Abstract:Natural gas hydrate is a type of solid clean energy with huge reserve， which is therefore considered as a substitute for traditional fossil fuels and have attracted much attention around the world. Due to its occurrence in low-temperature and high-pressure marine and permafrost environments， the key to achieving commercial exploitation is to find out economical and efficient exploitation methods. Based on the current research status of laboratory research， numerical simulation， and on-site experiments， the exploitation effects of methods such as depressurization， heat injection， chemical inhibitor injection， CO2 displacement and combination method were analyzed， and the advantages and limitations of each method were discussed. The existing exploitation methods are mainly limited by low reservoir permeability and poor thermal conductivity， and have not been able to achieve long-term continuous gas production. To address the above issues， the in-situ resistance heating method for reservoirs is proposed to improve thermal utilization efficiency， and it is believed that hydraulic fracturing and permeability enhancement technology is an effective measure to increase reservoir permeability and assist in efficient gas production through production methods such as depressurization； Regarding the potential instability of reservoirs caused by hydrate exploitation， it is believed that the use of CO2 replacement method can strengthen the reservoir， and the use of supercritical CO2 injection technology can improve the CO2 replacement rate.

Abstract:The 1998 Mallik 2L-38 Hydrate Research Well Program in Canada’s Mackenzie Delta, permafrost area, led by the GSC and the Japan JNOC, was successful, but the goal of producing from NGH was not accomplished for the warming weather and mechanic failure. GSC and JNOC, as well as other 8 partners returned to Mallik in 2002. Three wells-the 3L-38 and 4L-38 observation wells, and the 5L-38 production well were drilled. Gas hydrate core was gained from the 5L-38 well at 90% core recovery and geophysical studies were conducted. Two short-duration production tests were conducted—a depressurization test with MDT tool and a thermal injection test. One encouraging result was the 470m3 of gas production by a thermal injection technique employing hot fluid circulation. Then, for the 2007—2008 Mallik production test program, the 2L-38 well, drilled in 1998, was modified to establish a production test well by reaming, deepening, casing, and cementing. An ESP was set below the perforation zone to depressurize the formation by dropping the water level in 2007. During 125h of the test, at least 830 m3 of gas flow was obtained. The test results verified the effectiveness of the depressurization method even for such a short duration. The 2008 testing program at Mallik confirmed that continuous gas flow ranging from 2000 to 4000 m3/day was maintained throughout the course of the 68 day (139 hour) test. Cumulative gas production volume was approximately 13000m3, which data confirms that the depressurization method is the correct approach for gas hydrate production. The 2008 Mallik gas hydrate production program was a landmark in hydrates R&D in the world.