南海神狐海域天然气水合物试采成功后的思考

On the China's successful gas production test from marine gas hydrate reservoirs

2017年5月18日,国土资源部部长姜大明宣布,中国首次海域天然气水合物（即可燃冰）试采成功.一石激起千层浪,试采成功的消息立即霸屏式占据各大新闻传媒头条,各种消息观点甚嚣尘上.有观点认为,这将会改变中国的能源结构,引发新的能源革命,推动整个世界能源格局的改变;不少业内人士也指出,可燃冰试采成功可喜可贺,而要达到大规模商业性开采的程度还有很长的一段路要走;更有人担心试采成功的宣传言过其实,有炒作嫌疑.

China has succeeded in gas extraction from fine-grained gas hydrate reservoir in Shenhu area, northern South China Sea. In March 2017, China Geological Survey drilled the first test well at the water depth of 1266 m. Steady gas flow was obtained continuously from the test well from May to July （60 d totally）. The total volume of gas accu- mulated during this period reached up to 300000 m3, with an average of 5000 m3/d. The test well was later shut off by the operation team as 60 d greatly exceeded production schedule and the field trial accomplished all objectives. The success of this gas production trial demonstrates that gas hydrate hosted in fine-grained marine sediments can be exploited with current production technology. The success of the production test is based on a number of progresses made on the investigation and studying of gas hydrate in the northern South China Sea. The seismic indicators of gas hydrate, such as bottom simulating re- flectors （BSRs）, were first identified in the northern South China Sea in the late 1990s. Subsequently, four areas, namely Dongsha, Shenhu, Xisha and Qiongdongnan, were selected as the main potential targets for gas hydrate exploration and exploitation according to geologic and geophysical investigations. From 2007 to 2016, four gas hydrate drilling expeditions were conducted in the northern South China Sea. These expeditions confirm the pres- ence of gas hydrate in Shenhu and Dongsha area. They also show that gas hydrate bearing sediments in Shenhu area have larger thickness, higher saturation than those in Dongsha area. Besides, these sediments in Shenhu area exhibit a good lateral continuity, which facilitate gas hydrate exploitation. The above characteristics of gas hydrate reser- voir make Shenhu area become the first gas-hydrate production test target in the northern South China Sea. A range of difficulties have been encountered in the production test of Shenhu area compared to the field pro- duction trial of Nankai Trough in Japan. First, while gas-hydrate test site in Nankai Trough is located at a water depth of 600 m, the site in Shenhu area is located at a water depth of more than 1200 m, which increases operation risk as water depth rises. Furthermore, gas hydrates in Shenhu area are buried at a depth of 100-200 m below sea- floor, which is shallower than that in Nankai Trough （276-337 m below seafloor）. Such a shallow depth results in weak compaction and loose sediments, and therefore increases the possibility of seafloor instability during produc- tion operations. More importantly, gas hydrate reservoirs in Nankai Trough, which have an average saturation of 70%, is much more concentrated than that in Shenhu area. The hosted sediment is shale/silt in Shenhu area, which results in low permeability, while it is sand in Nankai Trough. In brief, the quality of fine-grained gas hydrate res- ervoir in Shenhu area is lower than that of sand-prone gas hydrate reservoir in Nankai Trough. The production test has succeeded in overcoming the above difficulties and realized 60 d＇s continuous and steady production. Therefore, the success of production trial in Shenhu area marks a great progress in the gas hydrate exploitation. Despite major advances in gas hydrate exploitation in recent years, there is still a long way to go in realizing the commercial production of marine gas hydrate. In order to utilize gas hydrate economically, safely and environ- ment-friendly, more work needs to be done on the improvement of gas-hydrate accumulation theory and explora- tion technique, as well as the updating of development techniques and environment protection methods during ex- ploitation.