Fracture-filling Hydrate Research Articles

Production Behavior of Hydrate-Bearing Sediments with Mixed Fracture- and Pore-Filling Hydrates

Most hydrate-bearing sediments worldwide exhibit mixed pore- and fracture-filling hydrates. Due to the high exploitation value, pore-filling hydrate production is the focus of current hydrate production research, and there is a lack of systematic research on the decomposition of fracture-filling hydrates and their effects on the evolution of temperature and pressure in hydrate-bearing sediments. If only the decomposition characteristics of pore-filling hydrates are studied while the fracture-filling hydrates decomposition and its effects on the hydrate-bearing sediments production process are ignored, the obtained research results would be inconsistent with the actual situation. Therefore, in this study, the effects of fracture-filling hydrates with different dipping angles on the hydrate production process were studied, and the necessity of considering the phenomenon of mixed pore- and fracture-filling hydrates in hydrate-bearing sediments was illustrated. On this basis, the simulation of a typical site (GMGS2-16) with mixed pore- and fracture-filling hydrates was constructed, and the production process was researched and optimized. The results indicated that: (a) fracture-filling hydrates formed in shallow fine-grained sediments and gradually approached the area of pore-filling hydrates, before a stable mixed zone was formed; (b) the occurrence of fracture-filling hydrates was conducive to the hydrate-bearing sediment depressurization production, and the promoting effect of the fracture-filling hydrate with smaller dipping angles was stronger; and (c) depressurization combined with heat injection could effectively compensate for the local low temperature and secondary hydrate caused by the mass decomposition of fracture-filled hydrates.

A new study of multi-phase mass and heat transfer in natural gas hydrate reservoir with an embedded discrete fracture model

Studies of the hydrate cores have shown that natural fractures can be frequently observed in hydrate reservoirs, resulting in a fracture-filled hydrate. Therefore, it is highly necessary for industries to predict the gas well productivity of fracture-filled hydrate reservoirs. In this work, an embedded discrete fracture model is applied to characterize the natural fractures of fracture-filled gas-hydrate reservoirs. The non-linear mass and energy conservation equations which are discretized with the finite-difference method are solved by the fully implicit approach, and the proposed model is justified by a commercial simulator. On the basis of the proposed model, we investigate the influences of natural fractures, fracture conductivity, and hydrate dissociation rate on the gas well productivity and the distributions of pressure, temperature, and hydrate saturation. The simulation results show that hydraulic and natural fractures exert significant impacts on the gas well productivity of the fracture-filled hydrate reservoirs, and the cumulative gas production is increased by 45.6% due to the existence of the connected natural fractures. The connected natural fractures can impose a more important influence on the gas well productivity than the unconnected natural fractures. The cumulative gas production is increased by 6.48% as Nnf is increased from 2 to 50, whereas the increase is 43.38% as Nf_con is increased from 0 to 4. In addition, A higher hydraulic fracture conductivity can be more favorable than a higher natural fracture conductivity for improving the gas well productivity, and a higher hydrate dissociation rate can lead to a lower temperature along fractures due to a more noticeable reduction of solid hydrate. This study provides a theoretical basis for developing fracture-filled hydrate reservoirs efficiently in the future.

Interfacial strength between ice and sediment: A solution towards fracture-filling hydrate system

Fracture-filling natural gas hydrates (NGH) are widely distributed in marine shallow clayey-silt sediment, and usually develop in forms of nodular, veins, layers, and massive pure hydrate. Interactions between these hydrate individuals and their surrounding sediment are crucial to evaluate potential submarine geohazards related to fracture-filling NGH systems. In this paper, we develop a novel apparatus to detect interfacial strength between hydrate individual and its host sediment. Shear strengths of three typical substances (i.e., massive pure hydrate, hydrate-saturated sediment, and hydrate-to-sediment interfaces) in fracture-filling NGH systems are examined and compared comprehensively by taking ice as substitute. Possible failure patterns of the fracture-filling NGH systems are discussed, and a tree-like model is proposed to describe formation mode and failure patterns of the fracture-filling NGH reservoirs. The results indicate that the load–displacement curves and shear surface characteristics of the ice-sediment interface differs significantly from those of the pure ice and ice-bearing sediment. The load–displacement curves of the ice-sediment interfacial specimens show double-peak loading characteristics, with multiple interlaced cracks on the shear surface. Additionally, the strength of the ice-sediment interfacial specimens is sensitive to shear rate, and decreases with increasing shear rate. These double-peak failure behaviors and rambling cracks in the shear surface are attributed to the nonlinear contact interfaces and two transitional sub-systems (i.e., silt-bearing ice and ice-bearing silt) in the specimens.

