Natural gas hydrate (NGH) is an ice-like crystalline compound formed by natural gas molecules and water molecules . It is widely distributed in the permafrost regions and marine sediments, which is considered as one of the potentially clean future energy. According to preliminary estimates, the total amount of energy in natural gas hydrate is two times larger than that of conventional oil and gas resources .
There are four main methods for hydrate production: depressurization, thermal stimulation, inhibitor injection, and CO –CH replacement . On July 9, 2017, China successfully completed its first pilot production of marine hydrate by depressurization in the Shenhu area, northern South China Sea. It was the first time in the world to exploit hydrates hosted in the clayey-slit sediments . The total volume of gas production was up to 3.09 10 m in 60 days [4,5]. Although this methane hydrate production has made a great progress, it is far from being commercialized.
Based on the data obtained from hydrate drilling expeditions in the South China Sea, a lot of numerical simulations were carried out. Su et al.  evaluated the production potential of hydrate reservoir at site SH3 by depressurization using a vertical well and investigated the sensitivity of hydrate exploitation under different bottom hole pressure, initial hydrate saturation, intrinsic permeability, and overburden's permeability. By using a single horizontal well in the middle of the HBL, which was set to constant pressure and temperature, the total gas production reached 3.46 10 m /d . Jin et al.  investigated the enhancement of thermal simulation on gas production and they also studied the effects of different well placements on gas hydrate production. Sun et al.  studied gas and water production rate, geomechanical response under different conditions including hydrate saturation heterogeneity, hydrate formation permeability, and gas formation permeability. Konno et al.  simulated the first marine hydrate production in Japan through numerical simulation. The simulation results showed that the gas production gradually increased with the expansion of the decomposition area, and the increase of hydrate formation permeability increased the ratio of gas to water.
Gas hydrates in Shenhu area are mainly hosted in clayey-silt sediments, which will make its multiphase flow more complex. But the effects of water permeability reduction index n on gas hydrate production were poorly studied. So, the main purpose of this study is to evaluate the effects of different n on gas and water production.
2. Simulation Model
The schematic depiction of gas production from the hydrate reservoirs in site SH2 in this simulation is shown in Figure 1. This model is divided into three layers, which are permeable overburden, gas hydrate bearing-sediments (the GHBS), and permeable underburden. The hydrate reservoir thickness is assumed to be 40 m, the value of intrinsic permeability k is set to be 10 mD, the porosity of the GHBS is assumed to be 38%, and the average hydrate saturation is assumed to be 40% . The thickness of the permeable overburden and the permeable underburden is set to be 30 m. The sediments lithology of overburden and underburden, such as permeability and porosity, are the same as the GHBS. The production well is in the center of this cylindrical hydrate reservoir with a radius of 0.1 m. Other properties and conditions related to the reference case are listed in Table 1. In this study, the perforation interval has a total length of Lp = 20 m, which is located in the middle of the GHBS. The production well has a constant pressure of 3 MPa.
The values of irreducible water saturation, irreducible gas saturation, and gas entry pressure are not fixed values; they will change with the particle size, clay content, and hydrate saturation. But in this simulation, those parameters are assumed to be fixed values. The relative permeability model used in this simulation is as follows :
(3) The capillary pressure function used is as follows :
3. Results and Discussions
Spatial distributions of physical properties in reservoir
Spatial distributions of pressure (P)
Figure 2 shows the evolution of the pressure distribution over time in the entire formation with P = 3 MPa. From the Figure 2, we can know pressure distribution: (1) in the first ten days of production, the water near the production well flows into the production well under the pressure gradient, and the effect of the depressurization is only within 20 meters; (2) after 365 days, when the water from the overburden and underburden enters the production well; (3) in 10 years of production, the range of hydrate decomposition is within 30 meters.
Spatial distributions of hydrate saturation (S )
The following conclusions results from Figure 3: (1) during the ten years of production, the range of hydrate decomposition is limited to 20 meters; The existence of hydrate reduces the effective permeability of clayey-slit sediments, which affects the range of the depressurization, and increases the time of gas flows to the production well; (2) in the 365th day, hydrate decomposition occurs at both the upper and lower boundaries of the GHBS, and the upper boundary of the GHBS decomposes rapidly, which is mainly caused by the high temperature fluid of the underburden and lower overburden flowing into production, and the temperature of the underburden is higher than that of overburden .
Spatial distributions of gas saturation (S )
A number of characteristics can be concluded from Figure 4. The gas saturation in the whole process of production is below 0.2, and the gas saturation gradually decreases with the production. In the early stage of production, gas is mainly distributed around the production well, but when the water from the underburden and overburden flows into the production well, the gas is mainly distributed on the lower decomposition edge. It can be seen from Figure 4 that the range of gas is larger than hydrate decomposition area, which confirms the reason for the formation of secondary hydrate.
Gas production and water production behaviors
As shown in Figure 5, there are many significant changes in three cases of different n. As n increases, the total gas production rate Q and water production rate Q gradually decrease. The reason is that the relative permeability of gas and water decreases rapidly with the decrease of saturation in higher n. And it is noticeable that Q declines rapidly at the initial stage, then slows down gradually, and finally keeps stable. In the whole production, when the values of n are equal to 2.5, 3.5, and 4.5, Q decreases from 1793, 1113, and 670 m /day to 341, 331, and 299 m /day, respectively. As time goes by, Q has increased significantly and the rate of increase has slowed down. The main reason for the increase of Q is that as the production proceeds, the area of hydrate decomposition increases, which leads to more and more water entering the production well. In the whole production, when the values of n are equal to 2.5, 3.5, and 4.5, Q increases from 75, 60, and 47 m /day to 205, 186, and 158 m /day, respectively.
Figure 6(a) shows that with the increase of n, the total CH volume V and the total water volume V have decreased significantly. When n increases from 2.5 to 4.5, V drops from 1.93 10 m to 1.34 10 m , and V drops from 6.69 10 m to 4.46 10 m . When n = 4.5, the V and the V are 69% and 67% of n = 2.5, respectively.
Figure 6(b) gives the information about the change of gas-to-water ratio R = V /V over time. Before 650 days, the R gradually decreases as n increases. After 650 days, the R values in three different cases are very close to each other. This phenomenon mainly stems from the fact that, in the later stage of production, as the decomposition front is far away from the production well, water and gas flowing into the production well tends to be stable, resulting in the same gas–water ratio in three different cases.
The effects of irreducible fluids saturation, such as irreducible water saturation and irreducible gas saturation, on gas hydrate production are studied in this article. The main conclusions can be drawn as follows. After 365th day, the water from the overburden and underburden enters the production well. In 10 years of production, the range of hydrate decomposition is within 30 meters. As n increases, both the Q and the Q increase. The reason is that the relative permeability of gas and water decrease rapidly with the decrease of saturation in higher n. In the early stage of production, as the n increases, the R decreases. In the later stage, there is no effect on the R .