3.1 Relationship between hydrate saturation and P-wave velocity
As mentioned above, hydrate dissociation around wellbore during drilling operation was assessed through distribution of hydrate saturation. Before hydrate dissociation experiment, relationship between hydrate saturation and acoustic velocity (i.e., Pv(Sh) in Eqs. (2)) needs to be explored first. To highlight the representativeness of experimental data, 8 of 40 transducers were randomly turned on every time the acoustic velocity measurement was conducted for hydrate-bearing sediment with specific hydrate saturation. The measurement results of acoustic velocity are given in Table 1. As observed in Table 1, for sediment with specific hydrate saturation, 8 velocity data present little difference.
Table 1 Measurement results for hydrate-bearing sediment with specific hydrate saturation
Transducers No.
|
Acoustic velocity of hydrate-bearing sediments with different Sh, km/s
|
Sh=0
|
Sh=0.08
|
Sh=0.16
|
Sh=0.24
|
Sh=0.32
|
Sh=0.40
|
Sh=0.48
|
1
|
1.397
|
1.433
|
1.541
|
1.787
|
2.065
|
2.418
|
2.853
|
2
|
1.393
|
1.438
|
1.536
|
1.783
|
2.062
|
2.407
|
2.846
|
3
|
1.391
|
1.429
|
1.539
|
1.786
|
2.057
|
2.426
|
2.848
|
4
|
1.402
|
1.431
|
1.538
|
1.791
|
2.069
|
2.415
|
2.842
|
5
|
1.395
|
1.435
|
1.543
|
1.788
|
2.062
|
2.410
|
2.840
|
6
|
1.389
|
1.436
|
1.542
|
1.784
|
2.071
|
2.421
|
2.845
|
7
|
1.401
|
1.435
|
1.544
|
1.776
|
2.063
|
2.418
|
2.838
|
8
|
1.406
|
1.432
|
1.540
|
1.793
|
2.067
|
2.416
|
2.861
|
However, statistical significance test is the premise for obtaining function Pv(Sh). To this end, the statistical analysis was conducted with “One way ANOVA” in IBM SPSS Statistics 25. The results of error analysis and significance analysis are shown in Table 2 and Table 3 respectively. From Table 2, we can see that the maximum standard error and maximum standard deviation are only 0.002645 and 0.007482 respectively. The errors may be caused by the slight lateral difference in hydrate saturation in sediment during preparation of hydrate-bearing sediment. Additionally, as observed in Table 3, the significance Pis 0.000, which is less than 0.050. Therefore, effect of hydrate saturation on acoustic velocity of sediment is statistically significant (F=96598.507, P=0.000<0.050).
Table 2 Error analysis results
Items
|
Sh=0
|
Sh=0.08
|
Sh=0.16
|
Sh=0.24
|
Sh=0.32
|
Sh=0.40
|
Sh=0.48
|
Average value, km/s
|
1.3968
|
1.4336
|
1.5404
|
1.7871
|
2.0645
|
2.4164
|
2.8466
|
Standard deviation
|
0.005874
|
0.002925
|
0.002669
|
0.003441
|
0.004472
|
0.005975
|
0.007482
|
Standard error
|
0.002077
|
0.001034
|
0.000944
|
0.001217
|
0.001581
|
0.002112
|
0.002645
|
Table 3 Significance test results (One way ANOVA)
|
Sum of squares
|
df
|
Mean square
|
F
|
Significance, P
|
Between group
|
14.382
|
6
|
2.397
|
96598.508
|
0.000
|
In group
|
0.001
|
48
|
0.000
|
-
|
-
|
Taking the average acoustic velocity as the standard, Figure 6 displays the relationship between acoustic velocity and hydrate saturation. It can be found from Figure 6 that acoustic velocity increases with hydrate saturation in the form of a nonlinear quadratic function. However, the specific quantification needs to be achieved through data fitting. After fitting operation in Excel, relationship between acoustic velocity and hydrate saturation was expressed by Eqs. (3).
(3)
Notably, for Eqs. (3), the correlation coefficient R2 is 0.994, indicating that hydrate saturation can be determined by inversion of acoustic velocity obtained in hydrate dissociation experiments.
