Pore Structure Characterization Of Coal-bearing Shale And Its Effect On Shale Gas Occurrence

Shale pore structure characteristic is the core content of reservoir evaluation,which plays significance role in investigating the generation conditions,aggregation mechanism and occurrence rules of shale gas.The theoretical research on pore structure and gas bearing property of coal-bearing shale needs to be further improved.In this work,drilling samples from several sets of coal-bearing shale in typical basins in China were taken as the research object,and comprehensively methods such as field investigation,experimental analysis,image processing and numerical simulation were taken to clarify the structural characteristics and evolution rules of coal-bearing shale and their influence on the occurrence of shale gas.The main achievements and understanding of the work are as follows:Based on the field exploration and drilling data,it is concluded that coal-bearing shales in China have a wide area,multi-era distribution,with single thin layer of<200 m and buried depth of<2000 m mostly.According to the organic geochemistry experiments(including TOC content determination,rock-eval pyrolysis analysis,vitrinite reflectance Ro determination,and maceral quantification)and X-ray powder diffraction experiments,the results show that:The coal-bearing shales are rich in organic matters with the TOC content of 0.9%?38.8%(mostly of>2.0%);most of ? type,with the ?13C of-26.7‰--22.5‰.Organic matter maturity range is large with Ro value of 0.5%-3.5%and Tmax of 317-542?.The clay mineral in coal-bearing shales is rich with the content range between 60%and 80%,in which the illite/smectite mixed layer is the major mineral;while the brittle mineral content is relatively low,mostly of<40%.Different kinds of shale pores were discriminated by the SEM/FE-SEM experiments and the pore classification and extraction method.In this work,the specific genetic mechanism,geometric morphology and spatial distribution characteristics of different types of shale pores were investigated.It was proposed that the pores with the diameters of 6-80 nm,200-350 nm and 6-10 m are well developed,which accounted for 46.5%,13.7%and 27.5%of the total pore volume,respectively.Interparticle pores and organic matter pores are the major pore types in the pore system,with the average visual porosity value of 5.9%and 5.2%,respectively.In addition,the petrophysical model was proposed to realize the quantitative characterization of various types of pores.The porosity of pores in organic matter layer,brittle mineral layer and clay layer decreased successively,with the porosity value of 1.52%,1.36%and 1.09%,respectively.Based on the fluid injection technologies combined high pressure mercury injection(HMIP),low pressure N2 and CO2 adsorption/desorption experiments,a whole aperture shale pore characterization model was proposed.Based on that,it was found that a large number of nanoscale pores are well developed in coal-bearing shales,with the average pore size of 20.2-135.0 nm;and pores with the larger size has the higher volume contribution,while the pore specific surface area contribution rate is on the contrary.the micro-and meso-pores predominate in the nanoscale pore network,with the pore volume contribution rate of 56.45%and 41.06%and surface are contribution rate of 41.30%and 46.48%.,respectively.In addition,type B,C and E hysteresis loops were formed in the N2 adsorption-desorption curves,indicating that mixed pore shapes such as slit shape,conical shape and ink bottle shape were the main development forms of shale pores,and these pores developed with god openness.In combination with N2 adsorption results and FHH model,the quantitative characterization of pore heterogeneity of coal shale was completed.The fractal dimension D1 and D2 values of shale pores are 2.156-2.563 and 2.674-2.773,respectively.The results demonstrate that the fractal characteristic of shale pores is obvious,the interior has the higher complexity and heterogeneity than the surface.Moreover,by combining with the characteristics of organic matters,minerals and pore structures and diagenesis process,and parameter fitting analysis experiment,the main controlling factors affecting pore structure development and fractal characteristics of coal-bearing shales were clarified.It was found that organic matter directly controlled the development of micropores,while clay minerals mainly affected the development of macropores.Compaction and cementation reduce pore space,whereas dissolution expands pore space.The effect of pores with smaller size on pore fractal characteristics was stronger,and the enrichment of clay minerals and organic matters increases pore heterogeneity.Based on the high-pressure thermal simulation experiments and natural evolution sample analysis experiments,combined with pore qualitative and quantitative characterization methods and reservoir organic matter and mineral analysis technologies,the structural evolution law of different types and scales of pores in coal-bearing shale was systematically clarified,and the five-stage pore evolution model of coal-bearing shale was constructed.(a)Stage I(Ro<0.7%):macropores are dominant,and the interparticle pores formed by the original shale matrix loosely packed are rapidly compacted and reduced under the action of-formation pressure.(b)Stage ?(0.7%<Ro<1.3%):large amount of organic matter micropores and mesopores are formed during the organic hydrocarbon generation.Meanwhile,the organic acid produced in the hydrocarbon generation will corrode the soluble minerals and formed a large number of dissolution pores.(c)Stage ?(1.3%<Ro<2.2%):After the first pyrolysis,the oil was filled with organic micropores and mesoporous,and the micropores and mesoporous were compacted and reduced.(d)Stage ?(2.2%<Ro<2.7%):Due to the secondary cracking of organic matter,new organic matter micropores and mesopores and new dissolve pores are formed.(f)Stage V(Ro>2.7%):Compaction slows down.and the pore of organic matter is gradually destroyed by carbonization.Combined with the molecular dynamics and rock and mineral composition model,based on the CH4 isothermal adsorption experiment,the coal-bearing shale gas content analysis and prediction model was established.It was revealed that the adsorption gas in coal-bearing shales was do1inant,with the content of 1.1-4.5 m3/t,accounting for 50%-85%.Shale gas occurrence characteristics simulation and prediction model is established,and the shale gas occurrence characteristics of adsorption and free gas in the different scales of shale pores are simulated and analyzed.The pore size distribution is of great significance to gas occurrence.It revealed that merely adsorption gas occurrence in the micropores.with the pore size increases,the adsorption volume is reduced and free gas content increased.At 3.5 nm approximately,the content of two phases of shale gas is nearly equal;and at 6.5 nm approximately,the content of the adsorbed gas content is low(<0.01 m3/t)and free gas content is stable.Combined the data of organic geochelical and mineral composition,the main controlling factors of coal-bearing shale gas enrichment and occurrence were clarified.It revealed that the abundance of organic matters and clays positively affect the adsorption and total gas content.The pore size in brittle mineral layer is large,which is beneficial to the occurrence of free gas.