An Evaluation of Gas Diffusivity Measurement in Reservoir Fluid from Low to High Pressure Systems for Oil Recovery Applications
Authors: Zainal Suzalina , Hon Vai Yee , Saaid Ismail Mohd. , Jelani Jamil 
Published: May 2016

Abstract
Gas diffusivity is a subject of interest in oil recovery projects. Its study is imperative in understanding the transport properties of two different species; gas and liquid. It enables the analysis of the rate of transport caused by concentration gradient. Considering that different gas and liquid systems have different diffusivities, there is a need to determine its value to be able to incorporate the effect of mass transfer in the design and planning of any gas injection and enhanced oil recovery projects; for instance, miscible gas flooding and non-thermal recovery of heavy oil by solvent injection. Diffusivity also assists in understanding methane hydrates formation and phase distribution process in a shut-in well. In reservoir engineering application specifically, diffusivity may be important when modeling gas displacement process at extremely low flow rate or in very small scale such as a micro-model.
Even though there are a number of reported diffusivity experiments conducted around the world, limited information is available under elevated reservoir conditions. In the Malay Basin specifically, there is no evidence of diffusion coefficient experiments conducted at reservoir conditions for the petroleum industry applications. This might be due to the difficulty in measuring diffusivity, especially in multicomponent liquid systems. Even so, experiments are necessary as there is no universal theory to calculate diffusion coefficients from other known properties of the system. Each binary system is unique from one condition to another.
This paper presents a review of reported experimental works on gas diffusivity. It compares different diffusivity measurement for different areas of oil recovery projects. This includes identification of laboratory apparatus and various experimental approaches and mathematical analyses of reported results. It then highlights the effects of pressure, operating temperature and binary system selection in validating the correct diffusivity test set-up before progressing to actual test involving multicomponent fluid system.

Introduction
Studies on gas diffusivities have contributed significantly in numerous petroleum engineering applications. The diffusion coefficient measurement is important as it is one of principle properties in the calculation of mass transfer rates. Interest in the measurement of diffusion coefficient dated as early as 1934. Hill and Lacey (1934) studied each methane and propane in quiescent liquid hydrocarbon systems at pressure and temperature range of 29 psia to 300 psia and 30oC to 33oC respectively. Following this study, more analysis was performed involving hydrocarbon and petroleum systems at much higher pressures and temperatures. However, these investigations were still limited to pressures much lower than the actual reservoir pressure due to equipment limitation. Moreover, there was some degree of difficulty in measuring diffusivity at high pressures for liquid system where the reported values have low accuracies (Riazi and Whitson, 1993).