Introduction As a highly potential clean energy, natural gas occupies an important position in the energy - transition process. However, many natural - gas reservoirs face water - break - through or water - flooding problems during production, forming a large amount of water - sealed gas that is difficult to extract, resulting in a decline in the production capacity of gas reservoirs. Fractured water - sealed gas reservoirs have the characteristics of low matrix - permeability of reservoirs, strong heterogeneity, and well - developed fractures, and their unsealing processes are influenced by multiple factors. Therefore, it is of great theoretical and practical significance to deeply explore the influencing factors and variation laws of unsealing characteristics in fractured water - sealed gas reservoirs.
Unsealing Experiments in Fractured Water - Sealed Gas Reservoirs Materials and Instruments Artificial sandstone cores, nitrogen, simulated formation water, wettability - reversal agents, etc. are used, and experiments are carried out through specific experimental devices. The experimental temperature is set at 50 °C. Unsealing Pressure - Difference Test Experiment Procedure It includes stages such as volume calibration, water - sealed slug preparation, pressure control in fractures, and gas pressurized injection. By changing factors such as core permeability, water - sealed slug length, pressure in fractures, and wettability, the influence laws of different factors on the unsealing pressure difference are studied. Interface Property Tests under the Action of Wettability - Reversal Agents The fluorocarbon surfactant is used to determine the gas - water interface tension and contact angle at different concentrations, so as to judge the wettability - improvement effect of the wettability - reversal agent.
Pore - Scale Simulation of Unsealing in Fractured Water - Sealed Gas Reservoirs Control Equations The momentum - conservation equation (Navier - Stokes equation) and the mass - conservation equation (continuity equation) are used to describe fluid motion. Interface - Tracking Equation The level - set method is used to track the gas - water interface to more accurately simulate the gas - water two - phase flow behavior during the microscopic drainage process. Boundary and Initial Conditions Hypothetical conditions are proposed, and the left - end inlet surface is set as the gas phase, and the right - end outlet surface is set as a constant - pressure boundary, etc., laying a foundation for microscopic seepage simulation. Model Establishment Based on CT - scanning technology, a high - precision 3D three - dimensional real - pore model is constructed to study the gas - water two - phase seepage characteristics and the key factors affecting the gas - drive drainage effect.
Results and Discussion Analysis of Influencing Factors of Unsealing Pressure Difference Permeability and Water - Sealing Degree The unsealing pressure difference increases with the increase of the water - sealed slug length, and the lower the permeability, the faster the increasing rate. This is because the lower the permeability, the smaller the pore size, and the greater the capillary resistance and the water - phase starting - pressure gradient. Pressure in Fractures and Water - Sealing Degree The unsealing pressure difference increases with the increase of the pressure in fractures. In low - permeability cores, when the water - sealed slug length increases, the unsealing pressure difference increases at a faster rate. This is because the increase in the pressure in fractures will increase the gas viscosity, increase the gas - water mobility ratio, make the gas - drive - water process closer to piston - like displacement, and make the breakthrough more difficult. Concentration of Wettability - Reversal Agent As the concentration of the wettability - reversal agent increases, the gas - water interface tension first drops rapidly and then gradually tends to be stable, and the contact angle of the core slice gradually increases. The wettability - reversal agent can improve the surface wettability of the core pore throats, reduce the capillary force, promote the gas to break through the water - sealed slug during the gas - drive - water process, and thus reduce the unsealing pressure difference. However, the reduction in the unsealing pressure difference in high - permeability cores is more significant. Prediction Model of Unsealing Pressure Difference for Water - Sealed Gas Based on 96 groups of experimental data, a prediction model of unsealing pressure difference under the influence of multiple factors is obtained by using the multiple - regression method, and this model can well fit the experimental data. Influence Characteristics of Different Factors on the Unsealing Process Pressure in Fractures The pressure in fractures has an important influence on the gas - drive drainage rate in matrix pores and the unsealing efficiency. The lower the fracture pressure, the faster the gas - advancing speed, and the more obvious the front - end fingering phenomenon, which limits the microscopic sweep efficiency of gas and leads to a decrease in unsealing efficiency. Concentration of Wettability - Reversal Agent As the concentration of the wettability - reversal agent increases, the gas - drive - water sweep range and the gas - appearance time at the outlet end first decrease and then increase, and the unsealing efficiency first decreases and then increases. This is because the wettability of the pore wall and the gas - water interface tension change simultaneously, which has a complex influence on the unsealing process. Wettability The wettability of the pore wall has a great influence on the gas - injection unsealing process and the microscopic distribution of fluids. Hydrophilic pores are conducive to the rapid unsealing of water - sealed gas reservoirs, but the microscopic sweep efficiency of gas is low; hydrophobic pores can improve the microscopic sweep efficiency of gas, and the unsealing efficiency is high; neutral wettability is between the two. Gas - Water Interface Tension The reduction of gas - water interface tension helps to reduce the unsealing pressure, promotes the rapid formation of gas - phase dominant channels, but will reduce the microscopic sweep efficiency of gas and slightly reduce the unsealing efficiency.
Conclusions Through experimental and simulation studies, the influence laws of factors such as reservoir permeability, wettability, water - sealing degree, and fracture pressure on the unsealing pressure difference and the microscopic gas - drive drainage process in fractured water - sealed gas reservoirs are clarified, a prediction model of unsealing pressure difference is established, and the variation law of fluid microscopic distribution and the microscopic drainage characteristics during the unsealing process are revealed. The specific conclusions are as follows: The unsealing pressure difference is an important index for evaluating the difficulty of unsealing in fractured water - sealed gas reservoirs, and reservoir permeability, water - sealing degree, and fracture pressure are the key factors affecting the unsealing pressure difference. The established unsealing pressure difference prediction model can well predict the unsealing pressure difference in fractured water - sealed gas reservoirs. The wettability - reversal agent can improve the wettability of the porous - medium wall, reduce the gas - water interface tension, promote the gas flow in the porous medium, and reduce the unsealing pressure difference. However, as the concentration of the wettability - reversal agent increases, the unsealing pressure difference first increases and then decreases. The unsealing efficiency is an important index for evaluating the unsealing effect in fractured water - sealed gas reservoirs, and the influence mechanisms of different factors on the unsealing pressure difference and unsealing efficiency are different. The increase in fracture pressure can improve the unsealing efficiency, the increase in the concentration of the wettability - reversal agent makes the unsealing efficiency first decrease and then increase, the transformation of the wettability of the pore wall from hydrophilicity to hydrophobicity can greatly improve the unsealing efficiency, and the reduction in gas - water interface tension will slightly reduce the unsealing efficiency. In conclusion, in - depth research on the influencing factors and variation laws of unsealing characteristics in fractured water - sealed gas reservoirs has important guiding significance for the efficient development and enhanced - recovery - factor - improvement of fractured water - bearing gas reservoirs.