Seismic modeling of multidimensional heterogeneity scales of Mallik gas hydrate reservoirs, Northwest Territories of Canada

Abstract

[1] In hydrate-bearing sediments, the velocity and attenuation of compressional and shear waves depend primarily on the spatial distribution of hydrates in the pore space of the subsurface lithologies. Recent characterizations of gas hydrate accumulations based on seismic velocity and attenuation generally assume homogeneous sedimentary layers and neglect effects from large-and small-scale heterogeneities of hydrate-bearing sediments. We present an algorithm, based on stochastic medium theory, to construct heterogeneous multivariable models that mimic heterogeneities of hydrate-bearing sediments at the level of detail provided by borehole logging data. Using this algorithm, we model some key petrophysical properties of gas hydrates within heterogeneous sediments near the Mallik well site, Northwest Territories, Canada. The modeled density, and P and S wave velocities used in combination with a modified Biot-Gassmann theory provide a first-order estimate of the in situ volume of gas hydrate near the Mallik 5L-38 borehole. Our results suggest a range of 528 to 768 Â 10 6 m 3 /km 2 of natural gas trapped within hydrates, nearly an order of magnitude lower than earlier estimates which did not include effects of small-scale heterogeneities. Further, the petrophysical models are combined with a 3-D finite difference modeling algorithm to study seismic attenuation due to scattering and leaky mode propagation. Simulations of a near-offset vertical seismic profile and cross-borehole numerical surveys demonstrate that attenuation of seismic energy may not be directly related to the intrinsic attenuation of hydrate-bearing sediments but, instead, may be largely attributed to scattering from small-scale heterogeneities and highly attenuate leaky mode propagation of seismic waves through larger-scale heterogeneities in sediments.

Application of conditional simulation of heterogeneous rock properties to seismic scattering and attenuation analysis in gas hydrate reservoirs

“…Stefani and De (2001) analyzed a wide range of vertical and horizontal well logs and observed the same statistical characteristics in reservoir scale well logs as in crustal logs (Goff and Holliger, 1999). Huang et al (2009) demonstrated the applicability of spectral-based simulation method to characterize heterogeneity of hydrate-bearing sediments honoring different correlation coefficients between multiple rock properties inferred from collocated well logs. They constructed petrophysical reservoir models to estimate hydrate volume as well as to simulate elastic seismic wave propagation and thus potentially provided a link between seismic expressions of heterogeneities to volume of hydrate in the pore space of poorly consolidated sediments.…”

“…A significant limitation of their study was that a nonconditional simulation method was employed so that the reservoir models were unable to represent the spatial variability of field observations and measurements. In this study, we extend the nonconditional simulation method used in Huang et al (2009) to a conditional simulation method. Chilès and Delfiner (1999) describe a variety of conditional simulation methods of continuous random functions that have been developed.…”

“…Sequential Gaussian and Matrix Decomposition methods can directly condition the model with measurements, whereas others such as Turning Band and Discrete Spectral methods must employ an additional kriging step to honor the data (Delfiner, 1976;Journel, 1974). We follow the spectral-based simulation method initially used in Huang et al (2009). One advantage of the spectral-based method over others is the computation efficiency inherited from Fast Fourier Transform (FFT).…”

In situ observation of gas hydrates growth hosted in porous media

“…In hydrate-bearing sediments, gas hydrates can be (a) free-floating in the sediment matrix, (b) contacting and existing sediment grains, but not cementing or (c) cementing and stiffening the bulk sediments [2,3]. The porous microstructure of gas hydrates governs the mechanical strength and hydraulic permeability of the hydrate-bearing sediments [4]. Moreover, for exploration of gas hydrates, the velocity and attenuation of compressional and shear waves depend primarily on the spatial distribution of hydrates in the pore space in sediments [5].…”

Direct Observation of THF Hydrate Formation in Porous Microstructure Using Magnetic Resonance Imaging

“…The microstructure of hydrates in the pore space of the sediment determines the velocity and attenuation of compression and shear waves, which are the important parameters of hydrate exploration. Moreover the microstructure of the hydrates in sediment is crucial in the field of exploitation, because it governs the mechanical strength and hydraulic permeability of the hydrate-bearing sediment [9].…”