Stress Sensitivity Experiments Of Coal Samples And Unsteady Percolation Model Of Water

So far, researches on the Coalbed methane(CBM) reservoir characteristics and the stress-sensitivity mechanism controlled by coal petrology cannot meet the technical requirements of CBM development, the establishment of CBM well production system is still at exploring stage, appropriate theory and methods related to macro- micro and initial- dynamic reservoir description are in urgent demand. In order to clarify the effective porosity and permeability characteristics and the stress sensitivity of coal reservoir restricted by coal petrology, and analysis the influence of stress sensitivity on single well production, medium – high and high rank coal samples were selected from the Southern Qinshui Basin and the Eastern margin of Ordos Basin, China, on the basis of the macroscopic description of coal samples, representative sample were picked out and their composition and structure were characterized by proximate analysis, density test, X-ray diffraction, and scanning electron microscopy, then the relationship of coal composition and fabric and the movable fluid seepage space were established; with tri-axial stress test and stress sensitivity test, the relationship between permeability and confining stress were described, and the coal petrologic factors controlling sample stress sensitivity were studied; finally reference and scientific basis were provided for reasonable production system.Indoor experimental results show that: coal samples selected from the Eastern margin of Ordos Basin develop orthogonal unfilled faces cleats and butt cleats, gas mainly diffuse from and through the micro-pores in matrix to macropores and macrofractures firstly, and then flow through open, unmineralized cleats and fractures to well base; as pores and fractures are blocked by carbonate minerals, gas mainly diffuse from and through the micropores in matrix to mesopores such as tissue holes, gas holes and intercrystal pores, et al, and then flow from the mesopores to well base. The elastic-plastic deformation of the flow channel is caused by primary deformation and structural deformation, and the deformation is influenced by macromolecular structure of coal, the contact type of coal component particles and micro salient of fracture inside surface. The fabric of the samples is the most important factor in determining the permeability and the change of permeability, the stability of the connecting area, the development of fractures, the macro heterogeneity of the samples and the occurrence of minerals influence the permeability and stress sensitivity of coal samples comprehensively. The mathematical model calculation results showed that in the draining water process of CBM wells, the dimensionless bottomhole pressure upturned with dimensionless time in stress sensitive reservoir, the stronger the stress sensitivity of the reservoir, the more quickly the bottomhole pressure and formation pressure decline; the initial permeability and stress sensitivity coefficient controls the size and the shape of the discharge area of single well, the single well control range enlarged with an increasing initial permeability and a decreasing stress sensitivity, and the initial permeability has larger effect than stress sensitivity.