Increasing Hydrate Saturation Research Articles

An easy and efficient way to evaluate mechanical properties of gas hydrate-bearing sediments: The direct shear test

Understanding the mechanical behaviours of gas hydrate-bearing sediments (GHBS) is important for their associated applications in wellbore stability, stratum deformation during exploitation, geological disaster prevention, and the risk assessment of replacing CH4 with CO2 in hydrate reservoirs and CO2 sequestration in oceans. However, triaxial tests on mechanical properties of GHBS are taxing and time-consuming. Here, we presented an easy and efficient way to evaluate these behaviours by using a self-developed direct shear apparatus. Then a series of direct shear tests on GHBS represented by CO2 hydrate-bearing silt were performed to investigate their mechanical behaviours and strength indices by changing the axial pressure, CO2 hydrate saturation, shear rate and hydrate synthesis temperature. Our results indicate that CO2 hydrate significantly strengthens specimens by cementing silt grains. In addition, when hydrate saturation increases, the cohesions are enhanced from 0.09 to 2.39MPa, and the internal friction angles increase and decrease at the range from 28.6° to 43.3° under the experimental conditions. These findings have direct implications for evaluating the stability and safety of natural gas hydrate reservoirs, CO2 replacement to extract CH4 and CO2 sequestration.

Effect of hydrate nucleation mechanisms and capillarity on permeability reduction in granular media

AbstractA model for water permeability reduction in hydrate‐bearing sediments is presented by considering capillary effect in hydrate nucleation. Both grain‐coating and pore‐filling cases are considered. The model is developed from a series of lattice Boltzmann flow simulations. Results show that the permeability decreases quasi‐linearly with increasing hydrate saturation during grain‐coating nucleation and that the permeability tends to be higher than predicted by previous analytical models, in which capillarity is not taken into account. The permeability follows unique reduction curve and is not so sensitive to initial sediment bulk density and grain size distribution. Simulations further show that there is a transition zone at Shyd = 0.3~0.4, where permeability reduction trend switches from grain‐coating model to pore‐filling model. Analyses of tortuosity and surface area confirm that the permeability reduction mechanisms result from pore‐channel blocking in grain‐coating case and pore size reduction in pore‐filling case.

The water retention curve and relative permeability for gas production from hydrate‐bearing sediments: pore‐network model simulation

AbstractThe water retention curve and relative permeability are critical to predict gas and water production from hydrate‐bearing sediments. However, values for key parameters that characterize gas and water flows during hydrate dissociation have not been identified due to experimental challenges. This study utilizes the combined techniques of micro‐focus X‐ray computed tomography (CT) and pore‐network model simulation to identify proper values for those key parameters, such as gas entry pressure, residual water saturation, and curve fitting values. Hydrates with various saturation and morphology are realized in the pore‐network that was extracted from micron‐resolution CT images of sediments recovered from the hydrate deposit at the Mallik site, and then the processes of gas invasion, hydrate dissociation, gas expansion, and gas and water permeability are simulated. Results show that greater hydrate saturation in sediments lead to higher gas entry pressure, higher residual water saturation, and steeper water retention curve. An increase in hydrate saturation decreases gas permeability but has marginal effects on water permeability in sediments with uniformly distributed hydrate. Hydrate morphology has more significant impacts than hydrate saturation on relative permeability. Sediments with heterogeneously distributed hydrate tend to result in lower residual water saturation and higher gas and water permeability. In this sense, the Brooks‐Corey model that uses two fitting parameters individually for gas and water permeability properly capture the effect of hydrate saturation and morphology on gas and water flows in hydrate‐bearing sediments.

