Study On Mechanism And Application Of Hydraulic Fracturing In Soft Seam Underground Mine

Coalbed methane development is of great significance to security, resources, environment, economy and other benefits. Although quantity of coalbed methane resources is large in our country, coal seam permeability is generally low. Low permeability characteristic is the main bottleneck of the coalbed methane development, and coal seam increasing permeability effectively is a key problem urgently to be solved in coalbed methane development. There are great advantages to increase permeability to medium hard and hard coal seams by hydraulic fracturing, but as to soft coal seams, whose elastic modulus, compressive strength and tensile strength is relatively low, and Poisson's ratio and pore compressibility is relatively high, hydraulic fracturing starting cracking and expanding laws have particularity, and the support of theory and practice is still lacking. Depending on the national science and technology major subject "technology and equipment to increase production for low permeability coal seam coal bed methane", and using methods synthetically of laboratory test, theoretical analysis, numerical experiment and field test, the thesis carried out the following research of hydraulic fracturing mechanism and application aiming at soft and low permeability coal seams.(1) Based on the physical and mechanical properties, and fracture characteristics test under the condition of different moisture of soft coal seams, crack propagation damage characteristics was studied, and soft coal seam's deformation and fracture mechanism was put forward, which laid the foundation for building hydraulic fracturing model of soft coal seams.(2) Based on the elastic-plastic mechanics and fracture mechanics theory, and from the points of deformation, occurrence and extension, the hydraulic fracturing process of soft and low permeability coal seams was divided into three stages:coal seam compression, crack fracturing and crack extension, and mathematics calculation model of toughness fracturing of soft coal seams was established, and the fracturing law and main controlling factors were obtained.(3) Based on the numerical method of particle flow, crack extension law of soft coal seam hydraulic fracturing was studied, and quantized influence effect of coal mechanical parameters to radius, opening degree and number of fractures during the process of soft coal seam hydraulic fracturing was revealed.(4) The numerical experimentation study of hydraulic fracturing parameter effect of soft and low permeability coal seams was carried out, by which quantized influence law of pump injection flow rate, injection pressure, injection time and so on to starting fracturing pressure, fracture radius, fracture opening degree, number of fractures was obtained; and which provided a basis for technology optimization of field hydraulic fracturing. (5) At Songzao mining area Yuyang coal mine, field test and validation research were carried out, by which increasing permeability effect of soft and low permeability coal seams hydraulic fracturing was inspected; and the adaptation of numerical experimentation for studying soft coal seams hydraulic fracturing was verified; and the adaptation and guidance of hydraulic fracturing mathematical model to the technology of field fracturing was confirmed.