Modelling On The Emission Process Of Gas In Shale And Analysis On Its Applications

Gas content is an important indicator for shale gas resource assessment.Studying on the emission process of gas in shale is a significant reference to the prediction of lost gas,which is the key to the precise prediction of gas content.The paper analyzes the main physical process of gas emission,and discusses the mechanism and process of both free gas and adsorbed gas when the core sample is being lifted in the drilling hole.Via field desorption curve,emission rate is analyzed and the relationship between mathematical model and geological parameters is founded so that the model could be applied on real formations.Via field desorption curve,a parameter about gas-bearing structure of the sample can be determined.The gas-bearing structure can be divided into free-type,adsorbtype and hybrid-type.Because of the compressed state under high pressure that the free gas in the formation is,the desorption of original adsorbed gas lags behind the diffusion of free gas.The free-type has a large amount of emitted volume in normal and formation temperature stage;the adsorb-type has scarcely any emission happened;while the curve pattern of hybrid-type is between the former two types.Shale gas emission rate curve can be obtained by shale gas field desorption curve while samples with three different gas-bearing structure have different shapes of emission rate curve It can be learned that shale gas emission rate curve of the process is at first increased and then decreased,which is a skewed curve.The main geological factors of gas loss process in shale are total organic carbon content,maturity,mineral composition and permeability,etc.It can be learned that total organic carbon content and permeability is positively related to the average emission rate.The main engineering factors are temperature,pressure,coring method,the drilling fluid,etc.The higher the temperature the greater the emission rate.Coring method indirectly affect emission process by influencing the emission time.Via time correction process,the shape of shale gas emission rate curve could be approximately fitted by two symmetrical curve fitting model.The quadratic curve fitting model suits for long-time and slow-speed gas emission of free-type or hybridtype samples;while Gauss function fitting model for short-time and rapid gas emission of adsorb-type or hybrid-type.Work area for prediction is the carbon containing argillaceous dolomite in Lower Sinian Doushantuo formation of#ZD01.The calculated lost gas content of quadratic curve model ranges in 0.0016-0.3596m3/t;while 0.0102-0.1214m3/t for Gauss function model.Comparing with USBM direct method,the method of fitting curve model can effectively take free gas content of the original formation into calculating.