Strengthening mechanism of cemented hydrate‐bearing sand at microscales

AbstractOn the basis of hypothetical particle‐level mechanisms, several constitutive models of hydrate‐bearing sediments have been proposed previously for gas production. However, to the best of our knowledge, the microstructural large‐strain behaviors of hydrate‐bearing sediments have not been reported to date because of the experimental challenges posed by the high‐pressure and low‐temperature testing conditions. Herein, a novel microtriaxial testing apparatus was developed, and the mechanical large‐strain behavior of hydrate‐bearing sediments with various hydrate saturation values (Sh = 0%, 39%, and 62%) was analyzed using microfocus X‐ray computed tomography. Patchy hydrates were observed in the sediments at Sh = 39%. The obtained stress‐strain relationships indicated strengthening with increasing hydrate saturation and a brittle failure mode of the hydrate‐bearing sand. Localized deformations were quantified via image processing at the submillimeter and micrometer scale. Shear planes and particle deformation and/or rotation were detected, and the shear band thickness decreased with increasing hydrate saturation.

The effect of hydrate saturation on water retention curves in hydrate‐bearing sediments

AbstractThe experimental measurement of water retention curve in hydrate‐bearing sediments is critically important to understand the behavior of hydrate dissociation and gas production. In this study, tetrahydrofuran (THF) is selected as hydrate former. The pore habit of THF hydrates is investigated by visual observation in a transparent micromodel. It is confirmed that THF hydrates are not wetting phase on the quartz surface of the micromodel and occupy either an entire pore or part of pore space resulting in change in pore size distribution. And the measurement of water retention curves in THF hydrate‐bearing sediments with hydrate saturation ranging from Sh = 0 to Sh = 0.7 is conducted for excess water condition. The experimental results show that the gas entry pressure and the capillary pressure increase with increasing hydrate saturation. Based on the experimental results, fitting parameters for van Genuchten equation are suggested for different hydrate saturation conditions.

Study on the mechanical properties of hydrate-bearing silty clay

The mechanical properties are important for the evaluation of stratum deformation and instability in hydrate exploitation. By using an integrated test apparatus for synthesis of hydrate sediment and tri-axial tests, a series of compression tests is conducted on the silty clay containing tetrahydrofuran hydrate (SCTH) similar to that of South China Sea. The stress–strain curves and Mohr circles are obtained at different degree of hydrate saturations and confining pressures. The silty clay containing tetrahydrofuran hydrate shows typical ductile behavior, the shear strength increases linearly with the increase of hydrate saturation and confining pressure. The secant modulus increases with the increase of hydrate saturation. The shear strength is analyzed based on the Drucker–Prager criterion, Mohr–Coulomb criterion, Lade–Duncan criterion, while the secant modulus is analyzed using the mixed law of composite materials. It is shown that the strength can be well described by the Drucker–Prager criterion and Mohr–Coulomb criterion, and the secant modulus is close to the harmonic average of the modulus of soil skeleton and hydrate in SCTH.

Mechanical behavior of hydrate-bearing pressure-core sediments visualized under triaxial compression

Geomechanical and geotechnical properties are essential for evaluating the stability of deep seabed and subsea production systems for gas hydrate extraction from marine sediments. In this study, natural gas hydrate-bearing sediment was subjected to triaxial compression tests (shearing) using a newly developed triaxial testing system (TACTT) to investigate the geomechanical behavior of sediments recovered from below the seafloor in the eastern Nankai Trough, where the first Japanese offshore production test was conducted in 2013. The sediments were recovered using a hybrid pressure coring system, with pressure cores cut using onboard pressure core analysis tools. The pressure cores were subsequently transferred to our shore-based laboratory and subsampled using pressure core non-destructive analysis tools (PNATS) for the TACTT system. Pressure and temperature conditions were maintained within the hydrate stability boundary during coring and laboratory testing. An image processing technique was used to capture deformation of the sediment sample within the transparent acrylic test cell, and digital photographs were obtained for each 0.1% strain level experienced by the sample during the triaxial compression test. Analysis of the digitized images showed that sediments with 63% hydrate saturation exhibited brittle failure, whereas hydrate-free sediments exhibited ductile failure. The increase in shear strength with increasing hydrate saturation in natural gas hydrates is in agreement with previous data from sediments containing synthetic gas hydrates.

