An experimental approach of elemental sulfur deposition in carbonate oil reservoirs

The existence of sulfur compounds in crude oils creates many problems of sulfur deposition in the vicinity of the wellbore hole, in well completion and/or production equipment, and in producing reservoir rocks. The major objectives of this experimental study are to investigate the influences of oil flow rate, initial sulfur concentration of crude oil, and reservoir rock permeability on elemental sulfur plugging in carbonate oil reservoirs. To achieve these objectives, actual crude oils were de-asphaltened to eliminate the effect of asphaltene deposition. Ten dynamic flow experiments were conducted using two actual crude oils of 0.78 and 1.67% sulfur concentrations. Viscosity of crude oils of different sulfur concentrations was measured under different conditions of temperature, The crude oils were flooded through actual carbonate cores of different permeability in the range of 2.34-28.16 millidarcy and under different flow rates of 0.5, 1.0, 1.5, and 2.0 cc/min. In-situ sulfur deposited was measured using Scanning Electron Microscopy (SEM) to provide the amount of sulfur deposited along the core samples. The results indicated that crude oil of higher sulfur concentration has higher viscosity than that of one of lower concentration. The deposition of elemental sulfur does not take place at the low rate of 0.50 cc/min, starts at 1.0 cc/min and increases as the flow rate increases up to 1.50 and 2.00 cc/min, respectively. In addition, the higher sulfur concentration of the crude oil increases the deposition of sulfur in carbonate oil reservoirs. The results also showed that permeability of carbonate reservoir rocks has a severe effect on sulfur deposition since carbonate rocks of higher permeability do not experience the problem of elemental sulfur deposition while the problem is more severe for lower permeability rocks. In addition, the depositional rate is accelerated rapidly as the rock permeability decreases. The obtained results of this study have important interest in identification of the most important factors affecting the elemental sulfur precipitation in heterogeneous carbonate oil reservoirs and robust implications in the development of reservoir simulation models.