High-temperature High-pressure Methane Adsorption And Shale Gas Occurrence In Lower Cambrian Shale,Upper Yangtze Area

Methane adsorption is the core of shale gas occurrence,which is of great significance to shale gas enrichment and accurate estimation of Gas-in-place?GIP?.In this study,Lower Cambrian dark shale from Upper Yangtze area was taken as the research object to study high-temperature high-pressure methane adsorption mechanism and shale gas occurrence in in-situ reservoir.Based on data collection,field investigation,experimental tests,model calculation and theoretical analysis,main conclusions have been drawn as follows:?1?The Lower Cambrian shale is widely distributed in Upper Yangtze area.The sedimentary environment of lower member of the target shale is deep-water to semi deep-water shelf,while that of upper member is shallow water shelf.According to the mineralogical and petrological characteristics,five types of lithofacies are identified,among which the carbon shale and organic-rich siliceous shale are high-quality lithofacies for shale gas reservoir.Mineral composition of the Lower Cambrian shale is complex,mainly including quartz and clay,meanwhile containing feldspar,carbonate minerals and a small amount of pyrite.Clay minerals are mainly illite and illite/smectite mixed layers.Kerogen of the Lower Cambrian shale belongs to type I as the value of carbon isotope(?¹³C)varies from-34.7‰to-31.0‰.The content of total organic carbon?TOC?is high,with an average of 3.65%;meanwhile the TOC content in the lower member is significantly higher than that in the upper member.Laser Raman spectroscopy can be used to characterize the maturity of shale,and the results show that the target shale is overmature.?2?Using specimen observation,microscope and field emission scanning electron microscope?FE-SEM?,the fracture morphology and type of the Lower Cambrian shale are studied qualitatively and semi-quantitatively.Five types of fracture,three types and six sub types of pore were identified,among which organic pores,intergranular pores at the edge of brittle minerals and microfractures are relatively developed.A part of pores have collapsed because of the overmature evolution of organic matter and strong compaction from diagenesis.The full-scale pore structure was quantitatively characterized in combination of high pressure mercury injection,low temperature nitrogen adsorption and carbon dioxide adsorption experiments.The results show that the porosity of the Lower Cambrian shale is low,and the pore size is small with developed micropore and insufficient macropore.The pore size distribution?PSD?exhibited unimoda peak type of micropore and bimodal peak type of micropore and mesopore.The pore structure is characteristized by fractal heterogeneity.For the Lower Cambrian shale undergoing deep burial and overmature evolution,organic matter is the material basis for the development of micro-nano pores,and the rigid skeleton structure built by brittle minerals such as quartz is an important guarantee to avoid the pore from being compacted and damaged in the process of deep burial.?3?High-pressure methane adsorption experiments at a series of temperature were carried out to analyze the quantitative characteristics,influencing factors,theoretical causes and geological significance of"abnormal adsorption"in supercritical methane excess adsorption at high pressures.The applicability of supercritical methane adsorption model was comparative studied.Furthermore,based on calculation of adsorbed phase volume and number of adsorption molecular layers and the analysis of adsorption thermokinetics,the adsorption mechanism of supercritical methane was revealed that methane adsorption in shale reservoir was composed of filling adsorption and monolayer adsorption.Supercritical methane adsorption on shale is temperature dependent,and the reversed temperature effect on excess adsorption occurs at a certain high pressure.With the increase of temperature,the maximum adsorption capacity and adsorbed phase density decrease linearly,while the pressure required for adsorption saturation increases continuously,and thus the adsorption capacity of shale decreases.According to the analysis of controlling factors on methane adsorption,micropores are direct and main factors,and organic matter can promote methane adsorption with higher surface free energy.?4?The accurate models of adsorbed gas and free gas content in in-situ reservoir were established,and the influence of adsorption model selection and calculation method on GIP estimation was analyzed,especially for deep shale gas.The different responses of adsorbed gas and free gas occurrence to various geological factors were illustrated by geological extrapolation of GIP estimation model.The geological application shows that,with the increase of burial depth,the adsorbed gas content first increases and then decreases,while the free gas content first increases rapidly and then increases slowly.There was an equivalence point at which the contents of adsorbed and free gas are equal,and the equivalence point moved to the deep areas with increasing water saturation.Furthermore,the dominant depth zone of shale gas occurrence was proposed,which is in order of shallow zone of adsorbed gas,transition zone between two phase and deep zone of free gas.The dominant depth zone can provide theoretical reference for efficient shale gas exploration and development in different deep reservoirs.?5?The history of burial,heating,and hydrocarbon generation of the Lower Cambrian Qiongzhusi shale in well QJ1 was analyzed,and the main gas filling period is from the late Devonian to the Late Permian.Combined with the analysis of reservoir evolution and storage capacity,four stages of dynamic conversion of shale gas occurrence in the Lower Cambrian shale reservoir are revealed,including immature biogas adsorption,mature pyrolysis gas adsorption,desorption conversion of adsorbed gas,and adsorption conversion of free gas.Further,the dynamic evolution and temporal-spatial matching of gas generation,gas storage and gas preservation were discussed,and geological causes of overpressure,underpressure and undersaturation reservoir were elucidated,respectively.Based on the test of breakthrough pressure,diffusion coefficient and permeability,the self-sealing property of reservoir and the diffusion/migration mode of shale gas were analyzed.Meanwhile,it is clarified that excessive evolution of organic matter and regional floor conditions with poor sealing are the mainly risk factors for the lower Cambrian shale gas enrichment,and we should focus on the moderate thermal evolution of organic matter and favorable preservation conditions in the Lower Cambrian shale gas exploration and development in Upper Yangtez area.There are 120 figures,27 tables and 278 references in this thesis.