Numerical Analysis of Production Behaviors and Permeability Characteristics on the Second Gas Hydrate Production Test in the South China Sea

Abstract

The dissociation of hydrate plays an important role in restructuring the pore space and controlling the permeability, which determines the gas recovery from offshore hydrate reservoir. Based on the second production test in the Shenhu area, China South Sea (2020), this study investigates the production behaviors and permeability characteristics of the hydrate reservoir by using the depressurization method with a single horizontal well. A permeability adjustment model is proposed to revise the absolute permeability, and then the changes in permeability characteristics with hydrate dissociation are analyzed on a field scale for the first time. The dissociation of the hydrate can significantly improve the intrinsic permeability, thus promoting fluid flow and gas production efficiency. The simulation results show that it achieves a cumulative gas production of 89.41 × 104 m3 for 30 days, which is very close to the second production test of 86.14 × 104 m3. Also, the produced gas-to-water ratio is in the range of 15–55, and the ratio of dissociation-originating gas to the cumulative gas production reaches 0.50 for the 30 day simulation. The invasion of the underlying water with relatively high temperatures can promote the rate of hydrate dissociation in the bottom of the hydrate-bearing sediment due to the poor thermodynamic stability. The sensitivity analyses also indicate that the parameters of absolute permeability, irreducible gas saturation, and irreducible water saturation have significant influences on gas production. These insights verify the feasibility of the depressurization with the horizontal well and can be beneficial for future hydrate exploration.

Long-Term Analysis of the 2020 Gas Hydrate Production Test in the South China Sea by Full Implicit Simulator of Hydrate

“…With different hydrate crystal growth habits for S H lower and higher than 10%, the k r – S H relationship and the hydrate saturation exponent n are continuous and consistent, as discussed in our previous study . The absolute permeability of the HBL is k 0 = 11.10 mD . As listed in Table , the initial hydrate saturations in the HBL1 and HBL2 are 0.310 and 0.117, respectively (Figure ), and the corresponding k H values are 2.38 and 6.63 mD (Figure ).…”

“…33 The absolute permeability of the HBL is k 0 = 11.10 mD. 29 As listed in Table 1, the initial hydrate saturations in the HBL1 and HBL2 are 0.310 and 0.117, respectively (Figure 3), and the corresponding k H values are 2.38 and 6.63 mD (Figure 4). 16 As already discussed above, the boundary conditions (pressure and temperature) of the grids in the regions of the uppermost, the lowermost, and the farmost keep constant with their initial physical and thermodynamic conditions.…”

“…The thickness of the overburden and the underburden are 191.8 and 192.0 m, respectively, which are much thicker than those used in previous studies. 22,24,29,32 The main reason for using this kind of thick boundary is that the range of the pressure transmission in the marine sediments is as large as hundreds of meters. To simulate the pressure gradient more accurately, thick boundaries in the hydrate reservoir are necessary.…”

“…As shown in Figure 5b, a maximum of R MW(sim) = 115.8 m 3 of CH 4 /m 3 of H 2 O is reached before a continuous decline begins at about t = 150 days. The final R MW(sim) at t = 8 years is 51.7 m 3 of CH 4 /m 3 of H 2 O, which is 1− 2 orders of magnitude of that in field-scale simulations (166.7 23 and 10−50 29 ST m 3 of gas phase/m 3 of aqueous phase) reported in previous studies using the depressurization method. The profile of the R MW(sim) curve reflects the ratio of the methane and water production rate, and it is consistent with the simulation results of the spatial distributions of S A , S G , k EffA , and k EffG , which will be discussed later.…”

“…In the International Gas Hydrate Code Comparison Study (IGHCCS2), White et al 28 compared the simulation performance of over 20 gas hydrate simulation codes on the experiments and the first production test in Eastern Nankai Trough, Japan. Xiao et al 29 analyzed the gas hydrate production test in 2020 in the South China Sea by the 30 day and 1 year numerical simulation, and a permeability adjustment model was proposed for the absolute permeability. The geometry and physical characteristics of the hydrate-bearing porous media, including porosity, permeability, aqueous/gas/hydrate saturations, etc., are highly complex and heterogeneous.…”