Fundamentals of Fluid Flow in Porous Media

Chapter 3

Diffusion Coefficient: Importance in Petroleum Engineering

In 2004, Alberta’s Oil Sands were recognized by international media, for the first time, as part of global oil reserves. This established Canada as second only to Saudi Arabia in the hierarchy of potential oil producing nations. While oil sands extraction is more expensive than conventional sources, continuing technological advances are reducing the importance of those cost differences. Moreover, conventional oil production in Canada is declining, underscoring the importance of the oil sands as a vital source of North American supplies (Timilsina et al., 2005).

To recover these resources steam injection is widely used for heavy oil and bitumen reservoirs. The advantage of the process is its high recovery factor and its high oil production rate. However, the high production rate is associated with excessive energy consumption approximately 1 million BTU/barrel, CO₂ generation, and expensive postproduction water treatment. Additionally steam injection has operational restrictions that do not allow its application in all types of reservoirs.

In order to overcome the problems associated to steam injection additional techniques have been developed to recover the heavy oil and bitumen. Among those techniques the vapor injection process (VAPEX), the cyclic solvent injection and the co-injection of steam and solvent (SAS, SAP, ES-SAGD and LASER) are the ones with the most promising future, thanks to the viscosity reduction of the oil phase, the change in absolute and relative permeability and the upgrading of the oil phase. The above processes involve the injection of solvent into the oil reservoir. The objective of the solvent is mainly to reduce the viscosity of the heavy oil or bitumen by mixing with it. This mixing process is a mass transfer process and its velocity is controlled by the diffusion coefficient. Therefore the diffusion coefficient is one of the most important parameters for the proper characterization of the solvent based recovery processes. Accurate diffusion data for these processes are necessary to determine:

• The amount and flow rate of solvent required to inject into a reservoir,
• The portion of reserves that have been affected by the solvent undergo viscosity reduction,
• The time required by the reserves to become less viscous and more mobile as desired,
• The rate of live oil production from the reservoir.

To find the diffusion coefficient value we must depend largely on experimental measurements of these coefficients, because no universal theory permits their accurate a-priori calculation. Unfortunately, the experimental measurements are unusually difficult to make, and the quality of the results is variable.