Energy Reports (Nov 2021)
Morphology identification of gas hydrate from pointwise Lipschitz regularity for P- and S-wave velocity
Abstract
The accurate morphology identification of gas hydrate-bearing sediments (GHBS) has great significance in practical exploitation and subsequent resource evaluation. Previous studies have disclosed two main morphologies for gas hydrate in sediments: pore- and fracture-filling. However, the existing identification methods of gas hydrate’s morphology rarely consider their intrinsic differences in distribution characteristics. In this paper, a new method is proposed to identify the morphology of hydrate according to the scattered distribution of fracture dips for fracture-filling GHBS. Firstly, numerical simulations are performed to study the relationships between the morphology of hydrate and the sonic velocities. Considering the dip variation is within a certain range for fracture-filling hydrate, the theoretical curves show that the resulting mutation degrees between P- and S-wave velocities are inconsistent in fracture-filling GHBS, which is different from pore-filling GHBS. Then the modified estimation method for pointwise Lipschitz exponent α is introduced to capture their differences. The cross plots of Lipschitz exponent for P-wave velocity, α(Vp), and Lipschitz exponent for S-wave velocity, α(Vs), indicate that most of the dots representing pore-filling GHBS are evenly distributed near the line α(Vp)=α(Vs), while the dots representing fracture-filling GHBS are scattered outside the line α(Vp)=α(Vs). Based on these characteristics, a ratio method is put forward to differentiate the two types of hydrate. These hypotheses and methods are verified using the measured P- and S-wave velocities log data at different sites in Leg 204, Ocean Drilling Program (ODP), in the United States. Finally, the results of this new method agree closely with core data.