Experimental determination of permeability of porous media in the presence of gas hydrates

Abstract Permeability variation, particularly in the presence of gas hydrates, greatly influences production of natural gas from hydrate reservoirs. A series of experiments were performed to investigate the variation of permeability in gas hydrates-bearing sediments. Carbon dioxide hydrate was formed in Ottawa 20/30 sand samples contained within a rigid cell using a partial water saturation formation methodology. Different initial water saturations were used to achieve hydrate saturations up to 45% and the corresponding water permeability was measured during steady-state flow. The experimental permeability results were compared with several theoretical models using both the quantitative and graphical analyses. A hybrid modeling approach based on the weighted combination of grain coating and the pore filling models was used to fit the measured experimental data. The experimental results were also compared to relevant experimental studies that used similar methods to form hydrates. Our analysis indicates a gradual reduction in permeability with increasing hydrate saturation, which is consistent with earlier studies. Further analysis using hybrid modeling suggests a progressive change in the hydrate formation morphology from cementing to that of the pore filling with increasing hydrate saturation.

Relation between relative permeability and hydrate saturation in Shenhu area, South China Sea

Nuclear magnetic resonance measurements in hydrate-bearing sandstone samples from the Shenhu area, South China Sea were used to study the effect of gas hydrates on the sandstone permeability. The hydrate-bearing samples contain pore-filling hydrates. The data show that the pore-filling hydrates greatly affect the formation permeability while depending on many factors that also bear on permeability; furthermore, with increasing hydrate saturation, the formation permeability decreases. We used the Masuda model and an exponent N = 7.9718 to formulate the empirical equation that describes the relation between relative permeability and hydrate saturation for the Shenhu area samples.

<i>In Situ</i> Measurement of Electrical Resistivity of Ocean Sediments Containing Gas Hydrates

An understanding of the physical properties of hydrate-bearing sediment is necessary for interpretation of geophysical data collected in field settings. We have conducted a laboratory experiment to measure the electrical property of initially water saturated sediment containing natural gas hydrate. When gas hydrate was formed from pore fluid in ocean sediment, bulk sediment resistivity was significantly increased. The resistivity of the sediment was largely changed below 20% hydrate saturation. With the increasing hydrate saturation, the resistivity of sediment was increased and the resistivity of pore fluid was decrease. In the final process of hydrate formation, the resistivity depression was found mainly due to the transition of gas hydrate morphology. The electrical resistivity of hydrate specimens varied from 1.930 Ohm.m to 3.950 Ohm.m for saturation ranging from 0% to 52.68%. Besides, the dependence of the resistivity index versus hydrate saturation is inconsistent with Archies law. The results of our studies have important implications for quantitative laboratory and field calibration of geophysical measurements within gas hydratebearing intervals.

Pressure core based study of gas hydrates in the Ulleung Basin and implication for geomechanical controls on gas hydrate occurrence

Pressure coring technology enables the recovery of natural gas hydrate-bearing sediments without pressure release and thus without gas hydrate dissociation. A total of 21 pressure cores were successfully recovered from the Ulleung Basin, East Sea, Korea, during the Second Gas Hydrate Drilling Expedition in the Ulleung Basin (UBGH2). Recovered pressure core samples were utilized to characterize hydrate occurrences and formation mechanisms with non-destructive physical property scans, X-ray CT scans, and controlled degassing experiments. Findings include: 1) the P-wave velocity in mud-rich sediments with grain-displacing hydrate increases with decreasing bulk density since hydrate displaces sediment grains and lowers the bulk density; 2) the P-wave velocity in sand with pore-filling hydrate is mostly affected by hydrate saturation rather than bulk density; 3) The P-wave velocity trend with hydrate saturations for pore-filling hydrates, follows pore-filling model at low hydrate saturations and gradually deviates from pore-filling model toward cementation model as hydrate saturation increases. 4) Gas recovery times during controlled core degassing experiments are proportional to both hydrate saturations and porosities. 5) At Site UBGH2-3, the thickness of both sub-horizontal and sub-vertical grain-displacing hydrates decreases with depth while the number of sub-vertical hydrates does not decrease with depth, implying that the formation mechanisms of sub-horizontal and sub-vertical hydrates differ while growth mechanisms are similar to each other in different growth directions; 6) the angle and thickness of grain-displacing hydrate-bearing fractures are a function of the overburden stress and the hydraulic conductivity, but the main governing factor is the overburden stress.

