Formation Damage due to Asphaltene Precipitation Resulting from CO2 Gas Injection in Iranian Carbonate Reservoirs

Abstract

Abstract Although CO2 injection is one of the most common enhanced oil recovery methods, that is widely used in Iranian reservoirs, it may change the reservoir fluid properties and cause some problems such as Asphaltene precipitation which will result in reduction of injectivity and productivity of oil wells and plugging wellbore and production facilities. This paper describes the effect of miscible CO2 flood on Asphaltene precipitation in Bangestan reservoir of Kupal field in southwest of Iran. The amount of Asphaltene precipitation in the mixture of oil and CO2 gas was measured by using PVT test and the mechanism of precipitation under the reservoir conditions was obtained by high pressure core flood test. Then, the experimental results were compared with the ones obtained from available asphaltene precipitation models. Introduction Asphaltenes are highly condensed polyaromatic structure or molecules, consisting primarily of carbon, hydrogen and minor proportion of hetero elements such as, sulfur, nitrogen, oxygen, etc. Asphaltenes are believed to exist in the crude oil partly dissolved and partly kept in finely dispersed colloidal suspension form that may be stabilized by non-polar resin fractions of the crude,[3] These higher molecular weight components of crude oil are in equilibrium at reservoir condition may start to precipitate once the stability of the colloidal suspension is disrupted by change in pressure or temperature during primary depletion or introduction of miscible gas and liquids and other oilfield operation. The injected carbon dioxide, when it contacts the reservoir oil, can cause changes in the fluid behavior and equilibrium conditions and also alter the asphaltene-to-resin ratio of crude oil which favors precipitation of organic solids, mainly asphaltenes.[4] Once asphaltenes have been precipitated from the oil during CO2 flooding, they may continue to flow as suspended particles, or they may deposit onto the rock surface causing plugging and wettability alteration of reservoir matrix which can severely reduce recovery efficiency, cause formation damage. Deposition begins with adsorption of flocculated asphaltene particles onto active sites on the rock surface, particularly onto high specific area clayey minerals such as kaolinite. This is followed by a hydrodynamic retention or trapping process of particles at the pore throats.[5,6] Deposition of solid asphaltenes causes a reduction of the pore space available for fluids. Other formation damage mechanisms may include permeability reduction and alteration of rock wettability from water-wet to oil-wet.[7,8]. It is also possible that deposited asphaltene particles may be re-entrained in the flowing oil stream due to a mechanical erosion or ablation effect if the interstitial velocity of the fluids becomes high enough.[9] This effect has also been observed in studies of wax deposition in pipelines.[10]