Experimental Study on Producing Pressure Difference and Flow Rate Sensitivity in a Coalbed Methane Production Well

Abstract

It has been illustrated that the production efficiency of coalbed methane (CBM) is significantly sensitive to the producing pressure difference and flow rate of fluids. To study the controlling mechanisms of producing pressure difference sensitivity and flow rate sensitivity of CBM, several medium- and high-rank coal samples were collected from three coal mines in the Qinshui Basin for the aim of carrying out relevant sensitivity experimental analysis. The results show that the permeability of coal samples shows varying laws with the changes of the producing pressure difference and flow rate, and both display a critical value for the permeability change. Specifically, the permeability of coal samples shows a rapid decreasing trend with the increase of the producing pressure difference after the critical pressure difference. However, at the beginning stages with the injection of a slow flow rate of fluids, the permeability in coals increases, but with the increasing injection speed of fluids, the permeability rapidly arrived to the peak value at the critical flow rate and then significantly decreased. The critical pressure difference in the study area ranges from 0.051 to 0.062 MPa, and the damage rate of permeability varies from weak to moderately strong. The average critical flow velocities in Sihe, Xishan, and Zhaozhuang coal mines are 6.92, 7.77, and 1.21 m/day, respectively. The flow damage rate of fluids in coals increases with the increase of porosity, permeability, and flow rate. It is also found that the larger the porosity and permeability of the coal reservoir, the smaller the critical pressure difference. Additionally, the damage rate of flow velocity for coal reservoir increases with the increased clay mineral contents in coals.

Experimental study on water saturation effect on coal sample permeability under different effective stresses

“…First, by carrying out the core flooding experiments, the impact of water saturation on the pore-fracture characteristics of core is analyzed, and it is pointed out that with the increase of the water saturation, the stress sensitivity of rock core is prone to stronger (Meng and Li 2013;Chao et al 2020;Zhang et al 2019); Second, the internal mechanism of the impact of water saturation on stress sensitivity is analyzed: with the increase of the water saturation, the bound water in the formation may be converted to movable water, which occupies the seepage channel. The gas-water flow greatly increases the seepage resistance, and the greater the permeability change (Liu et al 2015); the clay minerals in the reservoir expand with water, and the gas slippage coefficient of the rock sample decreases, thus the permeability decreases more obviously (Gao and Yu 2018;Meng et al 2022); the water saturation affects the mechanical parameters, which softens the rock samples and affects their deformation (Kang et al 2022); Last, based on experimental research, a characterization model of gas-water relative permeability is established to guide and evaluate the development and repair of the mine in combination with the actual production process (Shen et al 2019;Zhao et al 2017;Ji et al 2022).…”