Temperature Increase during the Depressurization of Partially Hydrate-Saturated Formations within the Stability Region

Depressurization experiments of methane-hydrate-bearing sediments were conducted to measure changes within the hydrate stability region before dissociation started. Pore-filling hydrate with a saturation of about 40% was formed in water-saturated silica sand samples with a porosity of 0.4. The initial pressure was kept constant at around 14.5 MPa, and the initial temperature was varied between 281.3 and 283.7 K. When the samples were depressurized at rates ranging from −1.2 to −3.9 MPa/min, temperature increases of 0.26–0.37 K on average were measured. These were caused by a decreasing aqueous methane solubility as well as the liberation of isolated free gas, leading to additional heat-releasing hydrate formation (ΔHMH-f = −51.86 kJ mol–1 of CH4 at 280 K). The results suggest that the solubility obeys Henry’s law as long as free methane is present in the pore space. The temperature changes during depressurization shift the equilibrium pressure, leading to an anticipated dissociation during the process. The increase in hydrate saturation, in turn, reduces the effective permeability of the formation, reducing the extent to which a formation can be depressurized by a single vertical wellbore. Sensitivity studies for the above state sediment conditions show that the induced increase in the temperature raises the equilibrium pressure of up to 0.7 MPa and raises the hydrate saturation by 7%, for an initial in situ pressure of 35 MPa and varying temperatures.

Methane recovery from natural gas hydrate in porous sediment using pressurized liquid CO2

The dynamics of CH4 replacement in natural gas hydrate with liquid CO2 was studied with a high pressure three-dimensional reactor. Five groups of hydrate samples were formed to investigate the effect of hydrate reservoir properties on CH4–CO2 replacement reaction. The results showed that CH4 in the hydrate gradually moves to the liquid CO2 phase while CO2 in the liquid phase penetrates into the hydrate under pressure–temperature conditions not only within the phase zone surrounded by (LCO2–VCO2), (water–HCO2–LCO2), and (water–HCH4–VCH4) curves but also that above (LCO2–VCO2) and (water–HCH4–VCH4) curves. The replacement rate and amount of CH4 increase with the increase of hydrate saturation in the sediments. Compared with injecting gaseous CO2 method, liquid CO2 injection is also benefit for the recovery of CH4 from hydrate reservoir with much free water or that without underlying gas room to the extent that the injection of liquid CO2 is kept by high gas saturation. The replacement percent of CH4 hydrate decreases with the increase of hydrate saturation, but increases with the increase of water saturation. A higher replacement percent is obtained in the zone surrounded by three phase curves of (LCO2–VCO2), (water–HCO2–LCO2) and (water–HCH4–VCH4).

Acoustic Velocity Log Numerical Simulation and Saturation Estimation of Gas Hydrate Reservoir in Shenhu Area, South China Sea

