Study On Theoretics Of Hydraulic Fracturing In Coal Bed And Applicatons

Exploitation of Coal-Bed Methane (CBM) is one of the hot research subjects caught widespread attention from domestic and international scholars in coal mining technology, which can not only reduce or eliminate the gas disaster hidden perils, but also supply a kind of high quality cleaning energy sources for human being. However, most of the CBM reservoirs in China are of low-permeability, so it is significance for improving the extract efficiency of CBM and raising the energy sources supply that to develop the technique about exploitation of CBM in this condition. The technique of hydraulic fracturing has been successfully applied to petroleum exploitation, while it is a new technique and in a beginning stage for exploitation of CBM and there are many problems need to be deeply investigated. Therefore, the mechanism of hydraulic fracturing in coal bed had been systematically studied in this dissertation by using the modern mechanics theory, similar simulation experiment and numerical analysis method, and the main innovative results are showed in the following aspects:(1) The permeability of standard coal sample, coal sample with through fracture and over-broken coal sample was tested respectively by using the MTS815.02 Electro-hydraulic Servo-controlled Rock Mechanics Testing System and the homemade testing instruments of over-broken rock seepage with compaction, and the change law of permeability coefficient of different coal samples with differential pressure and axial load are obtained.(2) Based on the deeply analysis of stress characteristics about surrounding rock of CBM well and material feature of coal, the porosity of coal is taken as the damage variable, combined with the damage model of Gurson, the damage constitutive equation of coal bed is established, and the estimated formula of propagating length of fracture in coal bed is given by the analysis of propagate process of one dimensional fracture.(3) The continuity equation, flow equation of fracturing fluid in fracture and the width equation of fracture in coal bed are given based on the P3D model of Palmer. Then, the fracture height equation considered the vertical flow of fracturing fluid in fracture and damage characteristic of coal material is established and the numerical solution methods are presented.(4) According to the damage model of fracture propagation, the seepage-flow coupled model of Realistic Failure Process Analysis system (RFPA2D-Flow) is adopted to simulate the hydraulic fracturing in coal bed, and the effects of ground stress status and injection pressure on initial fracturing and propagation law of fracture are analyzed, the results indicated that the direction of greater horizontal ground stress is the dominant orientation of fracture propagation when the two horizontal ground stresses are different, which provide a theoretical basis for the reasonable arrangement scheme of CBM wells.(5) Physical simulation test of hydraulic fracturing of coal had been designed, and the hydraulic fracturing test of coal samples and briquette samples had been carried out. Some results were obtained as follows: The fracture of coal samples propagate along the direction of the greater compressive stress; the propagation length of fracture increases with the increase of injection pressure and the initial fracture pressure increases with the increase of pump capacity.The achievements of this dissertation had been successfully applied to the CBM exploitation project in Jincheng coal mine field, and better social and economical benefits had been obtained.