Production of gas can result in drying of the near-wellbore region caused by gas expansion, which causes evaporation and hence salt deposition. The deposition of salt leads to a reduction in porosity and permeability of the rock in the near-wellbore region, which then leads to reduction in well productivity. The main objective of this work is to understand the role of capillarity on salt deposition and its impact on well flow efficiency.
We develop and solve the dimensionless-conservation equations for solid-salt saturation using numerical methods under radial-flow conditions. The results of the calculations show that when the capillary effects are strong, the salt accumulates near the wellbore, which leads to plugging and hence higher skin values. The salt saturation continues to evolve until a limiting value, in the rock pores, is reached. Higher values of the limiting saturation lead to a greater reduction in permeability and therefore result in a larger skin value for the gas well. The flow efficiency of a gas well, calculated as a dimensionless ratio of the flow with and without skin, decreases continually and asymptotically approaches a constant value.
In a simulation for a typical field case, we find that the largest increase in skin factor and corresponding decrease in flow efficiency occur in approximately the first 30 days, after which the changes are slower. When connate-water salt concentration is higher, the skin value is much higher and thus the well productivity is lower. Using the model developed in this study, the decline of gas-well productivity caused by salt deposition can be predicted and hence accurate timing of well operations, such as water wash, can be made. Also, the modeling study can be used to select remediation strategies such as wettability alteration to reduce capillarity or application of inhibitors, which can reduce the limiting salt saturation by preventing growth of salt crystals within the reservoir rock.