Gas hydrate model and free gas model are established, and two-phase theory (TPT) for numerical simulation of elastic wave velocity is adopted to investigate the unconsolidated deep-water sedimentary strata in Shenhu area, South China Sea. The relationships between compression wave (P wave) velocity and gas hydrate saturation, free gas saturation, and sediment porosity at site SH2 are studied, respectively, and gas hydrate saturation of research area is estimated by gas hydrate model. In depth of 50 to 245 m below seafloor (mbsf), as sediment porosity decreases, P wave velocity increases gradually; as gas hydrate saturation increases, P wave velocity increases gradually; as free gas saturation increases, P wave velocity decreases. This rule is almost consistent with the previous research result. In depth of 195 to 220 mbsf, the actual measurement of P wave velocity increases significantly relative to the P wave velocity of saturated water modeling, and this layer is determined to be rich in gas hydrate. The average value of gas hydrate saturation estimated from the TPT model is 23.2%, and the maximum saturation is 31.5%, which is basically in accordance with simplified three-phase equation (STPE), effective medium theory (EMT), resistivity log (Rt), and chloride anomaly method.

Compressional wave velocity measurements through sandy sediments containing methane hydrate

An experimental apparatus was built to measure P-wave velocity (vP) of sandy sediment during hydrate formation from brine and free methane gas. The influences of hydrate saturation, initial brine saturation, conversion ratio of water to hydrate, and grain size of sand upon vP were investigated. The experimental results demonstrate that vP strongly depends on both hydrate saturation and initial brine saturation, whereas the influence of the grain size of sand is unremarkable. During the formation of hydrate for different experimental runs at identical initial brine saturation, vP increases with the increase of hydrate saturation; while it decreases dramatically with the increase of initial brine saturation for a given hydrate saturation. The correlation between vP and conversion ratio of water into hydrate was studied, and it was found that the relation between vP and the conversion ratio of water to hydrate is approximately the same for different experimental runs, regardless of how big the differences in initial saturation of brine and the size of sand grain are. vP was calculated for four different hydrate distribution models. The results suggest that low conversion ratio of water to hydrate, hydrate mainly acts as a load component of a dry frame matrix, whereas at higher conversion ratio of water to hydrate it partly acts as a cement that coats grains at grain contacts, and thereby leads to a dramatic increase in vP. Compared with the hydrate saturation, the conversion ratio of water to hydrate has a large role in distributing hydrate in the pores of sediments.

Methane hydrate induced permeability modification for multiphase flow in unsaturated porous media

[1] An experimental study was performed using X-ray computed tomography (CT) scanning to capture three-dimensional (3-D) methane hydrate distributions and potential discrete flow pathways in a sand pack sample. A numerical study was also performed to develop and analyze empirical relations that describe the impacts of hydrate accumulation habits within pore space (e.g., pore filling or grain cementing) on multiphase fluid migration. In the experimental study, water was injected into a hydrate-bearing sand sample that was monitored using an X-ray CT scanner. The CT images were converted into numerical grid elements, providing intrinsic sample data including porosity and phase saturations. The impacts of hydrate accumulation were examined by adapting empirical relations into the flow simulations as additional relations governing the evolution of absolute permeability of hydrate bearing sediment with hydrate deposition. The impacts of pore space hydrate accumulation habits on fluid migration were examined by comparing numerical predictions with experimentally measured water saturation distributions and breakthrough curves. A model case with 3-D heterogeneous initial conditions (hydrate saturation, porosity, and water saturation) and pore body–preferred hydrate accumulations best captured water migration behavior through the hydrate-bearing sample observed in the experiment. In the best matching model, absolute permeability in the hydrate bearing sample does not decrease significantly with increasing hydrate saturation until hydrate saturation reaches about 40%, after which it drops rapidly, and complete blockage of flow through the sample can occur as hydrate accumulations approach 70%. The result highlights the importance of permeability modification due to hydrate accumulation habits when predicting multiphase flow through high-saturation, reservoir quality hydrate-bearing sediments.

