1. Introduction
Natural gas hydrate (NGH) has become one of the most promising clean energies with the characteristics of huge reserves, widespread distribution, and less pollution [
1,
2,
3]. The hydrate-bearing sediments (HBS) are natural soils containing gas, water, and hydrate [
4], which have strong relations with geological hazards such as submarine landslides [
5], production [
6,
7], and wellbore stability [
8,
9]. Consequently, it is indispensable to predict mechanical responses of HBS during the NGH development.
Synthesized and pressure-core samples containing hydrate have been investigated through the method of combining theoretical analyses and laboratory experiments [
10,
11]. Triaxial shear tests of HBS reveal influential factors and their controlling mechanisms on mechanical behaviors. Influential factors include but are not limited to hydrate saturation, effective confining pressure, fines contents, temperature, and micro-modes of hydrate distribution [
12,
13,
14,
15,
16]. Both the strength and stiffness are proved to be enhanced obviously with hydrate saturation [
17]. Characteristics of strain-softening and strain-hardening transform change with hydrate saturation and confining pressure [
18].
Compared with sand sediments, laboratory tests on silt sediments containing hydrate show differences in mechanical behaviors [
19,
20,
21], which indicate that the strength and stiffness are influenced not only by hydrate saturation but by the particle size and clay content [
22,
23]. Furthermore, experiments conducted on pressure cores found that mechanical properties of reconstituted samples containing hydrate are similar to that of pressure cores [
24,
25]. Thus, the investigation of reconstituted samples can deepen understanding of the mechanical behaviors of gas hydrate reservoirs [
17,
26]. Besides, Luo et al. [
27,
28] investigated the mechanical characteristics of artificial kaolin samples and remolded marine sediments. The results indicate that the hydrate-bearing clay can be used as a substitute for marine sediments from hydrate reservoirs. Meanwhile, the samples formed with gas hydrate are limited in lower saturation [
27]. However, it should be noted that present investigations on silt sediments containing hydrate are limited to lower saturation or artificial samples, which cannot characterize the strength and stiffness of reservoirs [
29]. Therefore, it is more accurate and reliable in evaluating the strength parameters of HBS by using reconstituted samples from the actual natural gas hydrate reservoirs.
This study presents a series of triaxial shear tests to analyze mechanical characteristics and failure mechanisms of sediments from the Shenhu area. The THF solution is used as a substitute to form hydrate. Failure mechanism is investigated at the micro-level to explain the characteristics of strain hardening. The failure strength will be analyzed and predicted based on the improved Drucker-Prager criterion. Furthermore, Young’s modulus E50 is estimated by analyzing experimental results and using empirical models. This work can provide a reference for the mechanical parameter estimation and risk management during natural gas hydrate development in the South China Sea.
2. Materials and Methods
2.1. Experimental Apparatus
Figure 1 illustrates the triaxial shear test apparatus. A detailed introduction of the experimental apparatus can be found in our previous publications [
14,
20,
30,
31]. The device is mainly composed of a hydrate synthesis unit, a loading unit, a pressure and temperature control unit, a fluid delivery unit, and a data acquisition unit. A cylindrical specimen can be prepared through the in-situ synthesis method in the pressure chamber [
21,
32,
33]. The specimen is 39.1 mm in diameter and 120 mm in height. The confining pressure can be adjusted from 0 MPa to 15 MPa. The temperature can be controlled from −20 °C to 40 °C with an accuracy of ±0.1 °C. The axial load capacity is 50 kN.
2.2. Materials
THF with a purity of 99.9% and marine sediment are used to prepare the reconstituted samples in this study. The host marine sediment is from the natural gas hydrate reservoir in the Shenhu area of the northern South China Sea. Its chemical composition is close to silty clay [
34,
35]. Its grain size distribution is shown in
Figure 2. It can be observed that the sediments are mostly composed of silt and clay. The fractions of the fine sand (63–250 μm), silt (4–63 μm), and clay (<4 μm) account for ~1%, ~63%, and ~36%, respectively [
36,
37]. The fractions of fine grains (less than 63μm) in the sediments are more than 99%. The porosity is about 43% and the density is 1.77 g/cm
3. The initial water content of sediments from the South China Sea is 16.48% [
18].
