Dynamic threshold pressure gradient in tight gas reservoir and its influence on well productivity

Flow in tight gas reservoirs shows significant non-Darcy behavior and threshold pressure gradient (TPG) due to complex geological conditions. The conventional TPG test always leads to a remarkable error because of gas slippage effect, especially for tight cores. In this paper, experimental approaches were carried out using specially designed core-flooding system to determine the accurate TPG under reservoir conditions. TPG variations of tight core at different developmental stages (different pore pressure) were investigated by using new experimental method. Results indicate that TPG of tight gas reservoir is highly effected by pore pressure and conventional experimental method will lead to a significant error: TPG obtained from conventional experimental method are much higher. The experimentally observed TPG under different pore pressure suggest that TPG is not constant during the development of reservoir, but varies with the change of pore pressure, we call it dynamic threshold pressure gradient (DTPG): TPG exhibits a substantial linear increase with decreasing pore pressure. New productivity model of tight gas reservoir with DTPG effect and stress sensitivity being taken into account was established and case studied on the basis of experiments. Results show that gas well productivity with consideration of DTPG is lower than productivity obtained from conventional model which considering constant TPG.