A new experimental system and method for periodically measuring permeability and strains of coal

Permeability and strain of reservoir changes dynamically with the increase of exploitation time. The evolutions of permeability and strains is of great significance to study the influence mechanism of multi-physical field coupling on reservoir permeability. However, published measurement systems cannot measure the permeability, overall strain and fracture strain simultaneously and periodically. And the experimental studies about the control of fracture strain and overall strain on permeability evolution are rarely reported. In this paper, we have developed a new experimental system and method, i.e., coal-gas multi-process coupling periodic test system and method, which can measure the permeability, axial strain, radial strain and fracture strain of rock and coal under different in-situ stress, gas injection pressure, temperature and humidity. Most importantly, these parameters can be measured periodically, simultaneously and automatically, and this is the unique function of the new exper-imental system that differentiates it from other reported experimental systems. The reliabilities of the results measured by the new experimental system and method were verified by a series of experiments. In addition, we completed an experiment for measuring the permeability of a low-permeability coal sample by periodic transient method on the system to study the evolution of permeability with gas injection time. The experimental results show that the permeability of the coal sample decreases with the increase of gas injection time in a negative exponential variation trend, and with the gas injection time increases from 168 h to 201 h, the coal permeability decreases by almost 18%, from 0.195 mD to 0.160 mD. It may be a long-term process from the initial equilibrium state to the final equilibrium state of the coal permeability evolution. This research provides a new system and method to measure permeability and strains simultaneously and periodically, and we hope it will bring some references to researchers.