Study on Acoustic Velocity and Resistivity Model of Shaly Sandstone Reservoirs Containing Natural Gas Hydrate
Author:Hu Xu Dong
Supervisor:zou chang chun
Natural gas hydrate is a kind of high-quality energy with great potential.It is regarded as an alternative energy source for oil and natural gas in the 21 st century,and has attracted wide attention all over the world.The main method for gas hydrate exploration and resource evaluation is geophysical exploration technology.Based on the reservoir characteristics of gas hydrate in the permafrost area of Qilian Mountains,this study carries out numerical simulation on acoustic and electrical aspects of gas hydrate reservoirs,and uses corresponding models to evaluate hydrate saturation.The thesis has achieved the following results:(1)The porosity of porous gas hydrate reservoirs in the study area is 4.3%,and the permeability is 7.2 × 10-3m D,which reveals that the reservoirs are extra-low porosity and ultra-low permeability.The clay content is as high as 19.4%,and it is in two forms: laminated shale and dispersed shale.Analysis of logging response characteristics of hydrate reservoirs in the study area shows that acoustic wave and resistivity are sensitive to gas hydrates(2)The acoustic velocity model of the hydrate reservoir is established by the improved Biot-Gassman theory.The numerical simulation results show that both Pwave and S-wave velocities decrease with the increase of shale content;P-wave velocities increase with the increase of gas hydrate saturation,and S-wave velocities decrease with the increase of gas hydrate saturation.Compared with the logging data of non-hydrated formation,the results obtained by the forward modeling are basically consistent with the actual logging data,which shows the validity of the model and its parameters.It can be used for preliminary assessment of hydrate saturation.(3)Based on the HB equation,dispersed-clay model and laminated-clay model are established respectively according to the distribution of clay in the study area.By comparing with the experimental data,it is shown that the two models can well describe the resistivity characteristics of rocks with different shale content in the study area.The numerical simulation results show that the resistivity of gas hydrate reservoir decreases sharply when the porosity is small.When the gas hydrate saturation is low,gas hydrates mostly exist in the pore surface;however,as the gas hydrate saturation increases,gas hydrates aggregate in the pore space and the conductive paths are blocked by gas hydrates,resulting in a dramatic increase in resistivity.(4)Based on the pore-bound resistivity equation,the resistivity model of shaly sandstone with gas hydrate is established,and the method is used to identify and evaluate gas hydrate reservoirs in the study area.The results show that the method can effectively identify thin-layer hydrate.The gas hydrate saturation of DK-C well ranges from 2.0% to 66.4%,with an average value of 40.5%.The study of petrophysical characteristics of gas hydrate reservoirs in Qilian Mountain permafrost region has important guiding significance for identifying gas hydrate reservoirs and evaluating gas hydrate reserves,which provides technical support for gas hydrate exploration and development in China.