Experimental Investigation into Three-Dimensional Spatial Distribution of the Fracture-Filling Hydrate by Electrical Property of Hydrate-Bearing Sediments

As a future clean energy resource, the exploration and exploitation of natural gas hydrate are favorable for solving the energy crisis and improving environmental pollution. Detecting the spatial distribution of natural gas hydrate in the reservoir is of great importance in natural gas hydrate exploration and exploitation. Fracture-filling hydrate, one of the most common types of gas hydrate, usually appears as a massive or layered accumulation below the seafloor. This paper aims to detect the spatial distribution variation of fracture-filling hydrate in sediments using the electrical property in the laboratory. Massive hydrate and layered hydrate are formed in the electrical resistivity tomography device with a cylindrical array. Based on the electrical resistivity tomography data during the hydrate formation process, the three-dimensional resistivity images of the massive hydrate and layered hydrate are established by using finite element forward, Gauss–Newton inversion, and inverse distance weighted interpolation. Massive hydrate is easier to identify than layered hydrate because of the big difference between the massive hydrate area and surrounding sediments. The diffusion of salt ions in sediments makes the boundary of massive hydrate and layered hydrate change with hydrate formation. The average resistivity values of massive hydrate (50 Ω⋅m) and layered hydrate (1.4 Ω⋅m) differ by an order of magnitude due to the difference in the morphology of the fracture. Compared with the theoretical resistivity, it is found that the resistivity change of layered hydrate is in accordance with the change tendency of the theoretical value. The formation characteristic of massive hydrate is mainly affected by the pore water distribution and pore microstructure of hydrate. The hydrate formation does not necessarily cause the increase in resistivity, but the increase of resistivity must be due to the formation of hydrate. The decrease of resistivity in fine-grains is not obvious due to the cation adsorption of clay particles. These results provide a feasible approach to characterizing the resistivity and growth characteristics of fracture-filling hydrate reservoirs and provide support for the in-situ visual detection of fracture-filling hydrate.

Shear wave velocity prediction based on LSTM and its application for morphology identification and saturation inversion of gas hydrate

Compressional wave velocity (Vp) and shear wave velocity (Vs) are important geophysical parameters for studying gas hydrate systems. However, the Vs parameter is not commonly used in logging-while-drilling because of the high costs of acquiring it. To address this issue, we propose a long short-term memory (LSTM) neural network for predicting a complete shear wave velocity curve at wells in Alaminos Canyon Block 21 (AC 21) based on the limited measurement of Vs logs. The LSTM method performs better than the LSF method when comparing the Vs curves generated by them with the measured one. The measured Vp and predicted Vs curves were applied to determine the distribution of hydrates using the ratio of Vp increment to Vs increment, and we projected velocity samples into the cross plot of modelled Vp and Vs by assuming pore- and fracture-filling hydrate reservoirs to identify the hydrate morphologies. The pore- and fracture-filling gas-hydrate-bearing reservoirs were proved at 541–633 feet below sea floor (fbsf) and 1200–1500 fbsf in hole AC 21-A, respectively. However, the gas hydrate saturations estimated using the joint usage of Vp and Vs at shallow and deep depths are less than 10% (at most 30% locally) and 10%–20%, respectively. Because less Vs data is measured, a transfer learning technique is introduced to estimate a Vs curve at well AC 21-B. Comparing velocity samples with the theoretical velocity curves, the morphology of the hydrate in well AC 21-B is observed to be a horizontal fracture-filling type, and its gas hydrate saturation is approximately 10%–20% at the depth intervals of hydrate occurrence. According to our numerical calculations, the hydrate occurrence at well AC 21-B shows a better continuity than in well AC 21-A. • A LSTM model is developed to predict a complete VS curve in JIP wells. • Morphologies and distributions of gas hydrates are determined. • Gas hydrate saturation is estimated based on the jointly usage of Vp and Vs data. • transfer learning to predict a Vs curve and further analyse gas hydrates.

