Formation Damage Caused by Simultaneous Sulfur and Asphaltene Deposition

Abstract

Summary Although many oil reservoirs are producing crude oils of different sulfur and asphaltene contents, deposition problems of sulfur and asphaltene components in porous media are investigated separately. The major objectives of this laboratory study are to investigate the simultaneous deposition of sulfur and asphaltene in porous media. To achieve these objectives, the influences of the following on the permeability damage of the reservoir rock were experimentally investigated:crude-oil flow rate,permeability of reservoir rock through which crude oil flows, andconcentrations of sulfur and asphaltene in the crude oil. A base run was conducted using the crude oil after removing sulfur and asphlatene. Ten dynamic flow experiments were carried out using different crude oils of different sulfur and asphaltene concentrations and under different flow rates. The crude oil was flooded through different rock permeabilities of 2.34, 6.23, 16.58, and 21.48 md and under different flow rates of 0.5, 1.0, 2.0, and 3.0 cc/min, respectively. No permeability reduction or pore plugging was measured for the base experiment. The results indicated that the increase of flow rate increases the formation damage because of simultaneous deposition of sulfur and asphaltene in the reservoir rock. Core samples of lower permeability showed more severe permeability damage than those of higher permeability for the same applied flow rate and the same sulfur and asphaltene content of the crude oil. Furthermore, the increase of asphaltene and/or sulfur content of the crude oil increases the rock damage. The attained results of this study highlighted the important role of formation damage of carbonate oil reservoirs containing oils with a considerable amount of sulfur and asphaltene. In addition, the study provides two empirical correlations capable of predicting the permeability damage rate as a function of flow rate or initial rock permeability. These correlations represent useful tools for semianalytical and simulation studies. Introduction and Review Almost all deep sour-gas reservoirs contain elemental sulfur as a dissolved species. Reduction in pressure and temperature induced sulfur precipitation by a reduction in the solubility of the sulfur in the gas and/or oil phase beyond its thermodynamic saturation point. These changes occur during production operations and can result in sulfur deposition in the reservoir, wellbore, and surface facilities. Deposition of elemental sulfur in the near-wellbore area may significantly reduce the inflow performance of sour-gas wells. In addition, oil reservoirs all over the world suffer from simultaneous sulfur and asphaltene deposition problems in porous media, which cause problems in production facilities. Investigation of sulfur deposition in porous media receives little attention, while asphaltene nature and deposition in porous media have been covered extensively. Furthermore, the investigation of simultaneous sulfur and asphaltene deposition has not been studied in spite of the expected problems of formation damage and facility corrosion.