Li Xiangfang

Review on desorption-diffusion-flow model of coal-bed methane

Literature reviews on matrix pores classification, gas and water phase behavior characteristics, kinetic behavior of methane in coal matrix pores, desorption model, diffusion model, percolation model, and gas production model of Coal-bed Methane (CBM) are conducted. It is concluded that the production process of CBM includes adsorbed gas desorbing, a small amount of desorbed gas dissolving into water and diffusing, a large amount of gas nuclei escaping after saturated, gas bubble and column forming, gas bubbles and columns percolating in matrix pores, free gas cross-flowing from matrix pores to cleat-fracture system, free gas flowing from matrix pores to wellbore (in case of low stage coal seam), and free gas percolating from cleat-fracture system to wellbore. As a reference to development of CBM and a theoretical basis on numeric simulation of CBM, it is valuable to the better understanding of gas production discipline of CBM.

A mathematic model for characterizing the relationship between coal reservoir pressure and fluid saturation

At present, the research on the gas water two-phase flow during the development process of coalbed methane reservoirs (CMB) is still weak. Numerous production prediction models and transient production data analysis methods have been developed. However, part of these previous models assume the single water or gas phase within the coal seams, the other part overlook the gradient of fluid saturation and pressure and another part investigate the effect of fluid saturation distribution on productivity by utilizing the concept of average saturation, which is obtained through the derivation of material balance equation in CMB reservoirs. Notably, the average saturation cannot capture the feature of saturation distribution in the coal seams. Therefore, the existed models do not take into account the influence of fluid saturation distribution characteristics on gas well productivity, which result in relatively large error between the present productivity model and the actual CMB wells. In these regards, according to the gas and water two-phase seepage differential equation, considering the stress sensitivity, matrix shrinkage, gas adsorption/desorption effect, this research establishes a mathematical model characterizing the relationship between formation pressure and saturation. Combined with gas-water two-phase relative permeability curve, the model can be solved to obtain the saturation distribution on the basis of reservoir pressure distribution. Moreover, the reliability of the model is verified against commercial numerical simulation software with excellent agreements. The proposed model fills the gap of CMB development theory, and significantly contributes to the precise calculation of the gas-water two-phase pseudo pressure. In addition, it lays a theoretical foundation for accurate prediction of CMB productivity.

Review on hydrocarbon generation, pores formation and its methane adsorption mechanism in shale kerogen

Now problems of shale gas development reveal that organic matter diagenesis, hydrocarbon generation process of kerogen, characteristics of methane and water occurrence in organic pores are all with complexities, which will seriously affect the desorption and production laws of shale gas. But existing research is shortage in these aspects, such as: the product characteristics of each thermal evolution stage of shale organic matter, the mechanism of hydrocarbon generation, organic pores type as well as formation mechanism. Besides, the occurrence modes of methane and water in organic pores are controversial, so further research is necessary. Literature reviews on process of kerogen formation, generation mechanism of oil/gas in shale kerogen, evolution characteristics of pores in organic matter and water distribution characteristics in pores of organic matter are conducted. During the biochemistry stage, kerogen is generated by bacterial action and condensation polymerization. Because the C-H bonds of aliphatics in kerogen release, aliphatic-intermolecular pores occur. During the thermo-catalysis stage, asphaltenes, liquid hydrocarbon and residual kerogen are generated by thermal-depolymerization, and cracking of asphalt turn out to be liquid hydrocarbon, so the marginal-oil pores (liquefied pores) form. During the thermo-cracking stage, liquid hydrocarbon crack into wet gas, and major pores in organic matter are marginal (or internal) -gas pores. During the deep high temperature stage, hydrocarbon transform into dry gas, owing to the release of C-H bonds in kerogen aromatics, pores in organic matter mainly include intermolecular pores in aromatic and gas pores. In addition, it is water that is involved in the pores of organic matter during each thermal evolution stage of shale kerogen, which shows a solid-liquid interface effect. As a theoretical basis on exploration and development of shale gas, it is valuable to obtain better understanding on production discipline of shale gas.