Abstract

Methane hydrates are solid, nonstoichiometric mixtures of water and the gas methane. They occur worldwide in sediment beneath the sea floor, and estimates of the total mass available there exceed 1016Kg. Since each volume of hydrate can yield up to 164 volumes of gas, offshore methane hydrate is recognized as a very important natural energy resource.

The depth extent and stability of the hydrate zone is governed by the phase diagram for mixtures of methane and hydrate and 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 increasing 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. It 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 is not determined easily by seismic alone.

The addition of electrical data, collected with a seafloor transient electric dipole-dipole system, can aid in the evaluation of the resource. Methane hydrate, like ice, is electrically insulating. Deposits of hydrate in porous sediment cause an increase in the formation resistivity. The data consist of measurements of the time taken for an electrical disturbance to diffuse from the transmitting dipole to the receiving dipole. The traveltime is related simply to the resistivity: the higher the resistivity, the shorter the traveltime. A sounding curve may be obtained by measuring traveltimes as a function of the separation between the dipoles and interpreted in terms of the variation of porosity with depth.

Two exploration scenarios are investigated through numerical modeling. In the first, a very simple example illustrating some of the fundamental characteristics of the electrical response, most of the properties of the section including the probable, regional thickness of the hydrate zone (200 m) are assumed known from seismic 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 ±ϵ given a measurement of traveltime at just one separation (800 m) to ϵ%. The rule holds over the complete range of anticipated hydrate content values.

In the second example, less information is assumed available a priori and the complementary electrical survey is required to find both the thickness and the hydrate content in a hydrate zone about 200 m thick beneath the sea floor containing 20 and 40% hydrate in the available pore space, respectively. A linear eigenfunction 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 ±ϵ given measurements of traveltime to an accuracy of ϵ% over a range of separations from 100 to 1300 m.

The value of the electrical information depends directly on the accuracy to which transient arrivals can be measured on the sea floor in water depths exceeding 300 m over a separation of the order of a kilometer, the error parameter e. While results of appropriate surveys, or even noise measurements, have not been published in the open literature, surveys on a smaller 100 m scale have been conducted by my group. Based on these data, I suggest that the value of ϵ may be of the order of 3%.

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