Abstract
Determination of the velocity sensitivity in coal reservoirs during the different production stages of coalbed methane wells is fundamentally crucial to adopt appropriate drainage technologies. To address this need, simulation experiments of coal samples from southern Qinshui Basin in China were conducted to test the variation of coal permeability with fluid flow. The pore structures were tested before and after the simulation experiment by using mercury injections, and the pore shape was observed using scanning electron microscope (SEM). The results show that formation water with fast flow may remove solid particles and that there is no velocity sensitivity under the experimental conditions of different coal samples and formation waters during the water production and depressurization stages of the coalbed methane well. There is a trend of the velocity sensitivity in the coalbed methane reservoir showing high concentration of solid particles during the stages of water production and depressurization. Coal permeability decreases with the increase of the fluid flow, there are different levels of velocity sensitivity in the coalbed methane reservoir during gas production of the coalbed methane well. The critical drainage flow should be within 11.26 m3/d during gas production of the coalbed methane well. The generation of the velocity sensitivity will make the pore structure of the coalbed methane reservoir poorly. During the stage of gas production, the formation water produces poorly, and the solid particles adhered to the surface of coal easily fall off and are deposited in the transition pore and micropore, which further results in the decrease of coal permeability.
Highlights
Coalbed methane (CBM) exploration and exploitation has rapidly developed in a number of countries such as the USA, Australia, Canada, and China (Moore, 2012; Pan and Wood, 2015)
It is observed that the dynamic coal permeability increases with increase of the fluid flow, the total increment of coal permeability is correlated to the value of fluid flow, and there is no velocity sensitivity no matter which coal samples were used and which adopted water samples were used
The velocity sensitivity of the CBM reservoir during different stages of the CBM well were based on the simulation experiments of coal samples from southern Qinshui Basin, China
Summary
Coalbed methane (CBM) exploration and exploitation has rapidly developed in a number of countries such as the USA, Australia, Canada, and China (Moore, 2012; Pan and Wood, 2015). CBM recovery has stimulated the fundamental studies of gas production affected by the variation of coal permeability during the CBM development process (Pan and Connell, 2012; Tao et al, 2012), simulation of the fluid flow in coal (Chen et al, 2016; Zhang and Wang, 2016), simulation of CBM production history (Salmachi and Yarmohammadtooski, 2015), damage mechanism and protection measures of coal methane reservoir (Huang et al, 2015; Liu et al, 2015), and drainage control on CBM recovery (Liu et al, 2013) In these studies, the problems resulting from the decrease of coal permeability due to the velocity sensitivity caused by unreasonable drainage of CBM were researched. This paper focuses on evaluating the potential damage from CBM reservoirs during different production stages of a CBM well caused by velocity sensitivity based on analyses of coal permeability, pore structure, and fracture structure, and gives an estimate of CBM drainage
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