Dynamic capillary pressure effects: experimental evidence and computational investigations

Laboratory data for the relationships between capillary pressure, saturation, and relative permeability are usually collected under static conditions. Flow processes, however, do not occur under static conditions. For the relations to be unique and thus independent of dynamics effects, the relations must be unaffected by flow conditions under which they are measured. There is ample experimental and theoretical evidence that this is not always the case, and that dynamic effects could be important. In this presentation, we first show that there is convincing experimental evidence that dynamically-measured capillary pressure curves deviate significantly from statically-measured curves. Often the deviation has been attributed to experimental difficulties and/or non-uniqueness of parameter estimation. We provide a different and physically-based interpretation of these deviations based on a capillary pressure - saturation relationship that takes into account dynamic effects. Data reported in the above-mentioned experiments are used to estimate the value of the dynamic capillary pressure coefficient appearing in this relationship. Subsequently, a dynamic pore-scale network model is used to investigate pore-scale processes which lead to such deviations. The model results are used to obtain values for the dynamic capillary pressure coefficient, and these are compared to coefficients derived from experimental evidence. We find that the numerical results give coefficients that have the same order of magnitude as those found in experiments.