The Experimental Researches And Well Logging Evaluations Of Marine Shale Gas-bearing Properties

Gas-bearing property is an essential aspect of logging evaluation on shale"seven properties".The calculation of adsorbed gas and free gas is the key to evaluating shale gas content,and gas saturation is the core parameter for free gas calculation.In this paper,the marine shale of Wufeng formation and Longmaxi formation in Jiaoshiba and Dingshan areas of Sichuan Basin is the research target.Given that the logging characterization of Langmuir volume and pressure is simple in the traditional adsorbed gas evaluation model,the new logging technology is seldom used to evaluate shale gas content,and gas saturation calculated by saturation model of argillaceous sandstone has a large error by comparing with the actual value.By performing the isothermal adsorption experiments,NMR experiments,resistivity influencing factors experiments,and rock conductivity experiments,the calculation methods of gas content and gas saturation are established.The research results can provide technical support for the logging evaluation of marine shale gas-bearing properties.In order to understand the petrophysical characteristics of marine shale,the lithology and mineral composition,physical properties and pore structure,geochemistry,formation fluid and logging curve characteristics of marine shale were analyzed by combinig core data,experimental data and logging data.The results show that shale’s mineral compositions include quartz,feldspar,calcite,dolomite,pyrite,and clay minerals.Clay minerals are mainly composed of illite/smectite,illite and chlorite.The shale has various types of storage space.The average radius of the shale is small,and the specific surface area is large.The average porosity is 4.27%.The type of organic matter is mainly I and II₁.The average TOC is 2.54%.The average of kerogen equivalent vitrinite reflectance（Ro）is 2.7%.The average water saturation is 42.36%,and the gas content of shale increases from Long 2 Member to the Wufeng formation.The total gas content of the Long 1 first sub-member and Wufeng formation ranges from 2.1 to 4.02 m³/t.The logging curves of shale show the characteristics of"four highs and three lows".In order to construct an adsorbed gas calculation model that comprehensively considers the effect of various factors,the isothermal adsorption experiment of shale was carried out,and the effect of TOC,clay minerals,temperature and water saturation on adsorption was systematically studied.An adsorption model of three-components that include organic matter,illite/smectite,illite and chlorite was established.In this model,each component’s adsorption capacity is calculated by the Langmuir model.The Langmuir volume of organic matter is related to TOC,organic matter maturity and temperature,the Langmuir pressure of organic matter is related to TOC and temperature;the Langmuir volume of clay minerals is related to clay content and temperature,and the Langmuir pressure of clay minerals is related to temperature.The adsorbed gas content of two wells was calculated by using logging curves.The result shows that the adsorbed gas of clay accounts for about 13%of the total adsorption content of shale.Therefore the contribution of clay to shale adsorption should not be ignored.With the increase of depth,the negative effect of temperature on adsorption increases,and the content and proportion of adsorbed gas decreases.More attention should be paid to the evaluation of free gas in deep shale gas.In order to explore the application of NMR logging on the evaluation of shale gas-bearing properties,one-dimensional and two-dimensional NMR experiments of shale under oil-saturated,drying,and water-saturated were carried out.By comparing the NMR T₂ spectrum under different fluid saturation states,The T₂ spectrum of shale was divided into two types.The T₂ spectrum of shale with large organic pore shows obvious three-peak,and the other without large organic pore is a single peak.With 3 ms as the T₂ cut-off value,a model to calculate shale gas saturation by using a one-dimensional NMR T₂ spectrum is established based on the gas bearing assumption.The error between the calculated value and the core analysis value is less than 10%.The gas content of shale with a high proportion of large-scale organic pore and low NMR wettability index is high.For the first time,the relation between daily production of a shale gas well and the volume of large-scale organic pore and NMR wettability index has been established,which provides a quick and new method for evaluating shale gas production.Based on the two-dimensional NMR T₁-T₂ measurement,a new fluid distribution map of shale is proposed,a calculation model of gas saturation is established based on this map.For the first time,the correction mode of shale lost gas is divided by combining NMR T₂ spectrum characteristics and the monomial fitting slope K of first-order desorption data.When K is less than 1000,the lost gas calculated by traditional model does not need to be corrected.When K is greater than 1000,a new lost gas calculation model that includes organic carbon content,T₂geometric average,loss time,formation pressure,temperature and NMR wettability index must be constructed.The processing result of well data shows that the new model significantly improves the calculation accuracy of lost gas,which is of great significance to the calibration of shale gas content and the re-understanding of shale gas resources.In view of that,there are low resistivity shales in the study area;the traditional saturation model of argillaceous sandstone has low accuracy in calculating the gas saturation of shale.The influencing factors of shale resistivity were systematically analyzed,and the research on the conductivity law of shale was conducted on the scale of meters,centimeters,and nanometers.The order of the influencing factors of shale resistivity is,conductive organic matter>low resistance thin layer>clay mineral>pore water>pyrite.A new conductive model of shale was constructed.In the model,the high resistance layer and low resistance layer are connected in parallel;the organic matter,pyrite,clay and pore water are mixed to conduct electricity in the high resistance layer.Based on the new conductivity model,a method is proposed to calculate gas saturation by gradually stripping the influencing factors of resistivity.The processing results of two wells show that the error between the calculated saturation by the new method and the core analysis value is the smallest,and the method has a good application prospect.