Study On Evaluation Method Of Gas Content Of Continental Shale In Area A

Gas content is an important basis for shale gas exploration and development.At present,there are few studies on the evaluation of gas content of continental shale in China.This paper takes the continental shale of typical area A in China as the key study area.Make full use of the data of common logging and special logging,core experiment,argon ion scanning electron microscope and isothermal adsorption experiment which can reflect the characteristics of shale.The factors affecting the gas content of continental shale are discussed.The reservoir parameter model is established.The actual well data processing and error analysis are carried out.A gas content evaluation method suitable for continental shale in area A is formed.The continental shale in area A is dominated by clay and quartz minerals,and the clay minerals are mainly illite and illite/ montmorillonite mixed layer.Both organic and inorganic pores are well developed.The sedimentary environment of continental shale in area A is complex,the sedimentary facies change rapidly and the heterogeneity is strong.There are many limestone and sandstone interlayers in the reservoir.The distribution of carbonate rocks varies greatly in the longitudinal direction.There is a phenomenon of low resistivity in all reservoirs.Due to these reasons,some common methods can not accurately calculate the parameters such as TOC,porosity,saturation and gas content of continental shale in area A.Through the study of mineral composition,richness of organic matter,organic matter maturity and pore structure of continental shale in area A,compared with marine facies and marine-continental transitional facies,it is clear that TOC is the main factor to control adsorbed gas content.The second is porosity.The maturity of organic matter is also one of the factors affecting adsorbed gas content.Factors such as specific surface area provide storage space for gas,and clay minerals also have an effect on the content of adsorbed gas in shale.The continental shale of Daanzhai member in area A is a deep shale reservoir with a depth of more than 3500 m.By comparing it with shallow continental shale,it is further clarified that TOC is the main factor affecting the content of adsorbed gas in shale.When the TOC content is the same,the adsorbed gas content of shallow continental shale is higher than that of deep continental shale.The maturity and pore structure of organic matter will affect the gas content of deep and shallow continental shale.The temperature and pressure of deep shale reservoir is higher than that of shallow shale,which affects the adsorption capacity of shale to methane.Methane adsorption capacity shale content generally decreases with the increase of temperature and increases with the increase of pressure.However,under the condition of reservoir,the adsorption capacity of methane by deep shale is close to saturation,and the influence of pressure change can even be ignored.Therefore,with the increase of temperature,the content of adsorbed gas in deep shale will gradually decrease,and when the total gas content increases,the proportion of adsorbed gas in deep shale will further decrease.The continental shale in area A has complex lithology,strong heterogeneity and low resistivity.It is not suitable to calculate TOC content in A region by conventional multivariate fitting and classical ?log R method.In this paper,on the basis of the original ?log R method,the selection of ? log R baseline is studied according to the overlap characteristics of acoustic wave and resistivity curve.Based on the natural gamma spectroscopy logging data,an improved ?log R method based on natural gamma spectrum curve is proposed.This method improves the accuracy of TOC calculation of continental shale in area A.At the same time,this method can also be well applied to marine shale.After further expansion,a double ?log R method based on natural gamma ray spectrum logging is proposed.This method improves the calculation accuracy of TOC in coal measures shale reservoir.On the basis of Herron porosity calculation formula,combined with element captured spectrum logging.Through the conversion of calcareous and siliceous data in element capture logging,the skeleton density model suitable for A area is re-fitted.A porosity model with variable skeleton parameters based on element captured spectrum logging is proposed.This method improves the accuracy of porosity calculation of continental shale in area A.Good results have also been obtained by using this method to calculate the porosity of marine shale.According to the good correlation between the gas saturation and the dipole array acoustic logging.On the basis of the overlap difference of density neutron logging,the ratio of P-wave and S-wave time difference is combined.A method for calculating gas saturation by overlap difference between P-wave and S-wave time difference and density neutron logging is proposed.This method improves the accuracy of gas saturation calculation of continental shale in area A.This method can also be used to calculate the gas saturation of marine shale.On the basis of the original evaluation method of shale adsorption gas,through the improvement of organic matter correction and TOC calculation accuracy,an adsorption gas evaluation method based on organic matter correction and TOC parameters improvement is proposed.This method improves the accuracy of calculating the adsorbed gas content of continental shale in area A.This research method has been popularized and applied in the exploration well of China Petrochemical Co.,Ltd.,and satisfactory results have been obtained.