Study On The Calculation Method Of Shale Gas Content

Gas content is an important basis for shale gas reserve evaluation,exploratory development and production forecast In this paper,the calculation of shale gas content is carried out from two aspects: direct method(in-situ gas-desorption)and indirect method(isothermal adsorption combined with experiment or well log).For the indirect method to calculate the adsorption gas content,based on the adsorption potential theory,considering the influence of temperature on shale adsorption,combining with the isothermal adsorption experimental data,a supercritical methane isotherm adsorption model was established,which can be used to predict the adsorption isotherms at different temperatures.In order to verify the accuracy of the model,the shale samples were selected for isothermal adsorption test,and the differences between the predicted values and measured values were compared.The established shale gas adsorption model has a good application effect in the isothermal adsorption of actual shale samples,indicating that the model can correctly characterize the isothermal adsorption law of actual shale samples under reservoir conditions.For the indirect method to calculate the free gas content,the effects of the adsorbed methane in organic pores and the water film in inorganic pores on free gas were considered.(1)The methane adsorption layer thickness in shale organic pores under reservoir conditions was calculated according to the adsorption phase volume.The calculation results show that the thickness of methane adsorption layer is small and negligible compared to the pore radius.(2)Based on the Derjaguin – Landau – Verwey-Overbee(DLVO)theory,a theoretical calculation model of water film thickness and disjoining pressure was established.In order to verify the practical application effect of the model,the relevant experimental data of predecessors were selected to calculate the theoretical value of the water film thickness according to the model.The theoretical value of water film thickness calculated by the model has a small error with the actual measured water film thickness by predecessors,indicating that the model has a high calculation accuracy.In the direct method,the applicability of the two mathematical models is analyzed for the methane gas escape in shale core lifting in the wellbore.Since the pressure on the core surface changes greatly during the lifting process and the apparent transport coefficient changes with time,the seepage model is not applicable to the gas loss process of shale in the lifting process.In this paper,the methane flow in shale core is simplified into a diffusion model.Considering the non-uniform decrease of the pressure on core surface,a complete calculation model of shale lost gas is established by solving a partial differential equations.Combined with the measured desorption data of samples,the optimization method is adopted to solve the lost gas volume and total gas content.