2.3. Experimental Procedures
To evaluate the mechanical characteristics of HBS and analyze the interactions between the THF hydrate and sediment particles, a series of triaxial shear tests were designed with hydrate saturation of 15%, 30%, 45%, and 60%, at the effective confining pressures of 1 MPa, 2 MPa, and 4 MPa, respectively.
Dry marine sediment (~210 g) is mixed with a certain amount of THF solution and pressed into the flexible bucket. The amount of solution can be predetermined according to desired hydrate saturation and the mass ratio [
21]. Next, the reactor is placed into an inductor after connecting sensors and pipelines. Afterward, the temperature is set at 1 °C, under which the THF would be transformed into hydrate. Finally, the dry nitrogen gas is injected into the pressure chamber and the pore pressure gradually increases to the desired value (i.e., 4.5 MPa), while the confining pressure is kept at 5.5 MPa, 6.5 MPa, and 8.5 MPa, respectively. Namely, the effective confining pressure is kept at 1 MPa, 2 MPa, and 4 MPa throughout the tests. The samples are sheared at a constant strain rate of 0.75 %/min. The data of loading force and deformation are acquired during tests.
3. Results and Discussion
3.1. Failure Strength
The basic stress-strain curves have been described in our previous study [
30], which is shown in
Figure 3. It can be seen from
Figure 3 that all stress-strain curves show the strain-hardening failure mode, and the curves can be divided into three parts, namely elastic deformation stage (I), transitional stage (II), and plastic deformation stage (III). In the elastic deformation stage, the deviator stress increases rapidly with axial strain increasing and the correlation is approximately linear. However, in the plastic deformation stage, it increases slowly as the axial strain reaches 3%. This paper would mainly emphasize the failure strength, Young’s modulus, cohesion, and internal friction angle.
Additionally, the mechanical properties of reconstituted clayey-silt samples are different from that of sand. The stress-strain curves of clayey silt show the strain hardening characteristics during the shearing process, while that of sand shows both strain hardening and strain softening characteristics. Besides, the failure strength is lower than that of sand under the same test conditions.
Generally, failure strength is considered as the maximum deviator stress of the samples at a specific confining pressure before the axial strain exceeds 15% [
38].
Figure 4 demonstrates the relationships between failure strength and hydrate saturation [
21,
22,
23,
27]. The results indicate that failure strength almost increases linearly with hydrate saturation in samples. That verifies the impact of hydrate cementation on the strength and stiffness of sediments [
39]. Besides, failure strength with the same hydrate saturation increases with the effective confining pressure rising.
3.2. Young’s Modulus E50
The Young’s modulus
E50 is defined as Young’s modulus at 50% of the maximum deviator stress.
Figure 5 displays the relationships between Young’s modulus
E50 and hydrate saturation. Young’s modulus increases remarkably with the increase in the hydrate saturation, which is affected by effective confining pressure. Meanwhile, the value of Young’s modulus varies from 19.4–141.1 MPa, indicating that the value is in a reasonable range compared with the previous studies [
21,
22,
23,
27].
In addition, both strength and Young’s modulus of clayey-silt samples are different from that of sandy sediments containing hydrate, which reflects the effect of sediment characteristics and hydrate types on reconstituted clayey-silt samples in this paper. The samples from the Shenhu area are different from the sandy sediments in sediment types, particle size distribution, hydrate types, cementation among particles, and so on.
3.3. Cohesion and Internal Friction Angle
Figure 6 exhibits the variation of cohesion of the sediments containing THF hydrate. The results show that the cohesion of HBS increases from 0.09 MPa to 1.28 MPa when hydrate saturation increases from 15% to 60%. The filling of pores with hydrate crystal and the cementation among sediment particles enhance the strength of HBS with hydrate formation, resulting in a significant increase in cohesion. The cohesion increasing tendency is similar to those observed in previous research results [
21,
22,
23,
26], which verifies the accuracy of these tests.
The relationship between the internal friction angles and hydrate saturation is depicted in
Figure 7. The variation range of internal friction angle is 8.3–12.3°. Compared with the previous studies, the tendency of increasing the internal friction angle is different from other results, but the value of the internal friction angle in this work is reasonable [
21,
22,
23,
26]. Additionally, the silty-clay samples containing THF hydrate have a low value in internal friction angle compared to hydrate-bearing sand, which illustrates that sediment types have a marked effect on strength parameters [
17].
