Diffusion controlled swelling of reservoir oil by direct contact with injection gas

Abstract

An approximate analytical solution has been developed to describe the diffusion controlled swelling of reservoir oil when it is directly contacted by an injection gas such as CO 2. The solution is based on the assumption of a variable power profile for the gas concentration in the oil phase which considerably simplifies the mathematical complexity of the finite-domain moving-boundary diffusion problem. The approximate solution is validated by comparing it with the exact solution for diffusion into a semi-infinite medium for short contact times. The approximate solution is used to evaluate the diffusion coefficient for CO 2 from previously reported experimental oil swelling measurements at high pressure, and to estimate time scales associated with diffusion on both the laboratory and field scales. The diffusion time scales compare well with reported flooding rates and core lengths used in laboratory tests to evaluate oil recovery. It is concluded that molecular diffusion is an important mechanism in the recovery of oil on the laboratory scale. However, it is less certain if this is also true on the field scale where there is considerable uncertainty with regard to the size of effective diffusive length scales.