Pore ​​structure Evaluation And Gas-water Microscopic Occurrence Mechanism Of Tight Sandstone Reservoirs In Penglaizhen Formation, Western Sichuan, China

The lithology of Penglaizhen Fm reservoirs is mainly tight sandstone in the MaJing-Shifang area of the western Sichuan Basin.The reservoirs are characterized by low porosity and low permeability,complex pore structure and strong heterogeneity,and et al,because of many tiny throats exist in the reservoirs,the reason of plenty of tiny throats in the reservoirs is that pore and throat are destroyed by large buried depth,complicated deposition and diagenesis which mainly containing compaction,cement.These characteristics resulted in complex distribution of gas and water,larger pore-throat capillary pressure,and made against the efficient exploitation of gas reservoirs.So in order to achieve efficient development of tight sandstone gas reservoirs,it have strong realistic meanings to carry out the study of pore structure and microcosmic gas-water occurrence.In this paper,Penglaizhen Fm reservoirs in MaJing-Shifang area of the western Sichuan Basin were took as an example to analyze the complex pore structure of tight sandstone reservoirs,based on the nuclear magnetic resonance,Rate-controlled Hg injection and High Pressure Hg injection experiments technique.And the saturation evaluation method which applied better for MaJing-Shifang area were built by using conventional logging data.Besides,basing on the pore structure evaluation,microcosmic gas-water migration and occurrence were researched by combing the microscopic visual displacement model.The study shows that the region’s tight sandstone reservoirs can be divided into four categories,and the most of pore volume of four categories were controlled by invalid throat that the radius is less than 0.1 μm,the invalid throat in 4th kind reservoir is more than 70 percent.The plenty of tiny throat will seriously affect the reservoir permeability,and affect the gas reservoir development;Aimed at the water saturation evaluation,the dual-porosity conducting volume model constructed on the complicated pore structure can better calculate the water saturation by combining with the classical Archie model.Meanwhile,a new method calculated the irreducible water saturation was proposed by using conventional logging data,it can be used to analyze mobility of water in tight sandstone reservoirs by combining with water saturation model;the mobile water saturation calculation in the gas reservoirs has a significant positive correlation with water production characteristics of reservoirs.With the increasing of mobile water saturation,the water production characteristic of gas-well can be divided into three stages;the migration and occurrence of gas-water in tight sandstone gas reservoirs are different in the accumulation and development process.Reservoirs which have better pore structure have lower irreducible water saturation,this kind of reservoir is helpful for the development of gas reservoirs in early stage.In the development process,the interference between gas and water is strong,so gas and water are apt to be produced simultaneously.And the existence of seal gas will influence production of gas-well.The thesis not only analyzed the pore structure of tight sandstone reservoirs,but also provided a reasonable water saturation evaluation.Moreover,the paper confirmed the migration and microcosmic occurrence of gas-water two phases in tight sandstone reservoirs,so that gave a scientific basis for the high-efficient development of the gas field.