| Shale contain abundant pore and fracture,the nanoscale pore,with high specific surface area and strong adsorption potential energy,provide a large number of adsorption sites for gas adsorption and adsorption space,which plays an important role in the accumulation of shale gas.Therefore,making the characteristics and the development state of micro pore system and fracture certain are crucial,which is not only benefit to deepen the understanding the accumulation mechanism of shale gas,but also conducive to promoting the understanding the exploration evaluation and development of shale gas reservoir.In addition,engineers and researchers are not only pay close attention to the mineral composition of shale,the space types and the physical properties of reservoir,but also concern about the mechanical characteristics and its parameters of shale for judging in line with the fracturing of rock physical standards or not.The shale from Sichuan basin is tested with total organic carbon(TOC)analyzer,X-ray diffractometer(XRD),X-ray spectrometer(XRF),scanning electron microscope(SEM),low temperature carbon dioxide(CO2)adsorption and low temperature nitrogen(N2)adsorption analyzer.The TOC contents,minral composition,mineral elements and the micro pore structure of shale were studied in detail.The results indicate that the shale studied have high TOC contents with 1.41~6.41 wt%,the main mineral in shale are quartz,clay minerals and carbonate minerals.The pore system can be divided into four categories: organic pore,intragranular pore,intergranular pore and microfracture,among them the proportion of micropore and mesoporous pore are larget with about 80%.The surface of shale have obvious fractal characteristics.The low temperature CO2 absorption isotherms of shale samples are type I.The volume of micropore are 0.5 cm3/100 mg,the specific surface area are between 14 to 20 m2/g,and the distribution of pore diameter have multi-peak.The low temperature nitrogen absorption isotherms of shale samples are type II.The absorption loopsare close to type H3 and H4,the pore structure of shale consist with cheese pores,ink bottle pores and other opening pores.The hysteresis coefficients of shale under the low nitrogen have significant negative correlation with TOC contents.The pore volume of shale based on the low temperature nitrogen absorption are 0.734~0.2199 cm3/100 g and the BET specific surface area are 4.77~21.731 m2/g.The mesopore hold the largest proportion with more than 43%,the micropore rank second and then the macropore.The total specific surface area and the total pore volume of shale show obvious positive correlation with the TOC contents,in addition,the specifical surface area of micropore and large pore,the pore volume of micropore and mesopore are all in good positive correlation with the TOC contents.The 3D pore system model of in-situ shale was constructed in this study with argon ion polishing,ZEISS focused ion beam and scanning electron microscopy(FIB-SEM)and AVIZO 3D software.The results show that the internal pore structure of shale interconnect the whole three-dimensional region and have a certain amount of isolated pores,while the connected pore have obvious anisotropy.The total porosity of the region in shale is 0.073 and the effective porosity is 0.056.The pore diameter range from 0.012 to 0.817 ?m with the average is 0.247 ?m,and the pore size distribution is the signle peak mode with the peak value is near 200 nm.The accumulative pore volumes show a monotonous increasing trend,and the frequency of throat diameter and throat length all show unimodal mode,and the coordination number of the pore is mainly concentrated on 2-6.The absolute permeability of each flowing direction are difference with the average absolute permeability are 1.73 x 10-6 m2,and the simulated permeability of the miro-pore systemof shale reservoir can be verified by kozeny-carman equation.The 3D velocity streamline are adopted to simulate the flow state and the ditribution of fluid in micro-pore system,besides,the pressure distribution of fluid in micro-pore system are also been analyzed.The volume method was adopted to measure the adsorption-desorption characteristics of methane and carbon dioxide on shale.The results show that the adsorption and desorption isothermal curves of methane and carbon dioxide were I curves,and the Langmuir model and the Desorption model could fit the adsorption and desorption curves well,respectively.The maximum adsorption capacity of methane on 8 shale samples is range from 1.8 to 3.7 cm3/g STP,while the maximum adsorption capacity of carbon dioxide are range from 3.7 to 9.4 cm3/g STP under the temperature at 45?.The adsorption-desorption curves have obvious hysteresis,and the hysteresis coefficient(HI)are used to characterize the degree of adsorption-desorption hysteresis.The hysteresis coefficient of carbon dioxide adsorption-desorption(HICO2)are larger than that of methane(HICH4),the lareger the hysteresis coefficient is,the more obvious the hysteresis phenomenon is.Based on the Desorption equation,the desorption curves of methane on shale are divided into four stages: low efficiency desorption stage,slow desorption stage,rapid desorption stage and sensitive desorption stage by the definition and derivation of the desorption efficiency.Moreover,the posistively trends between the adsorption of methane on shale and the TOC contents,the total specific surface and the total pore volume are found based on the methane adsorption data.The basic mechanical properties and the acoustic emission evolution characteristics of shale with different bedding angles are studied through uniaxial compression test and the Brazilian splitting test.The results show that the the stress-strain curves of shale under the uniaxial compression are typical type III curves with obvious brittleness characteristics.The shale with different bedding angles have weak layer characteristics,the uniaxial compressive strength of shale reach maximum value when the bedding angle is 90°,while the minimum uniaxial compressive strength appear at 30°.The failure pattern of shale at uniaxial compression test show as tensile splitting mode when the bedding angle is 0°,and the shear slip failure pattern appear at the bedding angle of 30° and 60°,while at the bedding of 90°,the shear and tensile comprehensive failure pattern are present.The cumulative AE counts curve shows a "gentle and steep increasing" mode at 0°,and a "stepped" growth mode at 30°,60° and 90°.Based on the acoustic emission amplitude analysis,the distribution of the low amplitude are founded throughout the process of uniaxial compression failure,and the medium and high amplitude acoustic emission signals are mainly produced near the marco cracks.The proportion of the low and medium amplitude acoustic emission signals are all more than 70%,and the ratio of the high amplitude are the lowest.The tensile strength of shale at the vertical direction to the bedding are obviously greater than that of shale at the parallel direction to the bedding in the Brazilian splitting test,and the failure mode can be divided into three types: tensile splitting failure pattern at the bedding surface,shear tensile comprehensivefailure pattern at the surface of matrix and bedding,and tensile splitting failure pattern at matrix.The cumulative acoustic emission curves show a "gentle and steep increasing" mode at 0° and 30° in the Brazilian spilitting test,while the distribution of acoustic emission signal at 60° and 90° are more scattered.The proportion of the medium and high amplitude acoustic emission signals are relatively larger in the vicinity of the peak stress,and the cumulative AE counts curves show as jumping "stepped" growth mode. |
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