Laboratory Measurements of the Effects of Methane/Tetrahydrofuran Concentration and Grain Size on the P-Wave Velocity of Hydrate-Bearing Sand

An experimental apparatus was developed to measure P-wave velocity (VP) of gas-hydrate-bearing sediment. Tetrahydrofuran (THF) was added to quicken the hydrate formation in the porous media and to synthesize hydrate-bearing sediments with uniform distribution. Methane acted as a free gas to participate in the hydrate formation. Five experimental runs were performed to examine the influence of sediment grain size and THF concentration on VP. The P-wave velocity and the amplitude for the first arrival wave signal were collected in real time during hydrate formation process. The experimental data showed that VP increases monotonically with the increase of hydrate saturation in the sediment pore space and finally tends to be a constant value. This final VP value increases with the increase of initial THF content, but the effect of sand grain size on VP is inconclusive. The variations of amplitude for the first arrival wave signal with elapsed time during hydrate formation illustrates that the amplitude increa...

Study of the permeability characteristics of porous media with methane hydrate by pore network model

The permeability in the methane hydrate reservoir is one of the key parameters in estimating the gas production performance and the flow behavior of gas and water during dissociation. In this paper, a three-dimensional cubic pore-network model based on invasion percolation is developed to study the effect of hydrate particle formation and growth habit on the permeability. The variation of permeability in porous media with different hydrate saturation is studied by solving the network problem. The simulation results are well consistent with the experimental data. The proposed model predicts that the permeability will reduce exponentially with the increase of hydrate saturation, which is crucial in developing a deeper understanding of the mechanism of hydrate formation and dissociation in porous media.

未固結堆積物のメタンハイドレート胚胎層の岩石物理モデル

It is well known that the methane hydrate exists below the sea floor over 500 m or the polar region under the frozen ground in the Earth. We made the cross-plot of the methane hydrate and Vp, Vs, or Vp/Vs from the well log data at the eastern Nankai Trough in Japan and the Mackenzie Delta in Canada. We found that as the methane hydrate saturation increases, Vp and Vs increase, and Vp/Vs decreases from these plots. We discussed the rock physics model of methane hydrate bearing layers. We calculated Vp, Vs, and Vp/Vs as the function of the methane hydrate saturation, based on four imaginary rock physics model of methane hydrate bearing layers. From the comparison between the real well sonic data and the calculated data from four models, we inferred that the rock physics model of methane hydrate bearing layers was matrix-supporting model at both the eastern Nankai Trough and the Mackenzie Delta areas. However, there were some error and difference with the Vp, Vs and Vp/Vs from the matrix-supporting model and real well sonic data between the eastern Nankai Trough and the Mackenzie Delta. We estimated the effect of Vp, Vs, and Vp/Vs for the change of clay content of matrix-supporting model. As the clay content decreases, the Vp and Vs increase, and Vp/Vs decreases. The clay content from well core data corresponds with the calculated Vp, Vs, and Vp/Vs as the function of the methane hydrate saturation and the clay content from the matrix supporting model. In case of the estimation of methane hydrate saturation from Vp, Vs, and Vp/Vs, we need to consider the geology, especially sand/clay ratio of the methane hydrate bearing layers.

The pore-scale distribution of sediment-hosted hydrates: evidence from effective medium modelling of laboratory and borehole seismic data

Abstract Much of our knowledge on hydrate distribution in the subsurface comes from interpretations of remote seismic measurements. A key step in such interpretations is an effective medium theory that relates the seismic properties of a given sediment to its hydrate content. A variety of such theories have been developed; these theories generally give similar results if the same assumptions are made about the extent to which hydrate contributes to the load-bearing sediment frame. We have further developed and modified one such theory, the self-consistent approximation/differential effective medium approach, to incorporate additional empirical parameters describing the extent to which both the sediment matrix material (clay or quartz) and the hydrate are load-bearing. We find that a single choice of these parameters allows us to match well both P and S wave velocity measurements from both laboratory and in situ datasets, and that the inferred proportion of hydrate that is load-bearing varies approximately linearly with hydrate saturation. This proportion appears to decrease with increasing hydrate saturation for gas-rich laboratory environments, but increases with hydrate saturation when hydrate is formed from solution and for an in situ example.