Investigation of productivity decline in tight gas wells due to formation damage and Non-Darcy effect: Laboratory, mathematical modeling and application

Abstract The tight gas reservoirs in the Southwestern part of China contain large abundance of gas resources with ultra-low permeability and porosity. However, the gas wells in this area show high early production rate but decline quickly, and follows by a long-term production at a lower rate. This paper aims to investigate the possible reason behind the production performance, and also reach a novel productivity model to describe the well performance in this area. This study begins with performing various formation damage experiments including salinity, velocity, acids, alkali, and stress on the core samples from this area. The formation damage through the measured permeability change in these experiments were evaluated and compared. The Non-Darcy effect of gas flowing through tight porous media is also investigated in this study. A novel model to evaluate productivity which couples the primary formation damage and Non-Darcy effect in this area is proposed based on the formation damage results and Non-Darcy effect. We finally apply the novel model to this gas field, and implement sensitivity analysis to find out the key parameters that affect the well productivity. Experimental results show that both salinity sensitivity and stress sensitivity lead to formation damage in this area. Comparing the degree of permeability decreasing suggest that stress sensitivity is the primary formation damage for the early quick production declines. Experimental results also show that there exists a threshold pressure for gas flowing through the tight porous media. Applications of the proposed model to this tight gas field and also for sensitivity analysis with the results indicate that, the degree of influence of stress sensitivity is much greater than that of threshold pressure gradient. In addition, the greater the stress sensitivity coefficient and threshold pressure gradient are, the greater the productivity declining rate will be.