Research On Pressure Characteristics Of Gas Reservoir And Mechanical Effect Of Stress Path For Shallow Gassy Sand

In this thesis, based on the geologic origin of shallow gassy sand and research method of experimental soil mechanics, a series of laboratory test for shallow gassy sand are carried out, in which the pressure characteristics of gas reservoir is disclosed and the mechanical behaviors of the shallow gassy sand under the conditions of various stress path are systematically studied. By means of numerical simulation analysis, the influence of shallow gas release is studied.Firstly, according to the feature of the regional geological evolution in Hang Zhou bay area, the occurrence law of shallow gassy sand is investigated. The geological reasons for abroad distributing of shallow gas in Hang Zhou bay area are special geological environment evolvement, exceptive sedimentary mode, favorable conditions of hydrocarbon-generating and geology characteristics, which also are source of shallow gassy sand's characteristic.Secondly, Carrying out experimental research about pressure characteristic of shallow gassy sand under different confining pressures with 4D stress path module of GDS triaxial system. The results show that there is remarkable influence on form of SWCC considering different confining pressures and water storage capacity of unsaturated sand declines gradually with infliction and accretion of confining pressure. A mathematical model of SWCC is advanced and establishing the relationship between SWCC and unsaturated apparent cohesion, which is applied to predict unsaturated strengthen of shallow gassy sand. Then to analyzed and forecast pressure of shallow gas reservoir with SWCC, and the forecasted result is higher if using SWCC without confining pressure.Thirdly, in order to analyze the mechanic effect of shallow gassy sand under the conditions of various stress paths, a series of triaxial test is carried out respectively, such as isotropic consolidation-active compression, isotropic consolidation-passive compression, isotropic consolidation-active extension, isotropic consolidation-passive extension, increasing of pore water pressure, decreasing of pore air pressure. The results show that stress-strain curve's shape and volume change characteristic are remarkably influenced by variety direction of confining pressure. The increase of confining pressure induces softening and shear-compaction, while the decrease of confining pressure induces hardening and dilatancy. There is a close contact between shear strength and stress, cohesion is higher when confining pressure decreases and internal friction angle is higher when confining pressure increases. The result also shows that it follows power law between shallow gassy sand's apparent cohesion and matric suction, and then shallow gassy sand's uniform shear strength formula of different stress paths is gained.Fourthly, By means of numerical simulation analysis, the influence of shallow gas release is studied. Release of shallow gas will arouse compaction distortion of tratum structure, which is studied by means of numerical simulation analysis. The result shows that value of sedimentation augments with increasing of gas reservoir pressure, thickness and with decreasing of depth of gas reservoir.Finally, some suggestions about carrying out construction in area of shallow gas reservoir, according to research result, are proposed.