Field-Scale Asphalt Precipitation during Gas Injection into a Fractured Carbonate Reservoir
Authors: Rassamdana Hossein , Mirzaee Nabiollah , Mehrabi Ali Reza , Sahimi Muhammad 
Published: December 2007

Abstract
The first field data, collected over an 11 year period, are presented for the effect of asphalt precipitation on the permeability and injectivity index of a fractured carbonate oil reservoir. The asphalt aggregates were formed during enhanced oil recovery by injection of rich gas into the reservoir. The data indicate that, at the initial stages of the operations the permeability and injectivity index decrease. However, at later times a pattern emerges in which they oscillate with the process time, with oscillation periods that depend on the heterogeneity of the reservoir. Hence, after the initial reduction the average injectivity index or the permeability does not vary greatly with the time, when the averaging is taken over long enough periods of time. The mechanisms that give rise to such oscillatory patterns are proposed, and computer simulations are carried out to test the proposed mechanisms. We also develop a model that can predict accurately the variations of the reservoir permeability and injectivity index with the time during the gas injection process.

Introduction
An important phenomenon in the petroleum industry is deposition of heavy organic compounds on the pore surfaces of oil reservoirs during enhanced oil recovery processes. Miscible displacement of oil by carbon dioxide or natural gas is considered one of the most efficient methods of increasing oil production. However, this process often causes some changes in the fluid flow behavior and the equilibrium properties of the fluids. Under certain conditions such changes lead to the formation of asphaltene and asphalt (asphaltene plus resin) aggregates, which are large molecular aggregates with complex structure and properties. Asphalt and asphaltene formation is a function of the composition of the crude oil and the displacing agents, and the pressure and temperature of the reservoir. Their deposition on the surface of the pores reduces the permeability of the pore space, which can lead to the eventual isolation of oil from the flowing fluid in the reservoir, and reduction of the efficiency of oil recovery processes. Because of the importance of enhanced oil recovery processes by miscible displacement formation of asphalts and asphaltenes and their precipitation on the pore surfaces have gained great attention. Deposition of such aggregates on the pore surfaces can also alter the wettability of the reservoir, a crucial factor in the distribution of immiscible fluids in a porous medium and displacement of oil by an immiscible fluid such as water. Over the past several decades the nature of asphalts and asphaltenes and the mechanisms of their precipitation have been studied intensively. In four previous papers we reported extensive new experimental data for the amount of asphalt and asphaltene precipitation, their molecular weight distributions, and the structure and mechanisms of their formation. We also proposed a simple yet highly accurate model for predicting the onset and amount of asphalt and asphaltene precipitation.
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