SS - Gas Hydrate: Numerical Investigation of Gas Production Strategy for the Hydrate Deposits in the Shenhu Area
Authors: Su Zheng , Moridis George J. , Zhang Keni , Yang Rui , Wu Nengyou 
Published: July 2010

Gas hydrates are solid crystalline compounds in which gas molecules are lodged within the lattices of ice crystals. Geophysical surveys and seafloor geochemical analysis in the past decade suggest that there is a significant potential for gas hydrate occurrence in the South China Sea (SCS). Shenhu area is considered as one of the most promising fields on the north continental slope of the SCS. Drilling and sampling at the site has indicated high methane hydrate saturations (up to 48%) in silty clay sediments with porosity of 0.38. The Hydrate-Bearing Layer (HBL) is overlain and underlain by zones of mobile water, and the system does not appear to be bounded by low-permeability strata. These characteristics define the production strategy, and appear to reduce the commercial production potential of the hydrate deposits.

In this study we assess by means of numerical simulation the production potential of hydrate accumulations in the Shenhu area. We simulate the hydrate dissociation and the corresponding gas production induced by depressurization at a single vertical well. We also test the gas production potential by alternately producing fluid and injecting hot water (huff-and-puff) at the well. To minimize gas losses through the overburden and excessive water production through proximity to the permeable, water-saturated zones, the production (perforated) interval is limited to the middle 1/3 section of the well within the hydrate layer.

Production from depressurization-induced dissociation based on a constant well pressure does not appear to be a promising approach in deposits characterized by the low hydraulic diffusion. This approach allows the production of hydrate-originating gas at decreasing rates (&lt;463m3/d for the permeability k=10mD) in the reference case. The production is invariably lower than would have been attainable in the presence of impermeable boundaries. The intrinsic permeability of the HBL is considered as the most insensitive parameter to enhance the gas production from the deposits. Huff-and-puff results in very low production rates (50-140m3/d) that are orders of magnitude much lower than generally acceptable standards of commercial viability of gas production from oceanic reservoirs. The study provides a first insight into the production potential of the Shenhu hydrate accumulations, and a basis for the analysis of the economic feasibility of gas production from that area.

Introduction

Background. Gas hydrates are crystalline substances composed of water and gas, in which a solid water lattice accommodates gas molecules in a cage-like structure (Sloan, 1998). Hydrate formation is promoted by low temperatures T and high pressures P, with significant heat exchange occurring during the reaction. Natural gas hydrates in geological systems involve mainly CH4, although hydrate-forming gases of thermogenic origin may also include CO2, H2S and N2 as guests (Sloan, 1998). The estimated amount of gas in the hydrate accumulations of the world greatly exceeds the volume of known conventional gas resources (Sloan, 1998). Because of their potential importance as an energy resource, CH4-hydrates are currently attracting significant attention. However, the role that gas hydrates may play in contributing to the world's energy requirements will depend ultimately on the availability of producible gas hydrate resources and the cost to extract them (Moridis et al., 2004).