Research On Logging Interpretation Method Of Gas Hydrates

Natural Gas Hydrate (Gas Hydrate) was widely distributed in permafrost and seafloor sediments as a potential energy in the 21st century. Geophysical logging methods had been widely used and played an important role in the exploration of gas hydrate as traditional resource evaluation methods. Logging interpretation method of gas hydrate was based on conventional oil and gas logging interpretation model. Owing to oil and natural gas hydrate different, the application of logging method presented obvious experimental color. Therefore, establishing logging interpretation methods suitable for gas hydrate reservoir for the development of gas hydrate were very important.This paper described in detail the concept, the formation conditions, the formation geological model and the distribution of gas hydrates in the world. Gas hydrate formation conditions included material conditions, physical and chemical conditions. Material conditions included natural gas, gas hydrate reservoir, gas hydrate cap. Physical and chemical conditions included the temperature and pressure conditions of gas hydrate formation necessary. Gas hydrates could be formed in a variety of different systems (open system or closed system) and different environment (submarine and terrestrial). Generally, the geological model of gas hydrate formation summarized as following: low-temperature freezing mode, transgressive model, fault model, self-diagenetic model, deposition patterns, and various filtration modes (pressure infiltration model, geothermal model, flow model ), in which infiltration was most common and important mode and it was the main formation model marine gas hydrate.This paper analyzed in detail the geophysical log response characteristics of the reservoir combining the characteristics of gas hydrates. Such as the decreasing of density, acoustic transfer, the significant increasing of resistivity, the slight falling of the neutron porosity. These features could effectively confirmed the existence of gas hydrates. Gas hydrates still had some other logging response characteristics such as: in the natural gamma log, the log response was generally low, caliper logging, due to the expanding of borehole during in drilling; the spontaneous potential curve occurs the slight negative shift at the gas hydrate reservoir.Qualitative identification of gas hydrate reservoirs have many ways. Such as: qualitative interpretation of seismic geophysical, qualitative interpretation of geochemical, qualitative identification in geology mark and geophysical logging methods. This thesis focuses on the geophysical logging identification of gas hydrate reservoir. The third chapte in detail introduced the logging response characteristics of gas hydrate reservoir. On the basis, this thesis completed qualitative identification of the several sites in ODP204. The voyage used the LWD and wireline logging to receive good quality logging data. Through qualitative analysis of drilling 1244, 1245,1250, gas hydrate reservoirs in the section were between 40 to 127mbsf, 50-131mbsf, 29-111mbsf in that the second logging unit. The strong evidence of gas hydrates layers:â—‹1 The logging resistivity logging was higher than the first units.â—‹2 RAB imaging logging appeared high resistivity zone in the second (shown in lighter color parts), marking out of the existence of the gas hydrate layers.â—‹3 Comparison of sonic log data, the time of transiting in target layer was higher than 1.65km/s, but un-gas reservoir was below 1.55km/s.â—‹4 Density logging data showed that gas hydrate reservoir density value was 1.7-1.8g/cm3.Accurating porosity values was a first task in quantitative interpretations of the gas hydrates layers. Porosity logging evaluation methods to gas hydrate reservoir included the resistivity logging, density logging, neutron logging, nuclear magnetic resonance logging etc the physical response closely related to the pore, but also supplemented to the other logging methods reflecting the nature of muddy such as spontaneous potential, natural gamma and the core analysis data used to evaluate the porosity etc. Then, core analysis porosity compared with the porosity obtained with different methods to find out that the results was closest to the core analysis porosity. This thesis made use of the logging data provided by 1244,1245,1250 stations of ODP204 voyage to calculate density logging porosity, resistivity logging porosity and the porosity of the two regression logging. The results showed that the absolute error and relative error of the porosity received by regression method had reduced significantly than density logging method and resistivity logging. This method increased significantly accuracy and agreed well with core analysis porosity.As the gas hydrate reservoir was a complex system, the estimation method of gas hydrate content should try to choose a variety of methods or models to calculate in order to improve the interpretation accuracy. This thesis calculated the concent of gas hydrate by two-water model (Archie), multi-component model and nuclear magnetic resonance logging evaluation model. The results show that the gas hydrate content calculated by Archie formula and the multi-component model was better than the result calculated by the nuclear magnetic resonance.