| Heat injection mining is an important method of coalbed methane extraction.To study the effect of sudden increase in ambient temperature on the permeability of coal when they are exposed to high temperature fluids,and to explore the prospect of application of multiple thermal effects in fracturing and increasing seepage of coal rocks.In this paper,the permeability and the development of pore and fissure structures of coal samples were tested before and after cyclic thermal shock by using equipment such as rheological testing machine and nuclear magnetic resonance(NMR)experimental system.The study applied the characteristics of NMR nondestructive testing to characterize the evolution of pore distribution,and combined digital image processing techniques with fractal theory to conduct statistics on the changes of macroscopic fractures of coal.The following conclusions can be drawn.(1)The thermal shock treatment can effectively increase the porosity of coal,and its effect on the pores is mainly shown in the macropore stage,where the primary pores and secondary pores are continuously expanded and connected to form more macropores and microfracture structures under multiple thermal stresses.With the increase of the number of cyclic thermal shock,the increment of total porosity,the increment of seepage pore porosity and the increment of seepage pore ratio all show an increasing trend,which indicates that cyclic thermal shock can promote the generation and expansion of seepage pores and provide favorable conditions for gas transportation.(2)During the cyclic thermal shock,the average permeability of the samples shows a logarithmic growth pattern.Compared with the increase of permeability produced by single thermal shock,the multiple cycles of thermal shock can increase the permeability more obviously,and the average permeability increase of coal by seven cycles of thermal shock reaches 100.02%.In the NMR permeability model,compared with the SDR model,the Coates model has higher accuracy in evaluating the evolution of coal permeability under cyclic thermal shock.In addition,it is found that the increment of permeability before and after thermal shock of samples is closely related to the increment of seepage pore porosity,based on which an empirical formula for estimating the increment of permeability is proposed.(3)The macroscopic fracture extension of coal samples after cyclic thermal shock exhibited good self-adaptive properties.The increase in the number of cyclic thermal shocks can effectively connect the relatively independent fracture structures,which makes the microfractures and small pore clusters rupture and connect to form an interwoven fracture network under the action of thermal stress.The surface fracture rate and fractal dimension of surface fracture increase with the increase of thermal shock cycles,indicating that the cyclic thermal shock can fracture coal rocks and provide a channel for coalbed methane transport.(4)The fractal dimension of seepage pores decrease after cyclic thermal shock.It could be because that,the pore-fracture structures of coal in the process of expansion and fusion,gradually formed the dominant gas migration seepage channel,and the structures of seepage pores tends to be simple and conducive to gas flow.(5)The mechanical properties of the coal samples decreased significantly after cyclic thermal shock,and the elastic modulus and uniaxial compressive strength of the coal samples decreased to a greater extent as the number of cycles increased.Due to the complex mineral composition structure of coal rocks,fatigue damage caused by thermal stress will occur under cyclic thermal shock,which makes coal rocks realize cracking and seepage increase.And with the increasing cycles,the efficiency of increasing permeability gradually decreases.This suggests that we should combine the reservoir conditions and reasonably set the plan in the process of enhancing coalbed methane extraction by using multiple thermal injection technology. |
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