An Experimental Study on Percolation Characteristics of a Single-phaseGas in a Low-permeability Volcanic Reservoir Under High Pressure

摘要

In this paper, we find that with the decrease in the average pore pressure in the process of gas production,both the slippage effect and the stress sensitivity effect will gradually increase; the increase in the slippage effect issignificant, while the increase in the stress sensitivity effect is not. In this paper, the Kalamay volcanic gas reservoir ofthe Junggar Basin in China was selected as the object of our research. The gas reservoir has typical fractured volcanicreservoirs, and the long-term percolation feature remains unclear. To study the percolation characteristics of singlephasegas under high pressure, the experimental method was designed to simulate these characteristics in the processof gas production by measuring the gas flow in the core and the input and output pressure at both ends. We carried outsimulation experiments of single-phase gas flow percolation characteristics under high pressure using 11 pieces of volcanicrock samples in three wells of the study area. The results show that as the core pore pressure increased, the permeabilityof low-permeability cores of the volcanic rock decreased significantly at room temperature. However, thisdecrease became more gradual, which means that the higher the core pore pressure is, the smaller the permeabilityvariation caused by gas slippage is; when the pore pressure is above 10 MPa, the permeability is nearly constant, slippageeffect significantly reduces in the process of gas percolation, so it can be completely ignored under these formationconditions. As the pore pressure decreases, the slippage effect and stress sensitivity effect will gradually increase;when the pore pressure is less than 10 MPa, the permeability appears to increase significantly, and this is especiallytrue for a pressure of 5 MPa. The main cause of this result is the slippage effect of gas seepage during the depletion ofthe gas reservoir, when the pore pressure is lower than a certain value. The valid stress changes of the core are notlarge, and the stress sensitivity is not strong, so the slippage effect plays a major role, which leads to an increase in thegas permeability during the late period of certain flow gas production.