No abstract

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 resource. 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 hydrate deposits involving a single HBL within fine-textured sediments and boundaries (overburden and underburden layers) which have the same intrinsic permeabilities with the HBL. 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 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.

The methane hydrate was formed in a pressure vessel 38 mm in id and 500 mm in length. Experimental works on gas production from the hydrate-bearing core by depressurization to 0.1, 0.93, and 1.93 MPa have been carried out. The hydrate reservoir simulator TOUGH-Fx/Hydrate was used to simulate the experimental gas production behavior, and the intrinsic hydration dissociation constant (K 0 ) fitted for the experimental data was on the order of 10 4 mol m -2 Pa -1 s -1 , which was one order lower than that of the bulk hydrate dissociation. The sensitivity analyses based on the simulator have been carried out, and the results suggested that the hydrate dissociation kinetics had a great effect on the gas production behavior for the laboratory-scale hydrate-bearing core. However for a field-scale hydrate reservoir, the flow ability dominated the gas production behavior and the effect of hydrate dissociation kinetics on the gas production behavior could be neglected.