Morphology identification of gas hydrate from pointwise Lipschitz regularity for P- and S-wave velocity

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.

An experimental study on gas production from fracture-filled hydrate by CO2 and CO2/N2 replacement

The fracture-filled gas hydrate commonly occurs in marine fine-grained sediments and the fracture of rock in permafrost regions. Gas production from fracture-filled methane hydrate by the CO2 replacement method has been investigated. The reservoir condition of permafrost-associated gas hydrate in Muri Basin, Qilian Mountains was taken as an example to set up the experimental parameters. CO2, CO2/N2 (3:1) and CO2/N2 (1:3) were selected for gas replacement. Based on the results from six sets of laboratory experiments, we can learn that the factors affecting gas replacement included gas composition for replacement, pressure and the morphology of the hydrate. When the pressure was lower than the equilibrium pressure of CO2/N2 binary hydrate and the equilibrium pressure of methane hydrate, there was a rapid decomposition process of methane hydrate and might be a transient water freezing process at the first few minutes. The ice around methane hydrate might inhibit the further decomposition of methane hydrate. A higher replacement rate and accumulative methane recovery ratio can be obtained when the morphology of methane hydrate was well distributed thin layer, which could be obtained with the ice layer by gradually increasing its temperature. The thin layer of hydrate is the main morphology of the fracture-filled gas hydrate found in the Qilian Mountain permafrost. Therefore, the gas production from hydrate by CO2/N2 (3:1) replacement might be a good choice for gas production from hydrate in Muri Basin, Qilian Mountain.

Identifying the morphologies of gas hydrate distribution using P-wave velocity and density: a test from the GMGS2 expedition in the South China Sea

Pore-filling and fracture-filling are two basic distribution morphologies of gas hydrates in nature. A clear knowledge of gas hydrate morphology is important for better resource evaluation and exploitation. Improper exploitation may cause seafloor instability and exacerbate the greenhouse effect. To identify the gas hydrate morphologies in sediments, we made a thorough analysis of the characteristics of gas hydrate bearing sediments (GHBS) based on rock physics modeling. With the accumulation of gas hydrate in sediments, both the velocities of two types of GHBS increase, and their densities decrease. Therefore, these two morphologies cannot be differentiated only by velocity or density. After a series of tests, we found the attribute as a function of hydrate concentration show opposite trends for these two morphologies due to their different formation mechanisms. The morphology of gas hydrate can thus be identified by comparing the measured with its background value, which means the of the hydrate-free sediments. In 2013, China's second gas hydrate expedition was conducted by Guangzhou Marine Geologic Survey to explore gas hydrate resources in the northern South China Sea, and both two hydrate morphologies were recovered. We applied this method to three sites, which include two pore-filling and three fracture-filling hydrate layers. The data points, that agree with the actual situations, account for 72% and 82% of the total for the two pore-filling hydrate layers, respectively, and 86%, 74%, and 69% for the three fracture-filling hydrate layers, respectively.

The mechanism and verification analysis of permafrost-associated gas hydrate formation in the Qilian Mountain, Northwest China

Muri Basin in the Qilian Mountain is the only permafrost area in China where gas hydrate samples have been obtained through scientific drilling. Fracture-filling hydrate is the main type of gas hydrate found in the Qilian Mountain permafrost. Most of gas hydrate samples had been found in a thin-layer-like, flake and block group in a fracture of Jurassic mudstone and oil shale, although some pore-filling hydrate was found in porous sandstone. The mechanism for gas hydrate formation in the Qilian Mountain permafrost is as follows: gas generation from source rock was controlled by tectonic subsidence and uplift--gas migration and accumulation was controlled by fault and tight formation--gas hydrate formation and accumulation was controlled by permafrost. Some control factors for gas hydrate formation in the Qilian Mountain permafrost were analyzed and validated through numerical analysis and laboratory experiments. CSMGem was used to estimate the gas hydrate stability zone in the Qilian permafrost at a depth of 100–400 m. This method was used to analyze the gas composition of gas hydrate to determine the gas composition before gas hydrate formation. When the overlying formation of gas accumulation zone had a permeability of 0.05 × 10−15 m2 and water saturation of more than 0.8, gas from deep source rocks was sealed up to form the gas accumulation zone. Fracture-filling hydrate was formed in the overlap area of gas hydrate stability zone and gas accumulation zone. The experimental results showed that the lithology of reservoir played a key role in controlling the occurrence and distribution of gas hydrate in the Qilian Mountain permafrost.

