PERMEABILITY ESTIMATES IN GAS HYDRATE RESERVOIRS OF THE NANKAI TROUGH

摘要

Various techniques have been developed to estimate reservoir permeability. A common methodology is to measure core permeability in the lab and to use this as a reference for other values derived from either a combination of local empirical relationships, formation pressure testing, magnetic resonance and/or geochemical logs. Recent developments in the use of gas hydrates as an alternative energy source has lead to increased interest in the understanding of hydrate-bearing reservoirs and their related permeabilities. Gas hydrates are solid icelike substances formed from a combination of water and natural gas in areas that are conducive to high pressure and low temperature. They are commonly found in shallow sediments in deep marine and below the permafrost in arctic regions. Much of the hydrate exploration to date has been in shallow unconsolidated sediments in the deep marine environment. The combination of estimating permeability in unconsolidated sediments plus the associated boundaries on gas hydrate stability presents a unique set of challenges. Core lab permeability measurements are difficult as the rock is generally too weak to maintain pore space integrity. Also, as hydrate is a solid with minimal intrinsic permeability almost all of the initial reservoir permeability is associated with the nonhydrate pore space. Hence any measurement of permeability needs to respect the in situ gas hydrate stability criteria of pressure and temperature and ensure that the hydrate does not start to dissociate to its constituents of water and gas. In this paper the authors review the approaches taken to derive permeability in the shallow marine hydrate bearing formations of the Nankai Trough where extensive log and core data have been acquired. Key items discussed are some unique core experiments, the use of wireline through-casing formation pressure tests and the permeability derivations from wireline log data.