EVALUATING MARINE GAS-HYDRATE SYSTEMS PART I: STOCHASTIC ROCK-PHYSICS MODELS FOR ELECTRICAL RESISTIVITY AND SEISMIC VELOCITIES OF HYDRATE-BEARING SEDIMENTS

摘要

There is an increased need for investigating marine gas-hydrate systems to estimate the magnitude of the energy resource represented by the hydrate and to identify any unstable seafloor conditions that may result from hydrate dissociation, which can jeopardize drilling activities. Deep-water gas-hydrate systems can be studied on large scales with geophysical techniques, such as seismic and electrical surveys. To evaluate near-seafloor gas-hydrate environments we first need to build rock-physics quantitative relations between measurable parameters, such as elastic and electrical properties of sediments containing hydrates, and gas-hydrate saturation. In this study we assume a model of isotropic, load-bearing hydrates, uniformly distributed in the near-seafloor sediments. This Part I of a 2-paper series presents a method for stochastic joint modeling of elastic properties and electrical resistivity of gas-hydrate sediments. The petrophysical parameters involved in the modeling are difficult to estimate and are uncertain. Therefore, probability distribution functions (PDFs) are used to account for the uncertainty associated with each of the petrophysical quantities involved in the modeling. Both electrical resistivity and seismic velocities depend on porosity of the sediments and hydrate concentration, and we refer to them as common model parameters. A Monte Carlo procedure is used to draw values for these common parameters from their associated PDFs and then compute the corresponding velocity and electrical resistivity values using Monte Carlo draws from the PDFs for each of the petrophysical parameters that are required for elastic modeling and for Archie equation for electrical resistivity. The outcome of this procedure is represented by many Monte Carlo realizations that jointly relate hydrate concentration, resistivity, and seismic propagation velocity. This joint relation varies with depth and it is non-unique and uncertain due to variability of the input parameters. These theoretical relations can then be used to estimate hydrate concentration in Green Canyon Gulf of Mexico through a joint inversion technique presented in the Part II.