Mechanism Of Hydraulic Fracture Propagation Of The Coal Reservoir In The Zhengzhuang Southern Qinshui Basin, China

Qinshui Basin is the Coal bearing basin which coalbed methane has a large-scale exploitation and utilization.Zhengzhuang and Anze are the study area of the paper.The coal petrology and quality,the characteristics of Geology,the distribution of tri-axial stresses and the propagation rule of hydraulic fracture in coal reservoir were investigated,then the evaluation of fracturing effect and optimization of fracturing parameters were studied.The paper mainly focused on the propagating of hydraulic fracture in coal reservoir and the building of fracture length model.To better understand these results and achievements,the detailed are as follows:(1)The research revealed that the coal is anthracite with the low ash content.The reservoir physical property show strong heterogeneity due to the development of Sitou fault on the southwest of Zhengzhuang block.(2)The analysis of mechanical parameters of coal reservoir showed that the tensile Strength has the best correlation with the compressive strength of air-dried sample,followed by Young's modulus.The study also found that tensile Strength between coal seam and the roof can affect the hydraulic fracture length and height.(3)In-situ stress distribution was researched by the method of In-situ stress test and well testing.Generally,In-situ stress is affected by the factors of burial depth and fault distribution.In-situ stress conditions have great effect on fracture pressure and the shape of hydraulic fracture.(4)This study performed in-situ tri-axial fracturing experiments on 6 block samples(300mm×300mm×300mm,including raw coal and artificial roof and floor)to simulate the hydraulic fractures propagation in the CBM reservoir.Totally four groups of experiments were used to simulate the scale and the approaching of pre-existing natural fracture,horizontal stresses difference coefficient,vertical stresses and injection flow rates influencing on hydraulic fracture propagation.Meanwhile,five series of numerical simulations were constructed to modeling the relationship between in-situ horizontal stresses and hydraulic fracture propagation.The result of numerical simulation show that the influence of K_h is higher than that of horizontal stresses difference in determining fracture propagating along?_H direction in coal seam.Actually the hydraulic fracture propagation is related to the fluid pressure and the critical hydraulic pressures at different interfaces such as pre-existing natural fracture,coal petrological layered interfaces,and the interfaces between the coal and rocks.(5)On the basis of classic filtration theory as well as the characteristic of stress sensitivity of coal reservoir,taking coalbed methane reservoir of Anze block in Qinshui basin as an example,the paper proposed the computing method of comprehensive filtration coefficient.By comparing with the fracture monitoring data,the suitable fracture propagation models were built for Zhengzhuang and Anze blocks.The research shows that the different models are fit for different areas,even the different structural location in the same area.The application of PKN fracture length model accorded well when natural fracture less developed and fracture propagation was confined in coal seam,while the application of KGD fracture length model accorded well when hydraulic fracture can cut through the roof and floor of coal seam.