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

Abstract

China conducted an offshore gas production test at the site SHSC2-6 in the Shenhu Area of the South China Sea in 2020. The geosystem and key parameters of this site, including the water depth, thickness of the hydrate and the free gas layers, pressure and temperature at key locations, and sediment porosity and permeability, are currently available. The hydrate dissociation is induced by depressurization through a single horizontal well located in the hydrate-bearing layer (HBL). The underlying free gas layer (FGL) below the HBL makes it a "two-gas coproduction" (natural gas hydrate and shallow gas) process. A novel numerical simulation code, the full implicit simulator of hydrate (FISH), is employed to develop the mathematical model and reproduce the 42 day field test in this marine hydrate reservoir. During the simulation, a permeability reduction model is used to fit the hydrate-bearing and the hydrate-free permeability in the field test. After strict history matching of the field test results (the mean absolute percentage error of gas production is 2.264%), we predict the long-term gas production behavior, including the gas and water production and the spatial distribution of pressure, temperature, phase saturations, etc. The results of the 8 year simulation indicate that the average gas production rate is 2.323 × 10 4 m 3 /day and the methane/water ratio is in the range of 50−120. This preliminary evaluation suggests that the hydrate deposits in the Shenhu Area are an unattractive production target with current technology. The main reason for this poor performance of gas production is the limited saturation of the free gas (smaller than 15%) in the hydrate reservoir. A future geological survey should focus on finding hydrate reservoirs with high gas saturation and large gas effective permeability. A key finding of this study is that the bottleneck of hydrate dissociation in the deposits is the low temperature caused by its own cooling effect. A thermal-assisted depressurization method is recommended as a potential strategy of gas recovery from marine hydrate reservoirs.

Control Factors for Gas Recovery in the Second Methane Hydrate Field Test in the Nankai Trough

“…In comparison to the field data of V G and V A , the mean absolute percentage error (MAPE) 14 of the simulated V G(sim) and V A(sim) was 12.04 and 11.54%, respectively. The coefficient of determination (R 2 ) was larger than 0.99.…”

“…in the above boundaries were set to fixed values in the simulation. The overburden and underburden were thick enough (244.2 and 184.5 m, respectively) for the simulation of pressure gradients in boundaries of the hydrate deposit …”

“…The deviations of the fluid production and component mass between the simulation and experimental results were all smaller than 10%. Using the FISH code, Li et al 14 calculated the 8 year gas recovery in the Shenhu area after excellent short-term history matching of field data. The average gas production rate was 2.323 × 10 4 m 3 /day, and the methane/water ratio was from 50 to 120 (m 3 of gas/ m 3 of water).…”

“…Numerical simulation is a low-cost method to evaluate the potential of gas recovery from hydrate deposits in natural fields. Numerical analyses of gas recovery in Mallik , and Shenhu area have been executed by TOUGH+Hydrate. Using the MH21-HYDRES codes, Yamamoto et al numerically simulated the field tests in the Nankai Trough and found that the heterogeneity of hydrate in the deposit caused the deviation of the predicted results.…”

ACS Editors’ Choice Virtual Collection from I&EC Research

“…10 Systems engineering articles examined dynamic optimization of combined cycle power plants 11 and systematic design of molecules and materials, 12 whereas transport-related articles reported new advances in using data driven models to describe flow in porous media 13 and fluidized beds 14 and methane hydrate dissociation in porous sands. 15 Several of the contributions report advances on the use of bioinspiration 16 or biopolymers 17,18 or smaller biomolecules 19 to create new materials or improve processes. Papers dealing with separations focus on CO 2 capture, 20 experimental methods for mixed gases, 21 flue gas desulfurization, 22 and production of small biomolecules.…”