Multi-Type Reservoir Flow Mechanisms in Deep Carbonate Gas Reservoirs: Insights from a Systematic Experimental Study of the Mao 2 Member, Maokou Formation, Sichuan Basin

This study aims to address the unclear flow mechanisms of multi-type reservoirs and the large productivity variations in deep carbonate gas reservoirs of the Maokou Formation in the Sichuan Basin. Existing studies have mostly focused on a single reservoir type or a single flow process and lacked a systematic comparison of multiple flow parameters for multiple reservoir types under unified experimental conditions. To fill this gap, core samples from the Hechuan area were classified into four types—fracture cavity, cavity, pore, and matrix—based on their reservoir space assemblage patterns. Under simulated reservoir conditions, a systematic series of core experiments were conducted, including threshold pressure gradient measurements, gas-phase flow tests, stress sensitivity evaluations, and gas–water two-phase flow tests, to reveal the differences in flow characteristics and the underlying microscopic mechanisms among the four reservoir types. The results indicate that the threshold pressure gradient exhibits a power-law negative correlation with permeability. Fracture-cavity- and cavity-type reservoirs are not affected by threshold pressure effects, whereas matrix-type reservoirs display extremely high threshold pressure gradients. The four reservoir types show significantly different nonlinear gas flow behaviors, manifesting various combinations of multiple flow stages, including a threshold-pressure-dominated stage, low-velocity non-Darcy flow, Darcy flow, deviation from Darcy flow, and high-velocity non-Darcy flow. With increasing effective stress, the permeability retention rate, in descending order, is cavity type (59.3%), pore type (49.0%), fracture-cavity type (45.5%), and matrix type (40.3%), with the same order observed for permeability recovery upon pressure unloading. Owing to the development of fractures, the fracture-cavity-type reservoir exhibits markedly superior water-phase flow capacity compared to the other three types. In pore-type and matrix-type reservoirs, where micro- and nano-throats serve as the primary flow channels, the relative permeability at the isopermeable point is low, and the increase in gas-phase permeability is significantly retarded. The characteristic parameters of two-phase flow exhibit a favorable logarithmic correlation with core permeability, and the irreducible water saturation, relative permeability at the isopermeable point, and width of the co-permeation zone show systematic variations with the reservoir type. Finally, categorized development strategies for the four reservoir types are proposed based on their distinct flow characteristics. The findings of this study can provide a theoretical basis and guidance for the rational development and evaluation of the Maokou Formation and similar deep carbonate gas reservoirs.