Abstract
Water jet technology is a key technology in the marine natural gas hydrate (NGH) solid fluidization mining method. As an important parameter in water jet breaking NGH sediments technology, the critical breaking velocity of NGH sediments is unknown. In the present research, an orthogonal design experiment is carried out to study the critical velocity of NGH breakage by water jet, using frozen soil and sand as experimental samples. First, the time it takes to reach maximum NGH breaking depth is determined. Then, ultimate breaking distance is studied with respect to the NGH saturation, jet pressure, and nozzle diameter. Following that, the variation of critical velocity with NGH saturation is analyzed. Eventually, a formula to calculate the critical velocity for marine NGH breakage by water jet process is established, and the undetermined coefficient (η) in the formula is calibrated with the experiment data. The results show that the ultimate breaking distance is mostly achieved within 63 s. The three experimental factors in order of the effect on the ultimate breaking depth (from high to low) are NGH saturation, jet pressure, and nozzle diameter. The critical velocities for marine NGH breakage corresponding to the NGH saturations of 20%, 40,%, 6%, and 80% are 5.71 m/s, 7.14 m/s, 9.60 m/s, and 10.85 m/s, respectively. The undetermined coefficient η in critical velocity formula is 1.44 m/s.
Highlights
Natural gas hydrate (NGH), named as “Flammable Ice”, is considered to be an alternative clean energy source in the 21st century because of its huge reserves [1]
The results show that the ultimate breaking distance is mostly achieved within 63 s
Since the experimental achieve themade maximum breaking depth is the time.inSince the experimental sample is sample is of frozen soil, the ultimate sample isbreaking submerged the water tank to maintain low made of frozenit soil, sampletois above submerged in the water tank to ambience maintain low temperature, should not bethe exposed zero degrees centigrade for temperature, an excessive it should not be exposed to above zero degrees centigrade ambience for an excessive time
Summary
Natural gas hydrate (NGH), named as “Flammable Ice”, is considered to be an alternative clean energy source in the 21st century because of its huge reserves [1]. The total amount of NGH resources in the world is about 3 × 1015 m3 , most of which is stored in the ocean [2,3,4]. Sandstone NGH reservoirs, which have a relatively high porosity and are easy to exploit, are usually selected for production tests [6,7,8]. Disseminated NGH reservoirs, accounting for more than 90% of the total NGH reserves, are difficult to exploit with the current technology because of low permeability [9]. The NGH resources in the South China Sea are about 85 trillion m3 [10]
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