Rock Physics And Physical Modeling Of The Coal-bed Methane In Qingshui Basin

Coal bed Methane(CBM) is a form of natural gas extracted from coal beds. In recent decades it has become an important source of energy in the United States, Canada, Australia, and other countries.The term refers to methane adsorbed into the solid matrix of the coal. Coalbed methane is distinct from a typical sandstone or other conventional gas reservoir, as the methane is stored within the coal by a process called adsorption. The Qingshui basin is one of the most concentration zones of Coal bed Methane in China. It is also a large coal seam gas field with higher commercialization level, great exploitation potential and higher exploration degree. There are more than1000CBM wells in the basin, and the productivity of the southern basin achieve to500,000,000to600,000,000m3per year.CBM is adsorbed to the surface of the coal. The adsorption sites can store commercial quantities of gas as part of the coal matrix. It is different to conventional petroleum resource, because in conventional reservoirs, oil or gas is made in the source rock, migrates to the reservoir rock, and its migration is stopped by cap rock. The reservoir-forming mechanism and development technology are important for the exploration and development of the coal bed gas which are quite different to the conventional gas.The research on reservoir genetic feature of coal bed methane is not very much. Domestic researchers have done some work, but there is no reservoir genetic theory to guide coal bed methane exploration.This study is dedicated to find the rock physical characteristics of coal bed methane enrichment layer. The author completed a lot of work on rock physical tests, which explain the characteristics of main coal beds in the Qingshui basin in terms of elastic parameters, fracture structure, anisotropy, heterogeneity and attenuation. This work is helpful to solve the problem in seismic exploration in coal bed methane.First, a series of tests are completed to obtain the characteristics of coal in the Qingshui basin. The variation range of coal metamorphic grade covers from low rank coal to high rank coal. The results show that coal samples are characterized with low density, low seismic velocity and low Poisson’s ratio. Velocities of compressional wave and shear wave are increased with the increase of confining pressure under a stable temperature. However, the increasing amplitude of compressional wave is higher than that of shear wave. Vp/Vs ratio increased with increasing of confining pressure up to25MPa. Micro-structure and anisotropic characteristics are measured to study the seismic wave attenuation.Second, we develop a series of isothermal adsorption tests to build a test process for bulk samples. Then, the author evaluate four theoretical models for bulk coal adsorption, such as Langmuir model, Freundlich empirical formula, BET model and Dubinin-Radushkevich mode. The results are used to fit a curve to quantify pressure vs adsorption quantity. As a result, the Langmuir model is the most fitable model for adsorption quantity calculation. Ultrasonic wave velocity tests are completed to study relationship between adsorption quantity, then the velocity of samples are built.Third, several seismic physical models are built for simulating the typical structure of coal bed in the Qingshui basin. Analysis for the seismic response of thin seam, small layer and collapse column are carried out based on the seismic data of physical model. The reflection wave of thin interbed is more complex than uniform coal seam. Because the reflection wave is usually a composite wave subjected to many factors like thickness of coal seam, number of layers. Understanding of the characteristics of reflected wave is premise for coal seam seismic interpretation. Resolution of coal seam thickness by using frequency tuning curves is higher, because of the low velocity and apparent wavelength of coal seam. When the thickness of single coal layer is smaller than A/8(λ is the wavelength), it is practicable to predict total thickness of coal seam by seismic wave amplitude, although it is difficult to identify thickness of single layer.