Gas-Diffusivity Measurement in Reservoir Fluid at Elevated Pressures Systems for Transient Shut-In Modeling

Gas-liquid diffusivity is investigated for many different oil recovery applications. Most interest is in the gas-oil diffusivity for enhanced oil recovery applications such as miscible gas flooding, immiscible carbon dioxide WAG (Water-Alternating-Gas) flooding and non-thermal heavy oil recovery by solvent injection. Other study includes the analysis of gas-water diffusivity to understand the methane hydrates formation. In designing an enhanced oil recovery process, for instance, further information on the rates and amount of mass transfer by diffusion is required. The focus is to have better insight of the mass transfer mechanisms, particularly at the reservoir level.

A more downstream application of gas-liquid diffusivity measurement is its application in transient modeling. The interest is at the production tubing, primarily in describing the phase redistribution process in a shut-in well. Gas-liquid diffusivity has been suggested to account for the molecular interactions and distribution between the gas and oil phase in a producing well during transient flow. Mass transfer being the transport phenomena, causes the molecules in the fluids to rearrange to attain an equilibrium system. This process of rearrangement is transient and driven primarily by the concentration gradient in the gas and oil system. The diffusion coefficient parameter which is a function of mass transfer introduces a time factor to the overall phase segregation process. Higher diffusion rate is anticipated to contribute to faster phase redistribution and contrary if vice versa. In transient modeling for shut-in condition, this information is useful to enable accurate simulation of fluid redistribution period prior reaching equilibrium.

This paper presents the gas-liquid diffusivity measurement and analysis involving Malay Basin’s hydrocarbon fluid samples and its applications to the transient shut-in modeling. It also highlights on the reliability of a published method designed for a heavy oil system when applied to Malaysian light to medium crude oil systems.