Theoretical And Experimental Study Of Capillary Pressure And Relative Permeability In Coal System

As the coal bed methane reservoir rock,coal has special properties.Coal has low porosity and low permeability.It is difficult to measure capillary pressure and relative permeability in coal-two phase fluids system;however they are indispensable parameters in reservoir simulation and production forecasting,therefore it is necessary to study capillary pressure mathematical models and calculating relative permeability from capillary pressure.In this thesis,effective porosity of coal were calculated by measuring bulk volume and pore volume of coal core plug,the results shown that effective porosity of coal is extremely low,lower than 5 percent averagely,in the meaning time,other methods about obtaining effective porosity are presented.Absolute permeability of coal was measured by nitrogen;man-made core plug was measured by distilled water in order to verify the experimental apparatus.Absolute permeabilities were obtained by linear fitting method.The results showed that absolute permeability of coal is also extremely low,micro-Darcy can be used.Capillary pressure of six coal samples were measured through mercury intrusion method,these data were normalized first,then fitted using the Brooks-Corey model,van Genuchten model and Li model,the core method of fitting is Solver in excel and least square method.The fitting results shown that B-C model cannot match the capillary pressure data of our core samples,the degrees of fitting by Li model are much better than those by V-G model.Li capillary pressure model is derived from fractal geometry.Details of the derivation process of Li model were presented.Even better fitting results were obtained by adding a parameter in Li model.Unsteady-state method of measuring two phase fluid relative permeability and JBN related methods were studied.The displacement experimental data of Jones et al were calculated in JBN related method,the calculated relative permeability were almost the same as those obtained by Jones et al,Jones' data were used in Eclipse and a "Visual C++program for numerical simulation,the modeled results of cumulative water injection rate,cumulative water and oil production rate and pressure difference showed good consistency with those obtained by Jones et al.Relative permeability of six coal samples were calculated through corresponding relative permeability models,an explanation of the calculated wetting phase relative permeability was presented,during the mercury intrusion process,wetting phase is compressed while non-wetting phase is flowing.This study helps to understand the law of coal-fluid systems,contribute to the efficiently development of CBM.