Numerical simulation of the short- and long-term production behavior of the first offshore gas hydrate production test in the South China Sea

Gas hydrate-bearing sediments at the first offshore gas hydrate production test site in the Shenhu area, South China Sea, have the characteristics of high irreducible water saturation and low permeability. The stratum structure of this production test site is complex, its gas hydrate-bearing layer (GHBL), three-phase layer (TPL) and free gas layer (FGL) all contain methane gas or gas hydrates, that make the production more complicated. According to the available geological data at the first offshore gas hydrate production test site in the Shenhu area, a 2D numerical simulation model is built to study the short-and the long-term production behavior of the gas hydrates in the Shenhu area. In the short-term, the total gas production from hydrates, when the TPL gas saturation is 0.078, is determined to be 2.75 x 10(5) m(3), which is close to the actual total gas production of 3.09 x 10(5) m(3). When the irreducible water saturation varies from 0.40 to 0.63, the total gas production changes from 1.41 x 10(5) to 2.75 x 10(5) m(3), and the total water production decreases from 1.04 x 10(4) to 8.7 x 10(3) m(3), which indicates that irreducible water is a favorable factor for gas production from hydrates in clayey silt sediments. In the long-term gas production from hydrates, the gas production rate first decreases, then increases, and finally decreases again. Gas hydrate reformation occurs at the interface between the TPL and the FGL, which will adversely affect the gas production from hydrates.