The effect of asphalt precipitation on flow behavior and production of a fractured carbonate oil reservoir during gas injection

The first field data, collected over an 11 year period, are presented which indicate the possible effect of asphalt precipitation on the permeability and injectivity index of a fractured carbonate oil reservoir. The asphalt a ggregates were formed during enhanced oil recovery by injection of a rich g as into the reservoir. The data indicate that, while at the initial stages of the operations the permeability and injectivity index decrease, at later times they appear to oscillate with the process time, with apparent oscill ations' periods that depend on the heterogeneity of the reservoir. Two clas ses of plausible mechanisms that give rise to such oscillatory behavior are discussed. One relies on the changes in the structure of the reservoir's f ractures, while the other one is based on asphalt precipitation in the rese rvoir. Computer simulations of flow and precipitation of asphalt aggregates in a pore network model of the reservoir are carried out. The results appe ar to support our proposition that asphalt formation and precipitation in t he reservoir are the main mechanism for the observed behavior of the inject ivity index. We also develop a stochastic continuum model that accurately p redicts the time-dependence of the reservoir's permeability and injectivity index during the gas injection process.