Abstract
Some tight sandstone gas reservoirs contain mobile water, and the mobile water generally has a significant impact on the gas flowing in tight pores. The flow behavior of gas and water in tight pores is different than in conventional formations, yet there is a lack of adequate models to predict the gas production and describe the gas-water flow behaviors in water-bearing tight gas reservoirs. Based on the experimental results, this paper presents mathematical models to describe flow behaviors of gas and water in tight gas formations; the threshold pressure gradient, stress sensitivity, and relative permeability are all considered in our models. A numerical simulator using these models has been developed to improve the flow simulation accuracy for water-bearing tight gas reservoirs. The results show that the effect of stress sensitivity becomes larger as water saturation increases, leading to a fast decline of gas production; in addition, the nonlinear flow of gas phase is aggravated with the increase of water saturation and the decrease of permeability. The gas recovery decreases when the threshold pressure gradient (TPG) and stress sensitivity are taken into account. Therefore, a reasonable drawdown pressure should be set to minimize the damage of nonlinear factors to gas recovery.
Highlights
Water-bearing tight gas reservoirs, as part of unconventional reservoirs, attract more and more attention
In comparison with non-water-bearing tight gas reservoirs, the gas recovery of water-bearing tight gas reservoirs is generally lower, and three factors strongly influence the development of waterbearing gas reservoirs and flow behavior
The second influencing factor is the stress sensitivity of the permeability, which is common for tight gas reservoirs but intensified by water existence
Summary
Water-bearing tight gas reservoirs, as part of unconventional reservoirs, attract more and more attention. The third influencing factor is the gas-water relative permeability, which is impacted by the variation of drawdown pressure These three factors may act together and affect the gas flow and production of tight gas reservoirs. They derived the following equation (or coefficient) to characterize the stress sensitivity: sL These studies all imply that rock permeability reduction due to the increase of effective stress could significantly affect oil well productivity, especially for tight formations. The third factor is the gas-water relative permeability, which changes with variation of drawdown pressure These three factors work simultaneously and affect the gas flow and production of tight gas reservoirs. This model provides theoretical basis for the development of tight gas reservoir
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