8. Reservoir Characterization

Methane Hydrate Quantification From Mud Line to Bottom Simulating Reflector

Jack Dvorkin(1) and Richard C. Uden(2)

(1) Stanford University CA, USA. (2) Rock Solid Images TX, USA.

Abstract

Methane hydrates exist in porous sediment between the sea floor and the bottom simulating reflector. The location and distribution of the sediment where hydrate may possibly form vary laterally and vertically in a very heterogeneous manner. Mapping the quantity of methane hydrate is a challenging task. This paper demonstrates a method that quantifies the total methane hydrate within sediment beds at and below seismic resolution. We also discuss possible solutions for beds thicker than the lowest seismic frequency – the so-called thick layer problem. This method uses a rock physics transform that connects the gas hydrate saturation in sediment to the elastic-wave velocity (Dvorkin et al., 2003). It is based on the Dvorkin and Nur (1996) effective-medium model which relates the elastic moduli of soft unconsolidated clastic sediment to the porosity, pore fluid compressibility, mineralogy, and effective pressure. When beds are below the seismic resolution, the problem is one of scale. Therefore, we pose the problem of hydrate reservoir characterization differently by seeking a scale-independent volumetric reservoir property and a scale-independent seismic attribute to quantify the hydrate volume. We have developed a cumulative seismic attribute (CATT) that is very close to being scale independent. This is a new type of attribute formed by integrating the seismic trace impedance. We show the CATT attribute applied to synthetic seismic data to quantify the vertical hydrate accumulation, which means that this class of attributes can readily be applied to real seismic data using seismic impedance volumes. Our approach is to admit that we cannot quantify reservoir property at a sub-resolution scale. Instead, we quantify the volume integral of this property (or the cumulative property) and introduce a new class of seismic attributes, the cumulative attributes, that can be related by means of rock physics to the cumulative reservoir property, which in this specific example is the accumulated volume of methane hydrate.

Last modified: Fri May 12 06:03:57 2006