Abstract
Coal fines migration and intrusion in coal fractures affect coalbed methane (CBM) wells performance by reducing reservoir permeability and production continuity. Physical simulations are conducted to investigate the permeability variation under different diameter coal fines intrusion at various flow velocities and confining pressures. The results show that the conductivity of fractures is dramatically reduced and hardly recover to its initial condition after coal fines intrusion. The permeability after coal fines intrusion (Pcfi) has no direct correlation with the increase of flooding velocity, while decreases with the increase of confining pressures. The fractures can be totally blocked by coal fines, while penetration also happened during the flooding process, causing permeability fluctuation. The permeability loss rates value for 80-120 mesh coal fines intrusion are generally <60% compared with the initial permeability, including the flow velocity of 2, 3, 4, 6, 8, and 10 mL/min with confining pressure of 6 MPa and the confining pressure of 2, 3, 4, 5, and 6 MPa with flow velocity of 3 mL/min. However, under 120+ mesh coal fines condition, the permeability loss rates are higher than 85% under most flow velocities and confining pressures. When coal fines become smaller, the permeability loss rates decrease to be lower than 45%, and part the coal fines are discharged with the water flow. Thus, coal fines proper dischargement can partly maintain the reservoir permeability during coalbed methane production. The results would be useful in understanding coal fines intrusion behaviors and its controlling strategies during CBM drainage.
Highlights
Coal is to be broken into pieces due to its brittle, lowstrength, and weakly consolidated nature [1,2,3]
Large amounts of coal fines generated during the tectonic evolution history, as well as the coalbed methane (CBM) production process of well drilling, completion, and hydraulic fracturing [4, 5]
The permeability without coal fine intrusion decreases with the increase of flow velocity and confining pressure
Summary
Coal is to be broken into pieces due to its brittle, lowstrength, and weakly consolidated nature [1,2,3]. Large amounts of coal fines generated during the tectonic evolution history, as well as the coalbed methane (CBM) production process of well drilling, completion, and hydraulic fracturing [4, 5]. Coal fines can block cleats and hydraulic fractures, leading to the severe reduction in fracture permeability [6, 7]. Coal fines occurrence can influence drainage continuity, causing dramatic borehole pressure fluctuations, and gas and water flow interruptions [8]. Coal fines blockage can affect the spreading of pressure decreasing funnel and the gas production performance [9, 10]. Controlling the stable output of coal fines is important for CBM wells production [11, 12]
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