Abstract
The different lithological combinations of the surrounding rock of coal seams play a key role in controlling the enrichment and migration of coalbed methane (gas), and their permeability and stress sensitivity have important theoretical guiding significance for the regional election and evaluation of coalbed methane development. In this paper, the HB-2 type coal rock sample pore permeability adsorption simulation measurement device is used to carry out the sensitivity experiment of the permeability of different surrounding rock combinations on the effective stress of the No. 3 coal seam roof in the Daping coal mine area, Luan, Shanxi. The sensitivity coefficient of permeability to effective stress, the maximum damage rate of permeability, and other parameters are defined to characterize the response mode of permeability to effective stress and the dynamic change rule of permeability, and reveal the control mechanism of effective stress on permeability change of different surrounding rock combinations. The results show that the permeability of the roof of No. 3 coal seam is highly sensitive to the effective stress, and the permeability of different surrounding rock combinations decreases with the increase of the effective stress, and there is a strong negative exponential correlation between the permeability and the effective stress; the stress sensitivity coefficients of different surrounding rock combinations under unloading stress are higher than those under loading; the permeability changes of specimens with different combinations of surrounding rock under the same experimental conditions are varied and the differentiation phenomenon is significant; the permeability is affected by lithology, pore fissure degree, and different combinations of surrounding rock, among which different combinations of surrounding rock are the main controlling factors for the dynamic change of permeability.
Highlights
Coalbed methane is a kind of unconventional natural gas that is self-generated and self-storage from coal seams and is mainly stored in coal seams and surrounding rock in an adsorbed state (Yang et al, 2014)
Where, K is the permeability of the surrounding rock combination, mD; Q0 is the gas flow at atmospheric pressure, mL/s, μ is the viscosity of N2, MPas, L is the length of the rock core, cm, P0 is the atmospheric pressure of the day, kPa, P1,P2 is the rock core inlet pressure outlet pressure, kPa, A is the sample cross-sectional area, cm2
In the loading stage, when the mudstone is the top or bottom of the combination, the permeability is more sensitive to changes in effective stress; when the mudstone is in the middle, the permeability is less sensitive to changes in effective stress, and the average stress sensitivity coefficient of combination 5 is 2.36 times the average stress sensitivity coefficient of combination 6
Summary
Coalbed methane is a kind of unconventional natural gas that is self-generated and self-storage from coal seams and is mainly stored in coal seams and surrounding rock in an adsorbed state (Yang et al, 2014). The geological conditions that affect the occurrence of coalbed methane include gas generation conditions (coal thickness, coal rank, coal quality, and reservoir physical properties) and preservation conditions (burial depth and surrounding rock). The permeability of the surrounding rock is a key indicator to characterize the capped ability of the surrounding rock, and the permeability of different lithological surrounding rock combinations varies greatly. It is of great theoretical and practical significance to research the permeability characteristics of different lithological surrounding rock combinations in coal seams for the regional election and evaluation of coalbed methane development. Lu et al (Lu, 2008) revealed the strength weakening and flushing reduction principle of combined coal rock through experiments; Zuo et al (Zuo et al, 2011a; Zuo et al, 2011b; Zuo et al, 2011c) conducted experimental studies on the damage mechanism, mechanical properties, and graded loading and unloading characteristics of deep coal rock monomers and combinations; Zhao et al (Zhao et al, 1999) concluded that adsorption and pore pressure jointly affect the permeability coefficient of coal by establishing the relationship between permeability and volumetric stress; Somerton (Somerton et al, 1975), Brace (Brace, 1978) and Tang et al (Tang et al, 2006) studied the effect of effective stress on coal permeability; SEIDLE et al (Seidle et al, 1992) proposed a matchstick model to explain the relationship between coal rock permeability and effective stress, and established an empirical model for coal rock permeability; MCKEE et al (Mckee et al, 1988) established a theoretical percolation model based on the compressibility of the coal rock matrix; P&M model (Palmer and Mansoori, 1998a) and S&D model (ShiDurrcan and Durucan, 2004) are currently the most representative permeability models; Yang et al (Yang et al, 2008) studied the permeability law of coal rock under different conditions at low and high temperatures, respectively
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