The purpose of this study is to evaluate a series of well log responses within an in-situ natural gas hydrate interval, specifically to develop a method by which formation characteristics such as porosity and hydrate saturation can be determined within a suspected hydrate zone.
Efforts have been undertaken to analyze the use of neutron log for quantification of hydrate deposits. Neutron porosity logs respond primarily to the amount of hydrogen present in the formation. With an increase in hydrogen content there will be an increase in the apparent neutron porosity. Theoretically, a hydrate zone should have a higher neutron porosity compared to gas zones. The increased neutron porosity reflects the higher concentration of hydrogen atoms in the hydrate state. The stoichiometric hydrogen content within a unit pore volume of methane gas, free water, hydrate and ice have been calculated. Hydrate contains volumetrically the greatest amount of hydrogen atoms in comparison to free water, ice, and methane gas. A. zone saturated with methane gas would contain the smallest number of hydrogen atoms representing the other extreme, with free water and ice existing as intermediates.
The sonic velocity log has also been examined in detail in order to evaluate the effect of hydrate on porosity calculations undertaken with data from the transit time device. A. detailed examination of a series of Pickett crossplots used to evaluate hydrate saturation have been included.
This paper concludes with an examination of one stratigraphically controlled hydrate occurrence in the Kuparuk Oil Field. The newly developed techniques have been used to calculate hydrate saturation and porosity within the hydrate interval in order to evaluate the new procedures. In addition, a procedure for estimation of natural gas in the form of hydrate deposits is included.
Gas hydrates are crystalline compounds of water and gas in which the ice lattice accommodates the gas molecules in a cage like-structure. Hydrogen-bonded water molecules form a three dimensional shell whose voids can be filled with a wide array of gas molecules, such as methane, ethane and argon.
The lattice structure depends on the shape and size of the gas molecules which are in contact with the water. The degree of gas saturation within the ice lattice is dependent on temperature pressure and gas composition. Typically, more than 90 percent of the available sites in the ice lattice are occupied in a pure methane hydrate.
The formation of hydrate occurs within a limited range of temperature and pressure conditions when sufficient concentrations of hydrate forming gases are present along with water. The pressure and temperature conditions suitable for the formation of natural gas hydrates occur in regions of permafrost and beneath the sea in outer continental margins and ocean basins (Kvenvolden et al.,1980). The fact that temperature and pressure conditions associated with permafrost may fall within the stability field of gas hydrates was first recognized by Katz (1945). Hydrates can occur not only in permafrost but also below the base of the permafrost at temperatures above the freezing point of water.