Critical dynamic conditions for gas migration in tight sandstone

Abstract

Physical simulation serves as a crucial method for understanding the mechanisms of underground oil and gas migration and accumulation. To gain a deeper understanding of gas migration mechanisms in tight reservoirs under deep geological conditions, experimental models and boundary conditions were designed using the tight sandstone gas reservoirs of the Upper Paleozoic Shanxi Formation in the Yan'an Gas Field as a case study. Based on ultra-low permeability rock multiphase flow nuclear magnetic resonance online simulation experiments, the study investigated the critical pressure and dynamic conditions governing gas migration in tight sandstone, while also analyzing the factors influencing gas migration and accumulation. Different types of sandstones from the Shanxi Formation were selected, including quartz clean sandstone, quartz-rich low-plasticity particle detrital quartz sandstone, plastic particle-rich detrital sandstone, and tuffaceous matrix-rich quartz sandstone samples, representing reservoir rock facies with different porosity and permeability distributions. Experiments with constant low injection flow rates, different flow velocities (flow rates), and different pressure differences were conducted. The findings indicate that the critical charging pressure of tight sandstone reservoirs is primarily influenced by rock facies and permeability. Dominant rock facies with higher permeability exhibit lower critical charging pressures. For instance, the critical injection pressure of pure quartz sandstone gas typically falls below 1.2 MPa, while it generally remains below 1.5 MPa even for plastic-rich granular lithic sandstones and tuff-rich hybridquartz sandstones with inferior physical properties. Furthermore, there exists no absolute lower limit for the gas charging physical properties of tight sandstone. However, the charging efficiency and gas saturation of tight sandstone are positively correlated with reservoir physical properties, particularly permeability. The more developed the dominant rock facies and the higher the permeability, the higher the charging efficiency and gas saturation.

Keywords

• rock facies
• displacement experiment
• charging pressure
• dynamic condition
• natural gas migration
• tight sandstone gas