CBM-reservoiring effect in deep strata

Abstract：Specific characteristics of deep CBM reservoiring in theory was analysed,and deep CBM content,permeability and fluid pressure properties and their geological controlling factors was systematically eluicdated.Due to the geostress mechanism transform,fracture occurrence and pattern were vertically zonated leading changes of coalbed permeability.Permeability models of deep coalbed incorporated respectively with swelling/shrinkage sorption,earth temperature,geostress and groundwater chemistry effects were constructed to analyse their occurrences.The models was established for analyzing effects of geostress,earth temperature and swelling/shrinkage on fluid pressure,and the significant differences among pressure constitutes between deep and shallow coalbeds was found.The fluid pressure induced by sorption and geostress are roughly equal with burial depth shallower than 1 200 m,and that induced by temperature is the weakest.With burial depth deeper than 1 200 m,effect of earth temperature on fluid pressure is ranked first,geostress ranked second and the weakest is that of coal matrix swelling/shrinkage. Sorption experiments under high pressure and temperature have revealed particularity of sorption.The study uncoveres that coal rank effect on sorption decreases with increasing burial depth and sensibility of sorption of high rank coals to reservoir pressure are lower than that of low to middle rank coal.Coupling factors of coal rank,temperature and reservoir pressure,effective diffusion coefficient and deep CBM content prediction models are construt.The model reveales that ＂critical depth＂ exsits in relationship between CBM content and burial depth,and critical depth is synthetically controlled by coal rank,formation temperature and fluid pressure.Cirtical depth at equal coal rank becomes shallower as the fluid pressure gradient increasing,while becomes deeper with decreasing of earth temperature gradient at equal coal rank and reservoir pressure gradient.It is also obvious that effect of reservoir pressure on CBM content is stronger than temperature when the burial depth is shallower than ＂critical depth＂,while that of temperature is higher than reservoir pressure as the depth deeper than ＂ critical depth＂. Experiments of fluid-solid interaction mechanics under temperature and pressure revealed particularity of mechanics properties and permeability and their geological influence factors.It is found effect of confining pressure on coal mechanics is the most obvious in any rank coal.Effect of fluid pressure on low rank coal mechanics is stronger than that temperature,while that on high rank coal is opposite.Further,prediction models for deep coal mechanics properties were constructed.Negatively exponent relationships are found between permeability and effective stress,elastic modulus,Poisson＇s ratio,while effect of temperature on permeability depending on coal rank is very complex.Through loading the geology factors via three steps,the prediction model is deduced for deep coalbed permeability by coupling sorption,temperature,geostress and fluid pressure. ＂Correction coefficient method＂ for deep coalbed reservoir pressure prediction is put forward,and ternary prediction indexes and model for reservoiring effects of deep coalbed methane are constructed.Deep coalbed methane reservoiring effect of the eastern Ordos Basin is further analysed,and found three types of advantageous and eighteen less reservoiring combinations are found,and their occurrences in the area are predicted.