Experimental and numerical modeling of sulfur plugging in carbonate reservoirs

Sour gas, mainly in the form of hydrogen sulfide, is produced in large amou nts from many oil and gas reservoirs in the United Arab Emirates. In additi on to creating problems in production lines, the precipitation of elemental sulfur in vicinity of the wellbore is often reported to cause wellbore dam age. While there have been several studies performed on the role of solid d eposition in gas reservoirs, the role of sulfur deposition in oil reservoir s has not been investigated. This paper presents experimental results along with a comprehensive wellbore model that predicts sulfur precipitation as well as plugging. Two separate sets of experiments, one for a gas phase sys tem and another for a crude oil system, were conducted to investigate the d eposition of elemental sulfur in (linear) carbonate cores. The gas flow tes ts were conducted with elemental sulfur being carried with nitrogen through limestone cores. Changes in gas flow rate were monitored while the injecti on pressure was held constant. A series of experiments generated valuable d ata for plugging with elemental sulfur. X-ray diffraction tests provided ev idence of sulfur deposition along the cores. The oil flow tests were carrie d out to observe sulfur precipitation and plugging in a carbonate core. The crude oil was de-asphalted before conducting these rests in order to isola te the effect of asphaltene plugging. Significant plugging was observed and was found to be dependent on flow rate and initial sulfur concentration. T his information was used in a phenomenological model that was incorporated in the wellbore numerical model. The data for the numerical model were obta ined from both test tube and oil flow experiments. By using a phenomenologi cal model, the wellbore plugging was modeled with an excellent match (with experimental results). (C) 2000 Elsevier Science B.V. All rights reserved.