Preliminary Evaluation On Shales In Sichuan Basin And Potential For Enhancing Gas Recovery By CO₂ Injection

Enhanced gas recovery?EGR?from shale by CO₂ injection is of significance for the research on Carbon Capture,Utilization and Storage.The study on CO₂-EGR in shale is on the primary stage all over the world and is mainly relied on the numerical simulation.However,the experimental simulation on CO₂ injection into shale and reservoir fluid flow is rarely reported in existed achievements.Based on field investigation and experiments in laboratory,the reservoir characterization was conducted on typical shales from Sichuan basin,including lower Silurian Longmaxi shale,upper Ordovician Wufeng shale and lower Cambrian Niutitang shale.Then experimental simulations were performed by the low-field NMR technique to the mechanism of CO₂-CH₄ interaction in shale and to evacuate the potential of CO₂-EGR in shale..The following conclusions were made,The detailed comparison on petrology,petrophysic and geochemistry of three shales indicates that thermal evolution of organic matter is pretty high and the smectite illitization process is on the final phase of ordered mixed-layer illite/smectite during the diagenesis for all study shales.Among the study three shales,the Niutitang shale has highest content of total organic carbon?TOC?and vitrinite reflectance?R_o?,but lowest porosity,permeability,brittleness index?BI?and gas content.The component of gas in Longmaxi and Wufeng shales is dominated by methane,while that in Niutitang shale is characterized as low methane,high CO₂ and high N₂.The sedimentary and sequence characteristics of study shales were investigated by the information from typical shale gas well,aiming to explore the reservoir heterogeneity of Longmaxi shale and sedimentary mechanism of Niutitang shale.The Longmaxi formation contains two third-order sequences?SQ1 and SQ2?,on the basis of systematic core observations and well logging analyses.The bottom SQ1 section has greater permeability,porosity,BI,TOC and R_o than the upper SQ2 section.Besides,the correlation of permeability and porosity in SQ1 is higher than that in SQ2,revealed the better connection between pores in SQ1 rather than in SQ2.Based on the investigation on vertical heterogeneity,the middle-bottom of SQ1 section is treated as the‘sweet section'in vertical,for its greater potential for hydrocarbon generation,higher permeability,porosity and BI.The sedimentary environment of Niutitang shale was reconstructed and characterized as weak hydrodynamic,deep water and oxygen-deficient reducing.The sediments sources for Niutitang shale were mainly from neritic and continental slope,as well as the suspended sediments in water.Besides,the majority of sedimentary represented a normal seawater environment,but several isolated areas were affected by submarine hydrothermal sedimentation.According to the transverse relaxation of low-field NMR,the T₂<¹ ms?P1?and T₂=¹-50 ms?P2?respectively represent the NMR signals from adsorbed and free methane in shale,which has the ability to identify the different phase of methane in shale.Thus,the NMR T₂ amplitude is able to monitor the variation of methane in different phase in shale.This study suggests that CO₂ can not only enhance the total gas recovery of adsorbed methane from shale but also accelerate the desorption rate of adsorbed methane in shale.The CO₂-EGR efficiency varies in different shale samples with a range of 16.2%³4.3%,with an average of 23.5%.Among all reservoir characteristics,high TOC,high methane adsorption ability,low permeability and low clay minerals content are favorable for CO₂-EGR technique in shale and tend to generate greater CO₂-EGR efficiency.Besides,the quartz content,porosity and R_o have poor correlation with the CO₂-EGR efficiency.