Acoustic Wave Propagation in a Borehole with a Gas Hydrate-Bearing Sediment

Abstract

A knowledge of wave propagation in boreholes with gas hydrate-bearing sediments, a typical three-phase porous medium, is of great significance for better applications of acoustic logging information on the exploitation of gas hydrate. To study the wave propagation in such waveguides based on the Carcione–Leclaire three-phase theory, according to the equations of motion and constitutive relations, a staggered-grid finite-difference time-domain (FDTD) scheme and a real axis integration (RAI) algorithm in a two-dimensional (2D) cylindrical coordinate system are proposed. In the FDTD scheme, the partition method is used to solve the stiff problem, and the nonsplitting perfect matched layer (NPML) scheme is extended to solve the problem of the false reflection waves from the artificial boundaries of the computational region. In the RAI algorithm, combined with six boundary conditions, the displacement potentials of waves are studied to calculate the borehole acoustic wavefields. The effectiveness is verified by comparing the results of the two algorithms. On this basis, the acoustic logs within a gas hydrate-bearing sediment are investigated. In particular, the wave field in a borehole is analyzed and the amplitude of a Stoneley wave under different hydrate saturations is studied. The results indicate that the attenuation coefficient of the Stoneley wave increases with the increase of gas hydrate saturation. The acoustic responses in a borehole embedded in a horizontally stratified hydrate formation are also simulated by using the proposed FDTD scheme. The result shows that the amplitude of the Stoneley wave from the upper interface is smaller than that from the bottom interface.

A Borehole Acoustic Calculation Approach with Gas Hydrate Saturation Inversion in Gas Hydrate-Bearing Sediments

“…An example of a gas hydrate-bearing sediment proves that Equation ( 13) can be used to invert the gas hydrate saturation of the reservoir. Figure 2a,b show the borehole waveforms at distances of 2 m and 3 m, which are simulated by the staggered-grid finite-difference timedomain method in our previous work [34]. A rectangular window was used to intercept the flexural wave.…”

“…The lack of research on the borehole acoustic field of multiphase porous media hampers the effective utilization of guide wave information. The authors of this paper [34] studied the monopole borehole acoustic wavefield of a three-phase porous medium using the staggered-grid finite-difference time-domain (FDTD) scheme and the real axis integration (RAI) algorithm.…”

Discrete Element Simulation of the Macro-Meso Mechanical Behaviors of Gas-Hydrate-Bearing Sediments under Dynamic Loading

“…The mechanical properties of hydrate sediments are governed by the hydrate distribution in addition to factors such as net confining pressure, saturation, and mid-major stress coefficient [30]. Hydrate in hydrate sediments exists in three main forms [31], as shown in Figure 3: (i) pore-filling (Figure 4a); (ii) acting as a sediment soil skeleton (Figure 4b), and (iii) cemented between soil particles in the form of colloidal material (Figure 4c). Among them, the results of Waite et al [32] and Brugada et al [17] both showed that the pore-filling type has less influence on the mechanical properties of hydrate sediments, and the influence of hydrate distribution factors on the simulation results can be excluded to the maximum extent.…”

“…Therefore, the pore-filling-type structure is used in the modeling in this paper. In this paper, the parallel bonding model is chosen to calculate the contact state between particles according to the model of cemented clustered hydrate sediment distribution proposed by Li et al [31], i.e., there is bonding not only between hydrate and hydrate particles, but also between hydrate and soil particles, as shown in Figure 5. Figure In this paper, the parallel bonding model is chosen to calculate the contact state between particles according to the model of cemented clustered hydrate sediment distribution proposed by Li et al [31], i.e., there is bonding not only between hydrate and hydrate particles, but also between hydrate and soil particles, as shown in Figure 5.…”