Sulfur deposition in sour gas reservoirs: laboratory and simulation study

Sulfur deposition in the formation, induced by a reduction in the solubility of the sulfur in the gas phase, may significantly reduce the inflow performance of sour gas wells and some wells in sour gas reservoirs have even become completely plugged with deposited sulfur within several months. Accurate prediction and effective management of sulfur deposition are crucial to the economic viability of sour gas reservoirs.In this paper, a dynamic flow experiment was carded out to investigate formation damage resulting from sulfur deposition using an improved experimental method. The core sample was extracted from the producing interval of the LG2 well, LG gas field in the Sichuan Basin. The experimental temperature was 26 ℃ and the initial pressure was 19 MPa. The displacement pressure continuously decreased from 19 to 10 MPa, and the depletion process lasted 15 days. Then the core was removed and dried. The core mass and core permeability were measured before and after experiments. Experimental results indicated that the core mass increased from 48.372 g before experiment to 48.386 g afterwards, while the core permeability reduced from 0.726 to 0.608 md during the experiment. Then the core was analyzed with a scanning electron microscope (SEM) and energy-dispersive X-ray mapping. The deposition pattern and micro-distribution of elemental sulfur was observed and the deposited elemental sulfur distributed as a film around the pore surface.In addition, a preliminary three-dimensional, multi-component model was developed to evaluate the effect of sulfur deposition on production performance, and the effect of production rate on sulfur deposition was also investigated. Simulation results indicated that the stable production time would be shortened and the gas production rate would be decreased once sulfur deposited in the formation. The increase in deposited sulfur at high flow rates may be attributed to a bigger pressure drop than that at low gas flow rates. Gas production rate has a severe effect on sulfur saturation in the grid of producing well located in sour gas reservoir. The work suggests sulfur deposition should be considered to correctly predict production performance and gas production rate should be optimized in order to control or retard sulfur deposition during the development of sour gas reservoir.