Coalbed Methane Reserve Mechanism And Insitu Gas Content Modeling In Low Rank Coal Reservoir

This dissertation is based on the Key Project of the National Science &Technology(2011ZX05034-001), and supported by the research teams at home and abroad that benefited by the Joint PhD program founded by the China Scholarship Council. The foundation of theory and knowledge to study the issue of “In-situ gas content oflow rank coal” were completed. Therefore, this study focused on the pore structure in low rank coal, mechanism of gas storage in low rank coal, in-situ gas content in low rank coal and its influence factors.Various techniques and ways were used to characterize the pore structure of low rank coalsin different levels.The properties of pores with size less than 100 nm were studied by using low temperature liquid nitrogen,and the pores with size more than 100 nm were characterized by mercury injection experiment. Nuclear magnetic resonance and CT scan were used to study wider range of pores and fractures in coal than the two ways mentioned above, which also was nondestructive to the samples. SEM qualitatively analyzed the characteristics of the occurrence of pores, fractures, and minerals. XRD and EDX quantitatively determined the mineral types and contents in coal.Based on wellknowledge of the pore structure characteristics of coal, a continuous distribution model of pore and fracture is constructed. The model overcomes the weakness that the different testing methods can only test the limited range of pores and data from different methodscould not be used synthetically. The pore size distribution were from the nano scale to micron level, and verified by a variety of test data.This model can be used to calibrate the porosity and water content, and to refine the storage space of low rank coal.The mechanism of gas storage more suitable for low rank coal reservoirs was presented, basing on the characteristics of gas adsorptionand free gas in low rank coal.It showed that the main control mechanism was affected by the pore structure(combination of various size of pores), and with the increase of pressure, the micropores achieved pore filling state extremely fast(which can be considered as a monolayer adsorption saturation state), meanwhile, mesopores and macropores were still in monolayer adsorption unsaturation state. The free gas storagewas dominated by the space for free gas, and the key competitors were adsorption gas and moisture. Matrix pores are the main space for free gas, and the matrix adsorption pores contained adsorption gas, free gas and moisture, fractures were mainly occupied by water, with few and ignorable free gas.As the existing free gas content calculation model based on limited understanding of gas storage mechanism and used inaccurate porosity data, in this study, a new free gas content calculation model was proposed. It was on the basis of suitable mechanism for low rank coal and the new continuous distribution model for pore and fracture. In new calculation model, the effect of different levels of porosity and moisture content on the free gas storage space was considered.The in-situ gas content of low rank coal was modeled by the new calculation model. The results showed that samples with lower adsorption gas content or with pore structure dominated by mesopores and macropores would have higher free gas content. With the increase of pressure, the free gas content can be more than 50% of the total gas content in low rank coal.The pore structure, coal rank, maceral composition, moisture and mineral were positively correlated with the content of adsorbed gas, but negatively correlated with the content of free gas. The adsorption gas content had strongest positive relationship with the clay mineral content, and the free gas content had strongest positive relationship withthe fixed carbon content. Pore structure has both strong correlation to adsorption gas content and free gas content.