Asphaltene-Induced Formation Damage: Effect of Asphaltene Particle Size and Core Permeability

Abstract

ABSTRACT The precipitation of asphaltene during CO2 miscible flooding can lead to production losses and reduced efficiencies. Having a clear understanding of the asphaltene deposition mechanism can help the oil industry to develop effective engineering practice to minimize asphaltene deposition and develop treatment program to restore well productivity. This paper presents results of phase behavior and core flood tests conducted at reservoir pressure and temperature conditions to identify the dominating factors associated with permeability reduction during CO2 miscible flood of a light oil reservoir. Initially, dynamic phase behavior of crude oil and CO2 mixtures were investigated to determine the threshold concentration of CO2 for onset of asphaltene precipitation. The relationship between CO2 concentration and the mass of deposited asphaltene particles were quantified by analyzing the images of the mixtures as well as by physical separation of the deposited asphaltene solids. Laboratory coreflood tests were conducted to investigate the effect of asphaltene deposition in reduction of core permeability. Variables investigated included, initial core permeability, CO2 concentration and quantity and size of asphaltene particles in the oil/CO2 mixtures. Porous media used included sandpack, reservoir carbonate cores and commercial Indiana limestones. The tests were conducted at 100 oC and 3000 psig with reservoir crude oil containing CO2 concentration in the range of 40–70 mol%. Phase behaviour results indicated that the mass and size of asphaltene particles precipitated from the mixtures were strongly dependent on the CO2 concentration. Coreflood test results showed that the severity of formation damage was related to the initial core permeability as well as the quantity and size of the asphaltene particles precipitated. The damage mechanism was found analogous to the 1/3 and 1/7 rule of thumb that related the size of particulates in injected water to potential permeability reduction. It was also observed that formation damage caused by shallow solid invasion was more readily removed by crude oil injection while remediation of formation damage caused by deep solid invasion, was more difficult. INTRODUCTION From previous studies reported by Fisher and others(1), the process of asphaltene precipitation followed several important steps:The first step is precipitation, this is when the solid particles form a distinct phase as they come out of solution. The quantity and size of solid particles at this stage could be quite smallThe second step is the flocculation stage, a process by which the small solid particles clump together and grow larger.The third stage is deposition, a point at which the particles are so large that they can no longer be supported by the liquid and therefore settle out on solid surfaces.