An Experimental Study on Percolation Characteristics of a Single-Phase Gas in a Low-Permeability Volcanic Reservoir Under High Pressure

Tang Xiaoyan

doi:10.2174/1874834101508010186

Abstract

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 is significant, while the increase in the stress sensitivity effect is not. In this paper, the Kalamay volcanic gas reservoir of the Junggar Basin in China was selected as the object of our research. The gas reservoir has typical fractured volcanic reservoirs, and the long-term percolation feature remains unclear. To study the percolation characteristics of singlephase gas under high pressure, the experimental method was designed to simulate these characteristics in the process of gas production by measuring the gas flow in the core and the input and the output pressure at both ends. We carried out simulation experiments of single-phase gas flow percolation characteristics under high pressure using 11 pieces of volcanic rock samples in three wells of the study area. The results show that as the core pore pressure increased, the permeability of low-permeability cores of the volcanic rock decreased significantly at room temperature. However, this decrease became more gradual, which means that the higher the core pore pressure is, the smaller the permeability variation caused by gas slippage is; when the pore pressure is above 10 MPa, the permeability is nearly constant, slippage effect significantly reduces in the process of gas percolation, so it can be completely ignored under these formation conditions. 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 especially true for a pressure of 5 MPa. The main cause of this result is the slippage effect of gas seepage during the depletion of the gas reservoir, when the pore pressure is lower than a certain value. The valid stress changes of the core are not large, and the stress sensitivity is not strong, so the slippage effect plays a major role, which leads to an increase in the gas permeability during the late period of certain flow gas production.

Highlights

Volcanic gas reservoirs have become one of the important areas for exploration and development of natural gas in China
When the average pore pressure is lower than 10 MPa, especially at a pressure of 5 MPa, the gas permeability of the core significantly increases, which is mainly caused by the slippage effect of gas seepage when the pore pressure is lower than a certain value during depletion production
With the increase of pore pressure, the decrease in the apparent permeability of core gradually reduced; i.e., the higher the average pore pressure of the core is, the smaller is the permeability change caused by gas slippage

Summary

INTRODUCTION

The seepage characteristics of volcanic gas reservoirs, such as the slippage effect, the threshold pressure gradient and the stress sensitivity, had been studied by some scholars [10]; the percolation mechanisms of volcanic reservoirs were studied All of these experiments were performed under atmospheric pressure conditions [11, 12], which is much different from the actual development conditions of a low-permeability volcanic reservoir. The research methods used in a low permeability reservoir, i.e., experiments to determine the percolation characteristics of a single-phase gas under high pressure using cores of a natural volcanic gas reservoir, closely approximate the dry-gas reservoir conditions; the research results are more in line with the seepage law of the gas formation condition, which provides the theo-. An Experimental Study on Percolation Characteristics retical basis for the efficient development of a volcano rock gas pool

EXPERIMENTAL PROCESS AND EXPERIMENTAL METHODS

CONCLUSION