Natural Gas Hydrate

Natural gas hydrate, or clathrate hydrate, a crystalline solid composed of natural gas and hydrogen bonded water molecules, is formed at relatively high pressure and low temperature conditions. Natural gas hydrate is extensively distributed in oceans, continental margins and some lakes. Because of large volumes trapped in shallow sediments, natural gas hydrate is a potential source of energy, submarine geologic hazards and a factor in global climate change.

1. My research work on this area includes:

1) Mechanical properties of gas hydrate deposit: a series of laboratory experiments were carried out to study the stress-strain curves and the strength of gas hydrate deposit before and after dissociation of gas hydrate with man-made samples and samples coring from China South Sea and Tibetan Plateau. Theoretical mode of constitutive relation and mechanical parameters (Modulus, strength) with contents and saturation were presented.

2) Instability of pipeline and seabed above gas hydrate deposit: experiments and numerical modeling were carried out to study the displacement and the bearing capacity of structures and the seabed safety after dissociation of gas hydrate.

3) Method for exploitation of gas hydrate: Seepage of multi-media in porous deposit and thermal conduction during dissociation of gas hydrate was mainly investigated. A decoupling method was presented based on time differences among seepage, thermal conduction, dissociation and stress propagation. A new exploitation named machine-heat exploitation method was presented.

2. Main advances are as follows:

2.1 Apparatus R&D

    An apparatus for synthesizing of gas hydrate and testing of mechanical properties was developed(Fig. 1) which could provide confining pressures ranging from 0 to 30 MPa with an accuracy of 0.5% (Fig.2) and temperatures from -20℃ to 20℃ with an accuracy of 2.5%. The maximum back-pressure provided by a gas-supply cylinder was 10 MPa. It can not only test custom stress, strain, pore pressure and volume strain, but also measure supersonic, temperature, water content and electric resistance. The whole apparatus are placed in a room with low temperature.

Apparatus for testing safety of soil layer and structures in it due to gas hydrate dissociation in a centrifuge (Fig.3a) was developed. The pressure cell can provide 5MPa gas pressure. The diameter and height of the cell are 90cm and 145cm, respectively. There located with soil stress transducer, pore pressure transducer, displacement transducer, temperature transducer and camera. A series of models for testing responses of soil layer and structure during and after dissociation of gas hydrate were developed (Fig.3b).

A set of one dimensional apparatus (Fig.4a) and a set of cylindrical symmetrical apparatus (Fig.4b) for experimental study of gas hydrate exploitation were developed. The one dimensional apparatus is with diameter of 4.0cm and length of 100cm. The maximum of confined pressure provided by the cell is 30MPa . Temperature range is -20-20℃. Using this apparatus, changes of temperature and pore pressure along the length during dissociation of gas hydrate under depressurization or heat injection or displacement conditions can be measured. Meanwhile, the changes of pore distribution and saturation of gas hydrate can be measured by placing the pressure cell in a CT (computer tomography). The cell of the cylindrical symmetrical apparatus is with diameter of 50.0cm and maximum depth 50cm (which can be adjusted). The maximum of confined pressure provided by the cell is 30MPa . Temperature range is -20-20℃.

                 

 Fig.1  Apparatus for mechanical test of gas hydrate                    Fig.2 Room with low temperature (4m2)

            

                    (a) Small scale model                                           (b) Model for experiment in centrifuge

Fig.3 Model for experiment on the safety of stratum and structure

(a)  One dimensional apparatus    (b) Cylindrical symmetrical apparatus

              Fig.4 Apparatus for experiment of gas hydrate exploitation

2.2 Study of mechanical properties

    A series of experiments were carried out by using apparatus for synthesizing of gas hydrate and testing of mechanical properties. Mechanical properties of gas hydrate deposit: a series of laboratory experiments were carried out to study the stress-strain curves and the strength of gas hydrate deposit before and after dissociation of gas hydrate with man-made samples and samples coring from China South Sea and Tibetan Plateau. Theoretical mode of constitutive relation and mechanical parameters (modulus, strength) with contents and saturation were presented.(Fig.5a-f)。    

(a)  Stress-strain curve of CH4 gas hydrate     (b) Stress-strain curve of CO2 gas hydrate

(c) Dynamic stress-strain curve of gas hydrate   

  After dissociation  

  (d) Stress-strain curve of gas hydrate

before and after dissociation

(e)  Modulus   (f) Internal friction angle

Fig. 5 Comparison of experimental data and theoretical model of mechanical constants

2.3 Study of structure safety and soil layer’s instability

   Experiments and numerical modeling were carried out to study the displacement and the bearing capacity of structures and the seabed safety after dissociation of gas hydrate (Fig.6). Two new types of failure of soil layer due to gas hydrate dissociation, layered fracture and outburst (Fig.7), were found. The corresponding critical conditions were presented. The displacement and bearing capacity of structures after dissociation of gas hydrate was studied (Fig.8).

 (a) Bubbles after dissociation of gas hydrate     (b) Settlement of sediment and pipe
Fig. 6  Photos of centrifugal experiment
 
(a)  Layered fracture (b) Outburst
Fig. 7 Two types of new failure phenomenon due to gas hydrate dissociation
 
 (a) Deformation of pipes (or wells) with
dissociation of gas hydrate
(b) Effected zone surrounding
pipes (or wells)
Fig. 8 Numerical results of the stability of pipes (or wells) and stratum surrounding pipes