The Exploration On The Mechanism Of Multiple Fracture And Acidification In Hydraulic Fracturing Of CBM Well

CBM, known as "gas", has been regarded as a kind of harmful gas during coal mining, and been used less. Along with the progress of science and technology and the adjustment of the world energy structure, world gradually realized that CBM is a kind of precious clean energy. Therefore, CBM as a kind of important unconventional natural gas is more and more attention from all walks of life. Hydraulic fracturing technology of vertical CBM Wells is one of the main stimulation measures during CBM exploitation, but the facts that many CBM wells'production capacity after hydraulic fracturing fails to reach the anticipated goal and adjacent Wells' production capacity namely large difference which belong to Huabei company of CNPC in Qinshui basin, show that the theory and technology of oil & gas hydraulic fracturing can't satisfy the need of CBM wells'hydraulic fracturing. Based on the analysis of coal'physical mechanics properties and fractures properties and the summary of previous research work, this paper argues that it may be not a single symmetric hydraulic fracture but multiple form complex hydraulic fracture when implementation of hydraulic fracturing in coal the features of which is many natural fractures, special properties mechanical and regional differences. In addition, that the part of the coal's fractures was filled by minerals such as calcite and dolomite also reduced the production of the CBM wells in a certain extent.The multiple fracture theory is the hydraulic fracturing frontier theory, especially for the coal the fractures, physical and mechanical properties of which is different from homogeneous sandstone. The study of multiple fracture theory is difficult and faces many problems. Based on the previous base theory research, this paper formed a set of ideas about researching the multiple fracture problem and acidification in CBM wells which can improve the production capacity. Then based on the basic theory of elastic mechanics, rock mechanics, fracture mechanics and fluid mechanics and the PKN model, this paper discussed how the construction condition(fracturing time, discharge and fracturing fluid properties, etc) and objective stratum condition influent the development of hydraulic fracturing fracture. The enhancement of coal's permeability and principal stress difference can make width and length of hydraulic fracture and steering radius decrease. The decrease of fracturing fluid's pattern index and viscosity and the increase of flow can make the width of hydraulic fracture increase, and the increase of width of hydraulic fracture is beneficial to the implementation sand-up fracturing. Compared with the original calculation of this mathematical model, the method proposed in this paper adopting time as base step was more simple and intuitive, besides it considered the change of filtration in initial phase and during fracturing process, which made the original PKN model improved.Based on the analysis of stress field of coal near by the hydraulic fracture, the paper found that sub-fracture of tension easily formed in the area near the well and sub-fracture of shear formed in the area far from the well more easily in process of coal fracturing.After the theoretical analysis, we known the multiple fracture theory, then this paper discussed the form of the multiple fractures in the coal seam which has many natural fractures. Based on the improved calculation of the PKN model, the paper established the extended model of multiple two-dimensional fractures considering closing stress interference, the change of filter, crack steering, flowing and so on at the same time. Then it obtained the change rule of multiple hydraulic fractures' width and length, the change rule of flow inside and the factors which control the link of several hydraulic fractures after the calculation of multiple hydraulic fractures which extend at the same time. Due to the change of steering radius causing by the different emissions allocation, two hydraulic fractures whose starting angles were different could link together while they were extend at the same time. This model was used in natural fracture of coal seam, namely the crack develop in different direction on hydraulic fracturing. Through combining with the simulation technology method, we presented the influence on multiple cracks by natural fractures and bottom-hole pressure in natural fracture of coal seam. In addition, this paper was put forward the theory of near wellbore advantage which was fit the study of fractures form of hydraulic fracturing in vertical CBM wells, and then it given the factors which influence the form of coal hydraulic fractures and the form of the hydraulic fractures on the theory of near wellbore advantage. After that this paper summarized and analyzed the effect of hydraulic fracturing by multi-fracture systematically.After the composition analysis of coal samples and known that part of the nature fractures of coal rock were filled by mineral such as calcite and dolomite, the paper combined with acidification experience of oil & gas wells, and it was put forward program of acidification in coaled methane well and done acidification experiment on coal samples. From the experimental results, we found that substances in the fractures were dissolved easily in acid water, and the permeability of the coal samples was increased several times, some reached more than 40 times. This afforded basic indoor theory for acidification in CBM well.The research of multiple hydraulic fractures is a system study, especially for the special nature of coal reservoir. This paper, only based on two-dimensional, discussed the possible form and factor of multiple hydraulic fractures and done the experimental study of coal acidification indoor. But there were many insufficiencies, and further study and exploration remains to be developed.