In the calculation of coalbed methane resources in mines, the reserves are used in the mined area and the proven reserves of coal are used in the unexploited area, which has the characteristics of dynamic change. Reserves are obtained according to the dynamic changes of "three quantities", namely:
M retention = m proven -m output -m loss
Among them:
M reserves-coal reserves, t;
M proven-proven coal reserves, t;
M output-calculate the cumulative output of unit coal, t;
M loss-coal loss, t.
(2) Dynamic gas content in mining area
1. Numerical simulation
The development of coal mine roadway and coal production have changed the in-situ stress field and fluid pressure field of coal seam, and broken the dynamic equilibrium relationship among free gas, adsorbed gas and water-soluble gas in coal seam. In the coal mining area, due to the pressure relief of coal seam, cracks open or form new cracks; Due to mine ventilation, gas pressure difference and methane concentration difference constantly occur between mine roadway gas and exposed coal wall gas. Under the action of concentration gradient and pressure gradient, coalbed methane desorbs and diffuses and penetrates into roadway or working face. With the continuous advancement of roadway and coal mining face, the gas content, permeability and reservoir pressure of coal seam in mining area show dynamic changes. Based on the research results of mine gas emission, gas drainage, tile pressure measurement and finite element numerical simulation, the dynamic of coal seam gas content in coal mining area is numerically simulated.
Coal mining area can be divided into coal seam mining influence area (horizontal mining influence area), adjacent layer mining influence area (vertical mining influence area) and coal resource residue area.
(1) Mining-affected area of this coal seam
The mining influence area of this coal seam includes the mining influence area caused by driving roadway and coal mining face. The dynamic gas content of coal seam in the affected area is related to the exposure time of coal wall (or the advancing speed of coal mining face) and the distance from the exposed coal wall. The velocity, flow direction and gas pressure at any point change with time, which is an unstable flow field and is often estimated approximately by numerical simulation.
1) finite element method.
According to the research results of elasticity, the influence zone of mining involves at most 5 times the width of roadway. According to decades of experience in the mining industry, the cracks and pressure relief zones around the roadway are finally about 3 ~ 4 times the width of the roadway. The residual gas content in the affected area of coal seam mining is a function of geostress, permeability coefficient of coal seam, exposure distance from coal wall and exposure time of coal wall.
According to Ding Guangxiang's finite element simulation results (1996), the fitting relationship between the pressure distribution in the mining-affected area at a certain moment and the distance from the exposed working face is logarithmic attenuation:
Evaluation of coalbed methane resources in China
Among them:
L-the distance from a certain point to the exposed coal mining face, m;
The fitting coefficients of A2 and B2, P0 and Pi have the same meanings as before.
According to the gas pressure, the dynamic gas content can be calculated by Langmuir equation.
2) Gas emission method.
The relationship between the measured gas emission coefficient of bare coal wall and time is as follows:
Evaluation of coalbed methane resources in China
Among them:
CQ0 and CQT-initial intensity coefficient of gas emission from coal wall and m3/m2 d coefficient of gas emission from coal wall after t time;
T—— exposure time of coal wall, d;
α-time dimension coefficient;
β-coefficient related to coal, permeability, etc.
3) Gas pressure test method.
According to the measured original gas pressure and residual gas pressure at a certain point in the coal body in the gas pressure relief area, the original gas content and residual gas content are calculated by Langmuir equation, and the relationship between coal seam gas emission and exposure time and distance from exposed coal wall is regressed by numerical simulation. At a certain moment, the gas emission in the pressure relief area of coal wall is exponentially decreasing with the distance from the exposed coal wall, namely:
Evaluation of coalbed methane resources in China
Among them:
N—— Gas emission of this coal seam,%;
A3, B3-fitting coefficient, where B3 is also called attenuation coefficient;
L—— distance from exposed coal wall, m
(2) Mining-affected area of adjacent strata
Under the pressure relief of coal seam mining, the coalbed methane in adjacent layers will be desorbed to varying degrees. In mine gas drainage, the drainage efficiency is measured by many factors, such as the distance from the mining layer, the mining height of the mining layer, the mining length of the working face, the rock properties between layers, in-situ stress and so on. The gas emission degree of adjacent layers is inversely proportional to the interlayer spacing. The emission range of the upper adjacent layer can reach 170m, and the emission range of the lower adjacent layer can reach 50 m ... Through regression analysis, the relationship between gas emission from upper and lower adjacent coal seams and interlayer spacing (h) is as follows:
η =-53.4438+0n (h)+275.438+0。
η =-40.191n (h)+157.62
(3) the remaining area of coal resources
Due to the pressure relief and the full exposure of the coal wall, a large amount of coalbed methane desorbed from the top slag of coal seam, bottom slag of coal seam and safe coal pillar in goaf became a part of resources consumed by exhaust air and was ventilated out of the mine. However, the residual gas in the coal seam gas content will still remain in the coal loss.
In this work, the sum of degassing before crushing and degassing after crushing in gas content determination is used to approximate the residual gas in gas content. When there are data of coal seam gas content in desorption method, the dynamic gas content in the residual area of coal resources in goaf is equal to the sum of degassing before crushing and degassing after crushing; When there is no data of coal seam gas content in desorption method, the dynamic gas content in the remaining area of coal resources in goaf is equal to the gas content multiplied by the empirical coefficient of residual gas, which is the statistical result of coal seams with the same coal rank in the known data area (Table 4- 10).
Table 4- 10 Empirical coefficient of residual gas in different coal ranks
In the large coal pillar under buildings, roads and water bodies, the numerical simulation of residual gas content can be carried out according to the relevant methods of mining affected areas in this coal seam or adjacent layers.
2. Empirical extrapolation method
The dynamic gas content is derived from the empirical relationship by using underground drilling to expose the gas content in the coal core measured at different times, and its mathematical model is:
C dynamic = f (c measured value, t)
Among them:
C dynamic-dynamic gas content of coal seam, m3/t;
C measured-the measured gas content in the underground borehole coal core at a specific time, m3/t;
T- time, month.