On the resource evaluation of marine gas-hydrate deposits using seafloor compliance methods

Abstract

SUMMARY Methane hydrates are solid, non-stochiometric mixtures of water and the gas methane. They occur worldwide in sediment beneath the seafloor and estimates of the total mass available there exceed 1016 kg. Since each volume of hydrate can yield up to 164 volumes of gas, off-shore methane hydrate is recognized as a very important natural energy resource. The depth extent and stability of the hydrate zone are governed by the phase diagram for mixtures of methane and hydrate, determined by ambient pressures and temperatures. In sea depths greater than about 300 m, the pressure is high enough and the temperature low enough for hydrate to occur at the seafloor. The fraction of hydrate in the sediment usually increases with depth. The base of the hydrate zone is a phase boundary between solid hydrate and free gas and water. Its depth is determined principally by the value of the geothermal gradient, and stands out on seismic sections as a bright reflection. The diffuse upper boundary is not as well marked so that the total mass of hydrate cannot be determined by seismic measurements alone. Ocean surface gravity waves induce a low-frequency, horizontally propagating pressure field which deforms the seafloor. The displacement of the seafloor depends on the oceanic crustal density and elastic parameters, particularly the shear properties. Seafloor compliance is the transfer function between seafloor deformation and pressure as a function of frequency. Compliance measurements made at specific frequencies are tuned to structure at specific depths. Methane hydrate, like ice in permafrost, changes the physical properties of the material in which it is found, decreasing the density while increasing the compressional and especially the shear velocities. We apply the method of Crawford, Webb & Hildebrand (1991) and show how the addition of compliance data, which is particularly sensitive to changes in shear velocity, can aid in the evaluation of the resource. Two exploration scenarios are investigated through numerical modelling. In the first, a very simple example illustrates some of the fundamental characteristics of the compliance response. Most of the properties of the section including the probable regional thickness of the hydrate zone, 200 m, are assumed known from seismic surveys and spot drilling. The amount of hydrate in the available pore space is the only free parameter. Hydrate content expressed as a percentage may be determined to about ±2ɛ given compliance measurements with ɛ per cent error. The rule holds over the complete range of anticipated hydrate-content values. In the second, less information is assumed available a priori. The complementary compliance survey is required to find both the thickness and the hydrate content in hydrate zones about 200 m thick beneath the seafloor, which contain up to 20 and 40 per cent hydrate in the available pore space, respectively. A linear eigen-function analysis reveals that for these two models the total mass of hydrate, the product of hydrate content and thickness, may be estimated to an accuracy of about 2.81ɛ and 1.83ɛ per cent, respectively, given compliance measurements with an accuracy of ɛ per cent.