8 Seafloor Gas Hydrates Monitoring System Arranged from within Borehole

Most gas hydrates are buried stably in the formations which locate 0–1000m below seafloor. The global climate, oceanographic and tectonic processes can affect the stability conditions of gas hydrate, resulting in highly dynamic changes of subseafloor environments. A coupled seafloor∕borehole observatory system can be used to monitor the chemical, microbiological and physical data to achieve a better understanding of these changes and the carbon cycling in gas hydrate-bearing continental margin. When the borehole instrumentations are deployed, the geomechanical stability of hydrate-bearing formation should be assessed if it can support the borehole instrumentations. Some drilling and completion methods will be taken to reinforce and stabilize the weak and friable hydrate-bearing formation, which can realize a long-term observation. These dynamics can only be understood through time-series monitoring of complementary parameters over space and time, and monitoring can be best accomplished. One of the goals of setting gas-hydrate observatories should be serving for estimating resource reserves and drilling well of gas hydrate exploitation in the future. Therefore such parameters as the density, resistivity, pore pressure, hydrate saturation, permeability, thermal conductivity, elastic modulus and etc should be obtained in the borehole monitoring system. According to the above analysis, the authors designed a new borehole monitoring system for marine gas hydrates and discussed the relatively key problems in its development.