China is one of the countries which take coal as the main energy source.Developing and utilizing coalbed methane resources can not only effectively curb coal mine gas disasters and reduce the greenhouse effect of methane on the atmospheric environment,but also optimize energy structure and alleviate energy shortage.In this paper,aiming at the coal and rock around the CBM well under the condition of stress field,the mechanical behavior of the coal and rock affected by fracturing modification is studied systematically and deeply.The results not only provide theoretical basis for safe and efficient exploitation of CBM and promote comprehensive utilization of CBM in China,but also have important practical significance for effectively solving productivity evaluation of CBM reservoirs after fracturing transformation.On the basis of extensive research on rock mechanics and fracturing simulation test at home and abroad,this paper designs a simulation test of coal-rock fracturing transformation using raw coal as test material.Through CT scanning,permeability testing and triaxial compression test of coal and rock samples after fracturing transformation,the fracture development law of coal and rock and its influence on coal and rock mechanics characteristics are analyzed.On the other hand,by using RFPA software and numerical simulation,a physical model for evaluating the variation of effective elastic modulus before and after fracturing modification is established.The research has obtained the following understandings:(1)In the process of hydraulic fracturing transformation,coal and rock mainly extend along the direction of primary cracks;when the fracturing strength is high,new micro-cracks are more likely to occur.These new micro-cracks mostly occur near primary cracks,and are nearly vertical to primary cracks,connecting to other primary cracks nearby.(2)Fracturing modification can effectively improve the permeability of coal and rock,but it is not the higher the fracturing strength,the more obvious the increase of permeability.There is no support of proppant.When the hydraulic pressure is removed,the expanded fracture closes under the action of in-situ stress,and the permeability reversal may remain unchanged or decrease.(3)The stress-strain curves of coal and rock samples after fracturing are very similar to those of coal and rock samples without fracturing.They can be divided into compaction stage,elastic deformation stage,plastic deformation stage and residual stress stage.With the increase of fracturing strength,the elastic deformation stage gradually shortens,while the plastic deformation stage becomes more obvious.(4)In triaxial compression test,with the increase of confining pressure,the compressive strength and elastic modulus of coal and rock samples with the same fracturing strength increase,and the Poisson’s ratio does not change significantly.Under the same confining pressure,with the increase of fracturing pressure,the compressive strength decreases,the elastic modulus decreases,and the Poisson’s ratio does not change significantly.(5)The distribution of elastic modulus after fracturing is consistent with that simulated by RFPA software by changing the fracturing density.The effective elastic modulus of coal and rock after fracturing is positively correlated with the simulated fracturing density.It can be used to predict the change of elastic modulus of coal and rock caused by fracturing modification through simulation software calculation.It can provide analysis reference for mechanics characteristics of coal and rock around coal-bed methane well after fracturing modification,and ultimately be used in the analysis of wellbore stability. |