Study On Permeability Model And Experimental Research Of Coal Rock After Hydraulic Cracking Reform

Hydraulic fracturing technology is an important means to improve the recovery rate of coalbed gas.The change of coal seam permeability before and after fracturing is the key factor to measure the hydraulic fracturing effect.At present,the research on hydraulic fracturing seepage mainly exists that most scholars use bituminous coal and other pouring specimens,and there is a big difference between raw coal specimens with natural cracks and pores.The size of the test specimen is small(≤200mm),and the boundary effect is greatly affected when loading the ground stress.The fracturing and seepage test process is not continuous,and after fracturing,it is necessary to take out the sample and do the seepage.As a result,the experimental results and coal bed gas production capacity prediction are not accurate,and there is no reliable theoretical basis for the design of fracturing schemes.Therefore,in this paper,the hydraulic fracturing seepage experiment is carried out using a large-size raw coal sample(φ100mm×200mm),and the direct seepage experiment is carried out on the basis of fracturing experiment,so that the fracturing seepage can be carried out in the same experimental system,as close as possible to the actual situation of fracturing seepage in the field.The experimental results are more consistent with the field,thus providing theoretical support for the prediction of coal bed methane production.The main research results of this paper are as follows:(1)This paper reveals the complex mesh cracks formed by fracturing the raw coal water force.High-energy CT scan was used to obtain the water pressure fracture form of the raw coal sample after fracturing.The results showed that the more developed raw coal sample of the natural joint fracture formed a more complex crack network after hydraulic fracturing.The fracture morphology consists of three forms:type a-the main crack perpendicular to the direction of the minimum main stress;Type B-vertical spreading secondary fissure produced on both sides of the main fissure;Type C-joints and natural fissures supported by water pressure.The cracks formed after hydraulic fracturing of relatively homogeneous briquettes are mainly relatively single type A cracks.(2)The expression of the net pressure coefficient that characterizes the complexity of the cracks and the model of the complex mesh crack seepage after fracturing of raw coal are derived.Based on the CT sweep map of cracks after fracturing of raw coal water force,the plane frame diagram of the intersection of hydraulic cracks and natural cracks is abstracted.From the theory of elastic mechanics,the relationship between net pressure coefficient,crack pressure and stress state is deduced.Then a new seepage model which takes into account such factors as surface roughness and bending degree,crack connectivity and isotropy is constructed based on the water pressure fracture form of raw coal,the seepage model of"matchstick"and the cubic law.(3)The linear relationship between the values of the coefficientαand the stress sensitive coefficient C_f and the net pressure coefficient R_n is obtained.The experimental results show that the value ofαremains constant with the change of the net pressure coefficient,which is about 0.854 constant;The value of C_f increases abruptly with the decrease of the net pressure coefficient.When R_n is greater than 1,the value of C_f is about 0.148,and when R_n is less than 1,the value of C_f suddenly increases to 0.227.