Fluid Pressure System And Sequence Control Of Permocarboniferous Coal Series In Linxing Block

Fluid pressure system of reservoir in coal series and its geological control are the key to understand the symbiotic characteristics of unconventional natural gases in coal series(coal series gas for short or CSG), which is of vital importance to the optimization of co-mining CSG technology. Taking Permo-carboniferous coal series in Linxing Block,northeastern margin of Ordos Basin as the research object,the causality among sequence stratigraphic framework, rock physical property cycle and superposed fluid pressure system is analyzed so that the horizon of key impermeable layers is indentified, the vertical occurrence of fluid pressure systems is revealed and the interference among the fluid energy systems under CSG co-mining is simulated. Based on the physical property cyclicity of coal series rocks in the well profiles, four key layers are found near the major sequence boundaries, which was deposited in the highstand systems tract(HST) of the third-order sequence SQ1, SQ4, SQ6 and SQ9. Key layer rocks are relatively low in porosity, permeability and CSG content but high in the reservoir pressure. Accordingly,five superimposed fluid pressure systems are identified in the coal series, which are highly consistent with the third-order sequence units. It was considered that the sequence stratigraphic frame could control the occurrence of key layers and supply a physical property base for the formation of superimposed fluid pressure systems. Key layers limit fluid exchange and energy transfer among the systems, resulting in the relative independency of the fluid pressure systems. Based on the measured data, the cross simulation scheme for the interference of coal reservoir pressure was designed, potential interference between fluid pressure system Ss2 and Ss4 was analyzed under the CSG co-mining. It was suggested that the CSG co-mining may be suitable when the fluid pressure difference between two systems is less than 1500 k Pa, and not when more than1500 k Pa due to the distinct interference. It was found that the interference would become more obvious with the increase of fluid pressure difference when the difference is higher than 1500 kPa.