Quantitative characterization of gas hydrate bearing sediment using elastic-electrical rock physics models

Abstract

Successful evaluation of reservoir resources in gas hydrate-bearing sediments depends on the accurate estimation of reservoir parameters, such as hydrate saturation, porosity, and clay content. Elastic and electrical measurements are affected by several rock and fluid properties and their joint interpretation can provide complementary information and improve the accuracy of reservoir characterization results. In this work, we propose a new rock physics inversion workflow based on 3D rock physics template of elastic and electrical attributes for the direct estimation of reservoir parameters. We first conduct a quantitative analysis of the available well log data and core data to understand the morphology of gas hydrate within the marine sediment, and choose the appropriate elastic rock physics model. Then, we construct the 3D rock physics template by discretizing the 3D space of reservoir parameters and implementing the suitable elastic and electrical models to calculate the Poisson's ratio, P-wave impedance and electrical resistivity for each combination of values of rock and fluid properties. Finally, we superimpose the measurements on the templates and use a grid searching method to find the nearest gridded node to the data point and minimize the objective function of the mismatch between data and model predictions. To investigate the feasibility of our proposed inversion scheme, we test a rock physics model based on effective medium theory and a modified Archie's equation, and invert for reservoir parameters of gas hydrate-bearing sediments at well 1250F and 1247B, in Hydrate Ridge. The good match between inversion results and core data suggests that the proposed inversion scheme is applicable to real data and allows combining elastic and electrical data for reservoir parameter estimation of gas hydrate-bearing sediments.