Nonlinear percolation mechanisms in different storage-percolation modes in volcanic gas reservoirs

The non-linear seepage mechanisms in volcanic gas reservoirs were studied using experiment results of rate-controlled Hg penetration, nuclear magnetic resonance, and core displacement tests. According to types of storage space and throats, combined with dynamic production characteristics, the volcanic gas reservoirs were classified into four storage-seepage modes, pore type, inter-granular pore type, micro-pore type and fracture-dissolution pore type. Slippage effect and stress sensitivity influence the seepage characteristics of volcanic gas reservoirs. Slippage effect is obvious in micro-pore type reservoirs, medium in pore type reservoirs, and insignificant in inter-granular pore type reservoirs. Stress sensitivities vary greatly in different storage-seepage modes. Fracture-dissolution pore type reservoirs have the strongest stress sensitivity, pore type reservoirs come second, and inter-granular pore type reservoirs have the weaker. In combination of dynamic production data, a relay gas supply mechanism model of multi-media volcanic gas reservoirs was established. In the high and stable yield period, when differential pressure dominates, gas is supplied from large pores and large fractures, and the single well production is high. In the yield decline period, when compaction plays a leading role, gas is supplied from medium pores and small fractures. In the low and stable yield period, when imbibition and diffusion are the main driving force, gas is supplied from small pores and micro-fractures.