As shown in
Figure 6 and
Figure 7, cohesion can be expressed as an exponential function of hydrate saturation. Internal friction angle and hydrate saturation are related to the quadratic function. The cohesion and internal friction angle can be calculated through Equations (1) and (2).
where
c and
φ are cohesion (MPa) and internal friction angle (°), respectively;
Sh represents the hydrate saturation, (%).
3.4. Failure Mechanisms during the Shear Process
The occurrence of hydrate leads to changes in microstructure and mechanical behaviors of HBS. Combining the controlling mechanisms and laboratory tests, the pore-scale mechanism of reconstituted hydrate-bearing clayey-silt samples from the Shenhu area is discussed to describe deformation characteristics and the impact of hydrate formation on strength parameters.
Figure 8 illustrates the SEM images of clayey-silt sediment (a–d) and sand sediment (e,f). The results show that different particles of clayey-silt sediment have characteristics of different shapes with rough and uneven surfaces, while the surface of the sand is relatively smooth. Besides, the existence of clay particles modifies the structure of host sediment and structure strength directly [
17,
22], which is mainly caused by the size, morphology, and cementation of clay. Therefore, the particles of clayey-silt sediment aggregate into clusters with adhesion force among particles.
At low hydrate saturation, the effect of hydrate on strength parameters is limited because of the low cementation among sediment particles induced by hydrate formation. Relative movement among sediment particles will occur easily under the combined axial and lateral loads [
22,
38]. Correspondingly, damage and breakage of hydrate may occur along with particle movement due to the changes in microstructures, such as damage and breakage of hydrate crystals [
17,
19]. These interactions between the hydrate crystal and sediment particles as well as changes in the microstructure of sediments govern deformation behaviors and mechanical responses [
40,
41], as shown in
Figure 9b.
At high hydrate saturation, the strength and stiffness of HBSs are enhanced evidently by the cementation of hydrate among sediment particles [
42,
43]. Several shear micro-planes appear through hydrate mass if the bonding strength of the hydrate and particle is higher than hydrate strength [
22]. Hydrate-particle cementation is destroyed along the hydrate-particle interface when the hydrate strength is higher, as shown in
Figure 9b. Then, damage to hydrate crystal and the relative motion of mineral particles occur. Moreover, micro-fractures are formed with shear plane development.
The breakage of hydrate bonding and relative motion of mineral particles appear during the shearing process and then cause microstructural changes [
44,
45]. Uniform deformation and cracks are easily produced in samples, leading to destruction and failure [
38,
46], as illustrated in
Figure 9a. During this process, strength and stiffness are enhanced obviously with the presence of hydrate.
In addition, the fine particles and impurities, which exist in the pores of the clayey-silt samples, can restrict the formation of hydrate, and limit the movement of particles [
15,
33]. That makes the mechanical behaviors of reconstituted hydrate-bearing clayey-silt samples from the Shenhu area differ from that of sandy sediments containing hydrate.
4. Estimation of Strength Parameters
4.1. Failure Strength Prediction
As mentioned above, the failure strength of the sediments containing THF hydrate is mainly affected by effective confining pressure and hydrate saturation. Combining experimental results and the Drucker-Prager criterion [
23,
38], the improved model of failure strength can be expressed in Equations (3)–(5).
where
Sh represents hydrate saturation, %;
K and
β represent model coefficients related to hydrate saturation. Both
K and
β can be obtained via correlations of experimental data.
For the shearing tests,
σ2 and
σ3 are equal, Equation (3) can be expressed as:
Based on Equation (7), the failure strength (
σ1 −
σ3)
s can be assumed as a function of hydrate saturation
Sh and effective confining pressure
σ3 [
23,
38], which is given by Equations (8) and (9).
where (
σ1 −
σ3)
f represents the failure strength of the HBS, MPa;
A and
B are model coefficients related to the hydrate saturation.
The prediction error is determined through Equations (10)–(12).
where
Errormin,
Errormax, and
Errorave are the minimum error, maximum error, and average error, respectively; (
σ1 −
σ3)
fpre and (
σ1 −
σ3)
fexp represent the prediction value and experimental value of the (
σ1 −
σ3)
f; (
σ1 −
σ3)
fprei and (
σ1 −
σ3)
fexpi are the ith prediction value and the ith experimental value of the (
σ1 −
σ3)
f.
