In this study, we use two gas hydrate wells in Canada to evaluate the existing rock physics models for gas hydrates and find the best-fit model for gas hydrate quantification using velocity information. The two wells have gas hydrate saturation estimates from resistivity and NMR data, respectively. We use the gas hydrate saturation estimates and model the P- and S-wave velocity responses using several existing rock physical models. The estimated velocities were then compared with the measured velocities from the well. The result indicates that the effective medium theory model which treats gas hydrates as load-bearing grains matches the well log data best. We apply this model to a 3D seismic volume in the deepwater Gulf of Mexico (GOM) and predict gas hydrate concentration using seismic inversion techniques.
It is observed that the formation of gas hydrates in shallow sediments will elevate the P- and S-wave velocities of the hosting rocks. However, how gas hydrates deposit in the hosting rocks remain unclear. Numerous rock physics models corresponding to different pore geometries were proposed to estimate the effect of gas hydrates on the elastic properties of the hosting rocks. There are large differences among the predictions from the different models.
To quantify gas hydrates using velocity information, it is essential to apply the most practical gas hydrate rock physics model. Two independent sources of information - gas hydrate saturation prediction from resistivity data and from NMR data - provide a means for validating the different rock physics models.
Gas hydrate drillings worldwide have shown that higher gas hydrate concentrations create an increase in the elastic properties of rocks. There are a number of rock physics models in the literature that attempt to quantify this effect. Figure 1 shows the major rock physics models of natural gas hydrates.
Figure 1. Existing micro-structural models of gas hydrate bearing sediments (GR-grain; GH-gas hydrate, edited from Dai et al., 2004) (Available in full paper)
Models 1 and 2 are the cementation models that treat the grains as randomly packed spheres where the gas hydrates occur at the contact point (Model 1) or grow around the grains (Model 2) (Ecker 1998; Dvorkin and Nur, 1996). Models 3 and 4 are variations of the cementation models, but consider the gas hydrate as either a component of the load-bearing matrix or filling the pores (Helgerud et al., 1999). Model 5 is an inclusion-type model that treats gas hydrate and grains as the matrix and inclusions respectively, solving for elastic moduli of the system by iteratively solving either the inclusion-type or self-consistent type equations. Models 1 through 5 all consider gas hydrate as homogeneously distributed in the sediments. However, evidence of gas hydrate coring reveals that hydrates often exist as nodules and fracture fillings in the shallow sediments. This geometry is illustrated in Model 6. No quantitative treatment of this geometric model exists in the literature.