地球物理测井在天然气水合物勘探中的应用

天然气水合物是一种潜在的巨大能源。美国、日本、俄罗斯等发达国家在该领域已经进行了大量的研究工作，试图从各个角度去探测天然气水合物的存在并评价其蕴藏量。地球物理测井在天然气水合物探测与储量评价领域发挥了重要作用并且随着以勘探天然气水合物为目的的钻井的增多，日益受到重视。基于国内在该领域研究较少的现状，综述了国外地球物理测井技术在天然气水合物勘探中的应用，详细介绍了常规测井方法定性识别和定量评价天然气水合物储层的孔隙度、饱和度方法和技术，基于含天然气水合物储层的碳氧比能谱测井解释模型以及成像测井在含天然气水合物井段的测井响应。并在此基础上分析了我国在该领域的研究思路。

APPLICATON OF GEOPHYSICAL WELL LOGGING TECHNOLOGY IN EXPLORATION OF GAS HYDRATE

    Gas hydrate is a potential huge energy. The developed countries, such as United State, Japan and Russia, has done lots of researches in this area to detect the existence of gas hydrate and evaluate the reserves of it. Geophisical well logging has played a important role in the detection and evaluation of gas hydrate and along with the increase of wells drilling for explore the gas hydrate, more and more stress has been laid on well logging. Based on the present situation of poor research in this field in our country, this paper summarizes the application of geophysical well logging technology in the exploration of gas hydrate. 
    Firstly, the techniques of qualitative identification and quantitative evaluation of porosity and saturation in the gas hydrate bearing reservoir by conventional well logging data are discussed detailedly. For the qualitative identification, on the log in a gas hydrate zone ,there is a relatively higher electrical-resistivity deflection than that in a water saturated zone, a relatively lower SP deflection in gas hydrate bearing zone than that in free gas zone. And the caliper log in gas hydrate zone shows a characteristic with oversized borehole for the reason of gas-hydrate decomposition. There is a decrease in acoustic transittime in comparision to that in the water or free gas bearing horizon. In the neutron porosity and density log, they shows a slight increase and decrease respectively. For the quantitative evaluation, the porosity in the gas hydrate bearing reservoir can be determined from core analysis, density log, neutron porosity log and resistivity log, but the porosity from core ananlysis and resistivity log is more accurate than that from other two methods for the bad borehole condition. The saturation of gas hydrate can be calculated using standard Archie equation and a quicklook Archie analysis method.
    Secondly, carbon/oxygen spectral logging is a good method to quantitatively evaluate gas hydrate saturation. Two interpretation models of carbon/oxygen spectral logging for the gas hydrate bearing reservoir are presented. The first one is gas hydrate carbon/oxygen reservoir model,from which the ratio of between different elements and then the gas hydrate saturation can be derived.The second one is the complex carbon/oxygen reservoir model,in which the effects of borehole fluid and carbon in shale are considered.
    Thirdly,image logging responses at the gas hydrate bearing interval are introduced.During leg 164 of the Ocean Drilling Program, The FMS were used and produced a high resolution resistivity image, from which the existence of gas hydrate was proved. In addition, the FMI image was achieved in the Mackenzie delta of Canada ,from which the occurrence of gas hydrate also was inferred.
    Based on above summarization, the research strategy in this area for our country are presented.In view of the present situation, our country should cultivate young scholars in this field, and fund for the research in this area. In the aspect of technology, our country should enhance the research on the theory and experiment of every well logging responses, especially about calibration of bad borehole condition.