods are preferable in engineering applications. The indirect methods are composed of the pressure‐decay method, the electromagnetic method, the PH method, density method, the swelling of a pendant droplet method, and visualized dye method, among others. The indirect physical data, such as pressure change and PH distribution, are used to measure the diffusion coefficient of the liquid/gas mixture.…”