Coalbed Well Hydraulic Fracturing’s Influences On Coal Mining

Coalbed methane (CBM) hydraulic fracturing vertical wells in Sihe coal mine aretaken as an example in this thesis. CBM vertical well hydraulic fractures’ form andinfluence factors for fracture extension were analyzed through ground monitoring,underground observation, physical stimulation experiments and numerical stimulationdata. Hydraulic fracturing influence zones were divided by different destructiondegree for coalbed and surrounding rock, single well and well group hydraulicfracturing influence models was established. Some advices for safety coal miningwere put forward for different affected zones.Hydraulic fractures are vertical or horizontal on the whole. Multi-fractures,“T”shape and “工” shape fractures can be formed around wellhole. The hydraulicfractures are main vertical, symmetry but non-equilong, distribution in northeastdirection mainly and northwest secondly. The maximum width of fractures are2~3cm,which located in coal reservoirs vertically and near wellhole horizontally. The form offractures are influenced by some inner factors, such as coal rock mechanics properties,in-situ stress state, geologic structure, coal body structure, roof and floor propertiesand external factors, such as hydraulic fracturing construction design. The whole formof fractures were controlled by in-situ stress state, the predict transformation depth forvertical and horizontal fractures is171m without considering the tectonic stress. Coalrock mechanics properties and coal body structure can influence the length, width andheight of fractures, roof and floor properties can influence the vertical extension offractures and control the “T” shape and “工” shape formed. The external factors aremain material and parameters for hydraulic fracturing design, including the influenceof perforation, viscosity, displacement and volume of fracturing fluid.Based on the analysis of destruction features for hydraulic fracturing on coalrock and surrounding rock,4basic extension types for fractures cross interface wereraised.27destruction types were established according to the number of hydraulicfractures, the degree of fracturing fluid filtration and affected degree by the stress offracture extension. There are only9types actually, which correspond to the4types ofinfluence zones by hydraulic fracturing: strong affected zone, general affected zone,weak affected zone and non affected zone. Based on the analysis of range and featuresfor different affected zones were discussed, the affected zones models for single welland well group were established. The adverse effects on coal mining by hydraulic fracturing were analyzed and models for safety coal mining were put forward.