| Under the problems of fossil energy depletion and environmental pollution,countries around the world are committed to research and develop new clean energy source. CBM is a kind of new high calorific value stored in the coal seam gas and pollution-free energy. With the improvement of understanding of geologic rule and the exploration and mining technology innovation, CBM development and utilization become the key to relieve the energy crisis. CBM resources in our country are rich,taking the third place in the world. It has a broad development prospect, but the development of technology of CBM has been the important factors which restricts the development of the CBM industry. There are still many problems of which the extraction of the "three soft" coal bed gas fields to be solved, which establish a suitable for "three soft" coal seam reservoir reconstruction technique is the key to improve the efficiency of CBM extraction.In order to optimize the construction parameters of hydraulic fracturing and improve the fracturing effect, it’s urgent to build a proper crack propagation model to precisely describe the initiation and propagation rules of cracks. Based on the geological conditions of the coal rock reservoir in the “Three Soft” mining area, this paper makes a preliminary study on the pressure control, extended model,permeability, fracturing time and productivity prediction of the hydraulic fracturing which remoulds reservoirs. The main work is reflected as following:Fistly, in this paper the stress field theory is used to describe the distribution of formation stress, and the influence of drilling on stress field near the borehole wall is analyzed from the point of view of elasticity. Besides, this paper considers the frictional head loss and perforation head loss of fracturing fluid, and respectively analyzes the crack initiation condition of vertical well and horizontal well, and establishes the relationship between crack initiation pressure and the ground pressure.Secondly, on the basis of analyzing the advantages and disadvantages of the traditional hydraulic fracturing model, combined with fracturing test in JiaozuoHenan coal-bed gas field, crack width hypothesis which is based on PKN model and the law of frictional pressure drop in the flow of cracks, the crack propagation model and permeability calculation model of the “Three Soft” coal seam hydraulic fracturing are constructed. The multiple verification is carried out by using the seismic survey method and the inversion calculation method.In addition, this paper analyzes the energy accumulation and dissipation in the process of crack initiation and propagation in coal-rock mass by using energy method.In the process of hydraulic fracturing, except for the frictional head loss and perforation head loss, the work of fracturing pump and the potential energy of fracturing fluid mainly is transformed into deformation energy of rock mass to make rock cracks extend. Accordingly this paper establishes the relationship between extension length of hydraulic fracture and fracturing time, and explores a new idea for time control of fracturing design.Lastly, this paper analyzes absorption and desorption characteristics of CBM,and forecasts the total resource and recoverable resources of CBM in Jiaozuo Wei Village mining area. On the basis of analyzing the later period discharge and mining data of GW-002 well, FracproPT is used in history matching of reservoir parameters of this CBM well to get more accurate gas-bearing properties and physical parameters,which are applied in later period productivity prediction. |
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