Quantitative Characterization of Flow Hysteresis for Fractured-Vuggy Underground Gas Storage during High-Rate Injection and Production Based on Multiscale Digital Cores.

The construction of the Southwest China Gas Storage Center to support the construction of 100 billion cubic meters of "Daqing" in Sichuan and Chongqing that can effectively keep pace with the rapid growth of natural gas demand, ensuring national energy security, is proactively promoted by PetroChina. However, the carbonate gas reservoirs in the Sichuan basin feature strong heterogeneity, such as fracture-vug development and a low-permeability matrix. Additionally, the mechanism of flow between the high-permeability fractured-vuggy zone and the low-permeability matrix in underground gas storage (UGS) facilities during high-rate injection and production remains unclear. These issues challenge the construction of the first fractured-vuggy UGS group in the Maokou Formation gas reservoirs at Mou and Lao in China; thus, the seismic, outcrop, core, and other data describing these UGSs are analyzed using 3D digital core models that represent fracture-vug combinations and are designed to quantitatively understand the microscopic flow mechanism of fluid in the fractured-vuggy zone and the matrix during high-rate injection and production. Several results were obtained. First, aiming at the high-rate injection and production scenario of fractured-vuggy UGS, the flow hysteresis suitable for engineering application is defined as the difference in the time taken for the high-permeability fractured-vuggy zone and the low-permeability matrix to reach the quasi-steady state based on the existing transient disequilibrium research of fractured media. Second, the results of the three representative fracture-vug conceptual models indicate that the scale and combination of the fracture-vug system are the main factors controlling flow hysteresis. Third, the characteristic parameter of flow hysteresis is defined and derived to quantitatively characterize the flow hysteresis of fluid in the high-permeability fractured-vuggy zone and the low-permeability matrix. The research achievements reveal the flow hysteresis of fractured-vuggy reservoirs during high-rate injection and production, establish a quantitative characterization method of flow hysteresis for fractured-vuggy UGS based on multiscale digital core technology for the first time, and provide a percolation theoretical basis for the scientific and efficient design of fractured-vuggy UGSs.