Study On AHP-EWM Coupling Model Evaluation Of Heterogeneity In Deformed Coal Reservoirs

The physical properties of coal reservoirs are the core problem of coalbed methane reservoirs,while the physical properties of coal reservoirs often show heterogeneity.It is of great significance to study the heterogeneity characteristics of coal reservoirs for realizing the large-scale exploitation of coalbed methane in China.Qianxi is rich in coal resources and is the most important coal industrial base in southern China.The geological conditions of coalbed methane in Qianxi area are characterized by"one weak,two more,three high and four big",and the physical properties of coal reservoirs show obvious heterogeneity,which seriously restricts the exploration and development of coalbed methane.Therefore,this thesis takes Longtan Formation coal in the coalbed methane block of Panguan syncline in western Guizhou as the research object,takes structural coal reservoir heterogeneity as the main line,and combines the structural development characteristics,coal reservoir pore fracture development characteristics and chemical structure characteristics to carry out quantitative evaluation research on coal heterogeneity with different deformation degrees.On this basis,it further discusses and establishes a comprehensive evaluation method of coal reservoir heterogeneity suitable for the study area,and obtains the following main understandings:1)With the increase of coal crushing degree,the micro-fracture density of structural coal gradually increases,and more than two groups of dominant fractures are developed in different types of structural coal samples,especially the dominant fractures（Group I）with higher metamorphic degree of crushed mottled coal and mylonite coal have an average density of more than 50 fractures/cm²,and the average fracture density of Group II also reaches 20-40 fractures/cm².Compared with the macroscopic fracture characteristics of coal,the quantitative observation method of microscopic fracture of structural coal can more effectively characterize the heterogeneity of fracture development of different types of structural coal.2)The fractal results of coal pores show that with the increase of coal deformation degree,the fractal dimension of adsorption pores（Sierpinski model）increases,while that of seepage pores（Sierpinski model）decreases.The fractal dimension at 2～6 nm of mesopores increases in the stage of crushing to flaky coal,but gradually increases in the stage of ductile deformation.The fractal dimension of mesopores at 6～100 nm increases during plastic deformation.According to the carbon dioxide specific surface area and FHH fractal model,it can be seen that the FHH fractal dimension of 0.6～1.4nm micropores increases from the primary structural coal to the broken coal stage,and gradually increases in the ductile deformation stage.The fractal dimension of FHH of0.3～0.6 nm micropores is close to 3,and FHH does not change obviously with structural deformation,which indicates that the heterogeneity of smaller micropores is stronger than that of larger micropores.3)The change of aromaticity can be divided into two stages:brittle deformation and ductile deformation:brittle deformation increases slightly from flaky coal to flaky coal;The ductile deformation sequence increases rapidly from crumpled coal to mylonite coal.The evolution of aromatic structure confirms that the fat structure decreases with the increase of tectonic stress.The ratio of CH₂/CH₃ decreased rapidly in the ductile deformation sequence,indicating the rapid cleavage of fat chain.The change trend of oxygen-containing functional groups in coal shows that the stable components gradually increase,while the unstable components decrease or even disappear.d（G-D1）of structural coal with different deformation degrees decreases in the early stage of weak deformation stage,increases in the late stage of weak deformation stage,and increases more obviously in the strong deformation stage;A_G/A_D1 increases with the increase of deformation degree,indicating that structural deformation makes the intensity of G peak weaken gradually.With the enhancement of structural deformation,L_a decreases gradually in the primary structure coal-flaky coal stage,and increases gradually in the flaky coal-broken spot coal-crumpled coal-mylonite coal stage,indicating that the ductility of aromatic layers decreases in the flaky coal stage.The increasing trend in the stages of toughness,strong brittleness and ductile deformation indicates the occurrence of stress polycondensation.Especially in the stage of ductile deformation,the accumulation of strain energy promotes aromatization and makes L_a rise rapidly.4)The AHP-EWM coupling evaluation model of deformed coal reservoir is constructed.combined with the non-uniform index method,the heterogeneity of deformed coal reservoir in Panguan syncline is comprehensively evaluated.It is pointed out that the pore fissure characteristic index of coal contributes the most to the heterogeneity of coal reservoir,followed by the metamorphic degree and chemical structure of coal.With the increase of coal deformation degree,the comprehensive index of heterogeneity of coal reservoir continues to increase;The heterogeneous types of coal reservoir are reasonably divided,that is,class I（I≤0.225）corresponds to the primary structure coal and cataclastic coal in the brittle deformation series coal;Class II coal（0.225<I≤0.335）corresponds to flaky coal in brittle deformation series;Class III coal（0.335<I≤0.48）corresponds to broken spot coal in brittle deformation series coal;Class IV Coal（0.48<I≤0.705）corresponds to the crumpled coal in the ductile deformation series;Class V coal（0.705<I≤0.99）corresponds to mylonite coal in ductile deformation series,and it is pointed out that the critical value of comprehensive index I of heterogeneity of brittle and ductile deformation coal is 0.48.The thesis has 69 figures,23 tables,and 323 references.