The use of huff and puff method in a single horizontal well in gas production from marine gas hydrate deposits in the Shenhu Area of South China Sea

The Shenhu Area is located in the Pearl River Mouth Basin, the northern continental slope of the South China Sea. It is expected that the Shenhu Area will become a strategic area of gas hydrate exploitation in China. Based on currently available data from site measurements, including water depth, thickness of the Hydrate-Bearing Layer (HBL), sediment porosity, salinity and pressures and temperatures at key locations, it is possible to develop preliminarily estimates of the gas production potential by numerical modeling. We used measurements of ambient temperature in the sediments to determine the local geothermal gradient. Estimates of the hydrate saturation and the intrinsic permeabilities of the system formations were obtained from direct measurements. The hydrate accumulations in the Shenhu Area are similar to Class 3 deposits (involving only an HBL), and the overburden and underburden layers are assumed to be permeable. These unconfined deposits may represent a large challenge for gas production. In this modeling study, we estimated gas production from hydrates at the SH7 drilling site of the Shenhu Area by means of the stream huff and puff method using a single horizontal well in the middle of the HBL The simulation results indicate that the hydrate dissociated zone expands around the well, and the hydrate formation occurs during the injection stage of the huff and puff process. The higher temperature of the injected brine appears to have a limited effect on gas production using the huff and puff method. Reasonable injection and production rates should be adopted to avoid the over pressurization and depressurization during each huff and puff cycle. Production is invariably lower than that attainable in a confined system, and thermal stimulation is shown to have an effect over a limited range around the well. The sensitivity analysis demonstrates the dependence of gas production on the level of the increment of the injection and production rates of the huff and puff process, the temperature of the injected brine and the existence of brine injection during the injection stage. (C) 2011 Elsevier B.V. All rights reserved.