Estimation of seismic attenuation of gas hydrate bearing sediments from multi-channel seismic data: A case study from Krishna–Godavari offshore basin

Seismic attenuation of gas hydrate bearing sediments is estimated from the multi-channel seismic data acquired in Krishna–Godavari (KG) offshore basin in the vicinity of Site NGHP-01-10, where ∼128 m of fracture-filled hydrate is confirmed by drilling/coring, to understand the effect of gas hydrate on the seismic attenuation. The seismic quality factor (Q), which is the inverse of seismic attenuation, is estimated from two methods – first, the spectral ratio method which utilizes the logarithmic ratio of the seafloor and bottom simulating reflector (BSR) spectra; and second, the peak frequency method which depends upon the variations of the peak frequency of the BSR spectrum as a function of traveltime. In the study area, the complex fault system has perturbed the seafloor resulting in noisy seafloor spectrum. As a result, the logarithm of the spectral ratio was also noisy and the Q-values obtained from spectral ratio method were found to be unstable. On the other hand, the peak frequency method is found to be relatively stable as it depends only on the peak frequency of the BSR which can be reliably obtained by conventional processing. This method yields an effective Q value of BSR which represents a weighted average of the interval Q-values of the water column and the gas hydrate bearing sediments. Since the Q-value of water column can be treated as a constant, the variations in effective Q-value can be interpreted in terms of the variations of Q in the gas hydrate bearing sediments for a constant thickness of water column.The estimated effective Q-values, along the inline and crossline seismic profiles, depend on several factors such as gas hydrate, free gas and the complex fault system. The combined interpretation of the quality factor and the interval velocity model helps to understand these effects in different zones of the seismic data. The attenuation of the background marine sediments is estimated from the analysis of crossline seismic profile. The gas hydrate bearing sediments, as inferred from the velocity model, show low attenuation as compared to that of the background sediments. In contrast, active migration of free gas across the fault system leads to increase in seismic attenuation within the gas hydrate stability zone. The complex fault zone may also lead to an increase in seismic attenuation. The blanked seismic zone shows a marginal decrease in seismic attenuation probably related to load-bearing form of gas hydrate. Our attenuation results are compared against the results obtained from waveform inversion, albeit with a higher resolution. The relative variation of Q estimated from the proposed method is close to the average Q-values estimated from full waveform inversion for most of the CDPs. The proposed methodology can be a useful technique to estimate seismic attenuation from multi-channel seismic data.

Logging-while-drilling and wireline velocities: Site NGHP-01-10, Krishna–Godavari Basin, India

At Site NGHP-01-10, Krishna–Godavari Basin, India, the downhole wireline logging tool measured higher porosity and sonic velocities in gas hydrate-bearing sediments compared to the logging-while-drilling (LWD) acquired data in two closely spaced wells (∼10 m). Using a rock physics model that assumes random fractures in unconsolidated sediments using Hashin–Shtrikman bounds, we show that difference in physical properties could be due to intra-site difference in the distribution and pattern of hydrate-filled fractures rather than differences in the volume of gas hydrate. Our fracture-inclusive model suggests that between the two holes the porosity and hydrate saturation of the background sediments is similar while porosity and hydrate saturation of the fracture systems change considerably, resulting in the change of the sonic log responses. Relative changes in hydrate saturation between sediments and fractures at 90 m below the sea floor (mbsf) depth (at a prominent seismic horizon), suggests that fracture-filling hydrate could have partly originated as pore-filling material in the sediments and grew as fracture-filling material as a result of fluid flow. The volume fracture profile of hydrate (fracture porosity × fracture saturation) between the two holes further suggests that the higher concentration of fractures in the wireline hole may not be a natural in-situ condition but rather a manifestation of the drilling compounded with time lapse in data recording between the LWD and wireline log data. Frequently reported, worldwide, intra-site variability in hydrate saturation from LWD and wireline logs in close proximity could be more related to interpretative methods than actual geologic variability.