Graphical Determination of the Henry's Constant and the Diffusion Coefficient of Gases in Heavy Oils Using Late-Time Pressure-Decay Data

Abstract

Solubility and diffusivity of gases in heavy oil, respectively quantified by Henry's constant (H ij ) and diffusion coefficient (D), are essential properties for the design of recovery processes that require the injection of gas or vapour solvents into the reservoir. Data, obtained from various experimental procedures such as the pressure-decay technique (PDT), are used to estimate these two parameters. The PDT uses a Pressure/Volume/Temperature (PVT) cell where the gas phase pressure declines as gas diffuses into heavy oil following an early-time and a late-time regime. Current approaches to analyze data from the conventional PDT are either graphical techniques based on early-time data or full numerical simulation. Early time data, the period in which the diffusing gas has not reached the bottom of the PVT cell, do not provide enough information to simultaneously estimate diffusion coefficient and Henry's constant. Hence, existing graphical procedures are limited to diffusion coefficient estimation.In this paper, a novel and simple graphical technique is proposed to estimate the diffusion coefficient and Henry's constant using the late-time data from pressure-decay experiments. The proposed method is based on modeling of gas phase pressure decay using Fick's second law and gas phase mass balance equations. The Integral method is used to provide an approximate, but analytical solution to the set of equations. The resultant solution is used to develop a simple graphical method, i.e. inverse problem, in which both diffusion coefficient and Henry's constant are directly estimated. The estimated parameters through the proposed technique for methane/bitumen and carbon dioxide/bitumen experiments are in close agreement with the values reported in the literature.