Pore Structure Characteristics And Comprehensive Evaluation Method Study Of Medium-High Rank Coal Reservoir

With the rapid development of China economy, the demand for energy increase constantly. Coalbed methane resources become more and more important as an alternative energy. However, the dual porosity media system of coal reservoir is more complex than conventional sandstone, the studies of pore and pore structure of coal reservoir are consequently lagged behind, which restricts the large-scale development of coalbed methane industry. The pore structure study of medium-high rank coal reservoir contributes to the development and exploration of coalbed methane resources in China.Taking Hancheng area as the study object. The macro and micro composition and industrial composition of medium-high rank coal reservoir were throughly studied by observation, industrial analysis and vitrinite reflectance experiment. Electron microscope scanning showed that pores and fractures were more developed in bright and semi-bright coal compared with semi-durain and dull coal. Pores such as plant tissue hole, hole between chips, stomata and mould pore are more common, fractures such as polycondensation fracture, extensional fracture, shear fracture and compressional fracture were also developed in this area. Based on pressured-mercury experiment results, the intrusive mercury curve was divided into open-seepage type, closed seepage type and open- adsorption type, which respectively accounted for 41.2%, 29.4% and 29.4%. The results of low temperature nitrogen adsorption experiment showed that the higher the bright coal content was, the more developed the micro hole of coal. Micro hole made the biggest contribution to specific surface area, and was helpful to the adsorption of coalbed mehtane. The influence factors of gas content were studied, with the increase of vitrinite reflectivity, the coalbed mehtane content showed low-high-low trend, and reached the highest when Ro was 1.8%. Adsorption gas was positively correlated with fixed carbon content, and negatively correlated with ash content, volatile content and minerals. Through grey correlation analysis, gas content had maximum correlation with fixed carbon, ash took second place, and volatile content had the least influence. According to nuclear magnetic resonance experiment, the T2 spectrum curve of study area was divided into three types: three-peak small pore type, hump type and two-peak large pore type. Curve with three peaks indicated that adsorption pores cannot communicate well with other parts, in which most fluid could pass through freely. Pores with hump type curve had a relatively good connection, and fluid could flow freely. Two-peak large pore type had both adsorption pores for methane accumulation and seepage pores for methane migration, which is the most favorable pore structure type.Each method can only describe the pore structure characteristics from one aspect, therefore, after analyzing the experiment results, 11 parameters were selected, the comprehensive evaluation index system of pore structure was established after standardized processing. The three-level hierarchical structure model was set up using Analytic Hierarchy Process. The weight of each parameter was determined by establishing the comparison matrix. The evaluation results showed that 5# coal seam was the best, 3# coal seam ranked second, and 11# coal seam was relatively poor, which were consistent to the actual situation, therefore, the method can be used for the comprehensive evaluation of coal reservoir pore structure.