Measuring the in Situ Hydrate Saturation from γ-Ray Transmissivity Changes during Local Dissociation

Abstract

The worldwide abundance of gas hydrates in areas with engineering activities highlights the need for an accurate understanding of how much hydrate the soil contains, before any subsequent assessments are made. For the currently deployed gas hydrate saturation measurement, techniques are based on absolute measurements, derived from either the elastic properties of the formation, its nuclear magnetic resonance, or its electrical resistivity. None of those methods is wholly satisfactory, because of either the required reference measurements or a high dependence upon empirical correlations. This study explores the possibility to determine the hydrate saturation from relative measurements, by deriving its initial value from changes in γ transmissivity during the dissociation process of the contained hydrate. A customized testing apparatus was built, in which hydrate-bearing sand samples of up to 40% pore space saturation were first formed and then tested. The γ transmission from a ⁶⁰Co source was measured by a scintillation counter, while the cylindrical samples were radially dissociated by heating from their axis. Seven independent dissociation tests with an initial hydrate saturation between 20 and 40% were conducted. The results show that the change in local density (γ transmissivity) is an exponential function of the initial hydrate volume in place.