Abstract
Due to the unique adsorption and desorption characteristics of coal, coal reservoir permeability changes dynamically during coalbed methane (CBM) development. Coal reservoirs can be classified using a permeability dynamic characterization in different production stages. In the single-phase water flow stage, four demarcating pressures are defined based on the damage from the effective stress on reservoir permeability. Coal reservoirs are classified into vulnerable, alleviative, and invulnerable reservoirs. In the gas desorption stage, two demarcating pressures are used to quantitatively characterize the recovery properties of permeability based on the recovery effect of the matrix shrinkage on permeability, namely the rebound pressure (the pressure corresponding to the lowest permeability) and recovery pressure (the pressure when permeability returns to initial permeability). Coal reservoirs are further classified into recoverable and unrecoverable reservoirs. The physical properties and influencing factors of these demarcating pressures are analyzed. Twenty-six wells from the Shizhuangnan Block in the southern Qinshui Basin of China were examined as a case study, showing that there is a significant correspondence between coal reservoir types and CBM well gas production. This study is helpful for identifying geological conditions of coal reservoirs as well as the productivity potential of CBM wells.
Highlights
Coal reservoir permeability is a key property controlling fluid migration
For unsaturated coal reservoirs, when reservoir pressure is greater than the critical desorption pressure, coalbed methane (CBM) cannot desorb from the coal matrix, and pores are filled with coal seam water, which pressure, CBM cannot desorb from the coal matrix, and pores are filled with coal seam water, which defines CBM production in the single-phase water flow stage
The above research shows that the coal permeability undergoes five stages in the single-phase water above research shows that the transition coal permeability undergoes five stages in the single-phase flow The stage: vulnerable stage, vulnerable stage, alleviative transition stage, alleviative stage, water flow stage: vulnerable stage, vulnerable transition stage, alleviative transition stage, and invulnerable stage
Summary
Coal reservoir permeability is a key property controlling fluid migration. Many studies and production experiences have shown that permeability is crucial for coalbed methane (CBM)production [1,2]. Coal reservoir permeability is a key property controlling fluid migration. Many studies and production experiences have shown that permeability is crucial for coalbed methane (CBM). Coal reservoir permeability varies significantly with variation in effective stress, matrix shrinkage, and gas slippage during CBM development [3,4,5,6]. Coal reservoirs are sensitive to pressure, and reservoir permeability decreases exponentially with increasing effective stress during the complete depressurization process [7,8]. Energies 2020, 13, 644 resulting in increasing permeability [9,10,11]. Gas slippage increases the apparent permeability when reservoir permeability is low, but the effect is small [12,13]
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