Fluid velocity sensitivity of coal reservoir and its effect on coalbed methane well productivity: A case of Baode Block, northeastern Ordos Basin, China

Abstract

The fluid velocity sensitivity (FVS) effect in coal reservoirs affects the production of coalbed methane (CBM) wells by generation of coal fines. In this work, coal samples from Baode Block, northeastern Ordos Basin were made into coal cores for FVS analysis under different initial permeabilities, injection volumes and injection intensities. Standard brine with a mineralization degree of 8% (the mass ratio of NaCl: CaCl2: MgCl2·6H2O=7.0:0.6:0.4) was chosen as the driving fluid. Also, the production responses of CBM wells to FVS at different stages of drainage were discussed. The results show that there is a critical flow velocity for the generation of coal fines. Below this critical flow velocity, coal fines can discharge with fluid, which increases the reservoir permeability. Over this critical flow velocity, FVS tends to occur in coal reservoirs, which blocks the effective seepage paths and reduces reservoir permeability. Additionally, the larger the flow velocity and injection volume are, the greater the permeability damage rate is. But the increasing rate of permeability damage slows down, revealing that the damage of FVS to permeability mainly occurs in the initial period of coal fines migration. Moreover, abrupt increase of flow velocity can damage reservoirs and give rise to massive coal fines generation. During the drainage process of CBM wells, the FVS effect is mainly affected by the dropping rate of working fluid level at the stage of single-phase water. There exists a period of massive coal fines generation when the working fluid level first drops near the coal seam. In the stage of two-phase gas and water, the deposition rate of coal fines is faster and the migration distance is shorter than that in the stage of single-phase water. Due to the difficulty of coal fines discharge, the FVS effect tends to occur in this stage. In the stage of single-phase gas, the gas flow can make coal fines agitate partially. But the FVS effect is relatively weak as coal matrix shrinkage can improve the reservoir permeability to a large degree.