Figure 10 demonstrates the comparison between the calculated strength and the experimental results of sediments with THF hydrate. It is observed that the improved Drucker-Prager criterion shows high fitting accuracy of failure strength. The shear stress of HBS under certain experimental conditions is matched by the improved criterion, with a maximum relative error of about 8.7% and an average relative error of about 3.3%, as shown in
Figure 10b.
Figure 11 displays the prediction of failure strength. It can be observed that the failure strength of sediments containing THF hydrate can be approximately calculated by Equation (9) with knowing the stress state and hydrate saturation. Furthermore, improved strength calculating model and strength analysis could offer a precise prediction of mechanical parameters for numerical simulations and key geological issues during gas hydrate exploitation of the South China Sea [
7,
8,
10].
4.2. Young’s Modulus Prediction
Young’s modulus
E50 mainly depends on the hydrate saturation and effective confining pressure [
12,
47]. Afterwards, Miyazaki et al. [
48] established a model to predict the initial elastic modulus, as shown in Equation (13).
where
Ei represents the initial elastic modulus, MPa;
σ3 is the effective confining pressure, MPa;
ei(
Sh) represents the initial elastic modulus under
σ3 of 1 MPa, which is a function of hydrate saturation.
Similarly, Young’s modulus
E50 also can be estimated by proposing a calculation model. However, the assumed
ei(
Sh) limits the applicability and accuracy. Thus, we define this variable in the formula related to both hydrate saturation and effective confining pressure, which is given as:
where
f and
g represent the functions of the effective confining pressure.
Based on the experimental data, the empirical equation can be defined as follows:
Figure 12 exhibits the prediction of Young’s modulus
E50. The average error is 8.0%. A comparison between calculated and experimental results shows that the calculation model has very good applicability and high forecast accuracy, as shown in
Figure 12a. Furthermore, this model can be used to predict Young’s modulus by considering hydrate saturation and stress state [
19,
38], as displayed in
Figure 12b.
To summarize, both failure strength and Young’s modulus can be decided quickly based on empirical equations obtained from experimental data. However, the precision and practicability of prediction are limited due to a lack of field data. Thus, more high reliability and accuracy tests are necessary both in the laboratory and field.
5. Conclusions
The stress-strain curves of the THF hydrate-bearing clayey-silt sediments showed obvious strain-hardening characteristics. The deviator stress increases rapidly at the small axial strain level (<3%) and then grows slowly with the increase in strain, which is obviously different from that of hydrate-bearing sand.
The failure strength and Young’s modulus of reconstituted THF hydrate-bearing clayey-silt samples from the Shenhu area have a similar variation tendency to that of hydrate-bearing silty sediments. However, the differences in the value of strength parameters can be observed, which reflects the effect of sediment types, particle size distribution, hydrate types, and so on.
Macro-failure behaviors of reconstituted clayey-silt samples containing hydrate reveal the microstructure variations and microscopic interactions between hydrate crystal and mineral particles. Meanwhile, failure mechanisms of clayey-silt sediments containing hydrate are mainly determined by the collective effect of hydrate formation and the microstructure of sediments.
Failure strength is predicted by improved Drucker-Prager criterion with knowing hydrate characteristics and stress state. Moreover, the proposed calculation model based on experimental data can be used for predicting Young’s modulus of hydrate-bearing clayey-silt sediments from the Shenhu area. It should be noted that high-reliability laboratory tests and empirical models are required for the numerical simulation and field operation due to lacking in-site data during the development of natural gas hydrate resources in the South China Sea.
Author Contributions
Conceptualization, L.D. and H.L.; methodology, Y.L.; validation, Q.M., G.H. and N.W.; investigation, L.D. and J.W.; data curation, L.D. and Y.L.; writing—original draft preparation, L.D.; writing—review and editing, H.L., J.W. and N.W.; visualization, L.D.; supervision, H.L. and Y.L.; project administration, N.W. and J.W.; funding acquisition, Y.L. and N.W. All authors have read and agreed to the published version of the manuscript.
Funding
This research was jointly supported by National Natural Science Foundation of China (No. 41976074), Key Laboratory of Gas Hydrate, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences (No. E219kf1301), Graduate School Innovation Program of China University of Petroleum (YCX2019020), and China Scholarship Council (No. 202006450082).
Conflicts of Interest
The authors declare no conflict of interest.
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