Evaluation of the Gas Production Potential of Marine Hydrate Deposits in the Shenhu Area of the South China Sea
Authors: Li Gang , Moridis George J. , Zhang Keni , Li Xiao-Sen 
Published: June 2014

The Shenhu Area is located in the Pearl River Mouth Basin, the northern continental slope of the South China Sea. In 2007, gas hydrate samples were recovered during the scientific expedition conducted by the China Geological Survey in the area. Using numerical simulation and currently available data from site measurements, including the water depth, thickness of the Hydrate-Bearing Layer (HBL), sediment porosity, salinity and pressures and temperatures at key locations, we developed preliminarily estimates of the production potential of these hydrates as gas-producing resources.

We used measurements of ambient temperature in the sediments to determine the local geothermal gradient. Evidence from this and other field studies showed that the initial pressure distribution followed the hydrostatic gradient. Direct measurements from core samples provided estimates of the initial hydrate saturation and of the intrinsic permeabilities in the various strata of the system. The hydrate accumulations in the Shenhu Area appear to be Class 3 deposits (involving only a HBL) with a permeable overburden and underburden, and may pose significant challenges to gas production.

We investigated gas production from the Shenhu hydrates by means of depressurization and thermal stimulation using a single horizontal well placed in the middle of the HBL. The simulation results indicated that the hydrates dissociate along cylindrical interfaces around the well, and along horizontal dissociation interfaces at the top and bottom of the HBL. Production is invariably lower than that attainable in a confined system, and thermal stimulation is shown to to affect only a limited region around the well. The sensitivity analysis demonstrates the dependence of production on the level of depressurization, the initial hydrate saturation, the intrinsic permeability of the HBL, the temperature of the well, and the initial temperature of the HBL. A general observation is that gas production is low and is burdened with significant water production, making the hydrate accumulations at the Shenhu Area unattractive production targets with current technology.

Introduction

Background. Natural gas hydrates (NGH) are solid, nonstoichiometric compounds formed by host water molecules with small guest molecules, such as CH4, C2H6, C3H8, CO2, H2S, etc. (Sloan and Koh 2008). Natural gas hydrate deposits involve mainly CH4, and occur in the permafrost and in deep ocean sediment, where the necessary conditions of low temperature and high pressure exist for hydrate stability. Recent seismic explorations and geological researches show that natural gas hydrates existing in the sediment constitute a large natural gas reservoir that invites consideration as a potential strategic energy resource (Collett 2004; Klauda and Sandler 2005). Techniques for gas production from hydrate deposit are based on three major dissociation methods, i.e.:depressurization, involving a pressure P decrease below the dissociation pressure at a given temperature T of the hydrate;thermal stimulation, involving raising T above the dissociation temperature at a given P of the hydrate;inhibitor effects, based on the use of chemicals, such as salts and alcohols to shift the hydrate P-T equilibrium.

Among these possible methods for hydrate dissociation for gas production, depressurization appears to be the most efficient (Moridis and Reagan, 2007a; 2007b; Moridis et al., 2008; 2009a,b; Kurihara et al., 2005).