3.2 Evolution characteristics of hydrate dissociation around wellbore
Hydrate dissociation is an important factor causing wellbore instability while drilling in hydrate reservoir. Therefore, it’s necessary to deeply explore the evolution characteristics of hydrate dissociation around wellbore during the drilling operation. Based on the experimental conditions (default case) in Table 4, Figure 7 displays the distribution nephogram of hydrate saturation in sediment at different experimental moments. As observed in Figure 7, hydrate saturation at the position with the same distance from borehole on any path is basically equal to each other at the same experimental moment. Therefore, distribution of hydrate saturation in sediment during the experiment can be represented by that on any path. In this study, experiments are all based on the conditions shown in Table 4if no specific statement is made.
Table 4 Experimental conditions of default case
Experimental condition
|
Unit
|
Value
|
Hydrate saturation
|
-
|
0.24
|
Stabilizer concentration
|
wt%
|
0.30
|
Mud pressure
|
MPa
|
14.84
|
Mud temperature
|
K
|
296.40
|
Environment temperature
|
K
|
288.40
|
Reservoir pressure
|
MPa
|
14.55
|
Circulation flow rate
|
L/min
|
2.0
|
Total experimental time
|
h
|
48
|
Figure 8 demonstrates the distribution evolution of hydrate saturation along Path-1 around wellbore. From Figure 8, we can see that dissociation of natural gas hydrates firstly occurs in area near borehole due to the disturbance of drilling fluid at the beginning of experiment. When the experiment goes on for 0.5 hours, hydrate saturation at the borehole wall decreases to 0.172, and width of the dissociation transition area is 0.52 times the borehole radius (expressed as 0.52rb). After that, caused by the continuous disturbance of drilling fluid, hydrate dissociation gradually occurs outward along the radial direction, and the dissociation transition area thereby widens. When the experiment has lasted for 1.0h and 4.0h, width of the dissociation transition area reaches 1.13rb and 2.96rb respectively. However, natural gas hydrates at the borehole wall don’t completely dissociate until 12.0 hours after the experiment starts, which means that the completely dissociation area appears since this moment. Meanwhile, width of the dissociation transition area is 5.91rb when the experiment has lasted for 12.0 hours. In addition, with the continuation of the experiment, hydrate dissociation continues to occur at locations further away from the borehole. When the experiment was over, width of the completely dissociation area has reached 1.34rb, and the rest of the sediment is in the dissociation transition area.
During the experiment, the dissociation rate of natural gas hydrates is not constant. The dissociation rate of natural gas hydrate along Path-1 for different experimental moments is as shown in Figure 9. Notably, the dissociation rate of natural gas hydrates used herein is obtained by:
(4)
where, dt is time interval, dSh is the change of hydrate saturation in dt time interval.
As observed in Figure 9, at the beginning of the experiment, gas hydrates in area near borehole dissociate fastest. The maximum dissociation rate is 0.0113s−1 when the experiment goes on for 0.5 hours, and the position where gas hydrates dissociate fastest is the borehole wall. As the experiment remains, the position where fastest hydrate dissociation occurs gradually moves away from the borehole, and the maximum dissociation rate also dropped sharply. At the experimental moment of 12.0 hours, the maximum dissociation rate is 0.0032s−1, which is only 28.32% of that when the experiment is carried out for 0.5 hours. What's more, at the end of the experiment, positions with the maximum hydrate dissociation rate are about 4.25rb away from the borehole wall, and the maximum hydrate dissociation rate has been only 0.0013 s−1. Thereby, it can be inferred from Figure 9 that if the experiment continues after two days, hydrate dissociation rate at all positions will be lower. The mechanism why hydrate dissociation gradually weakens in experiment can be explained by Figure 10. As observed in Figure 10, hydrate dissociation is an endothermic reaction, so hydrate dissociation will cause the decrease in reservoir temperature. Besides, dissociation products (mainly methane and water) of gas hydrates can also lead to the increase in local pore pressure. Both the decrease in reservoir temperature and the increase in pore pressure caused by hydrate dissociation will inhibit its further dissociation. Furthermore, the above changes of reservoir temperature and pore pressure caused by hydrate dissociation will also restrain the heat transfer to the position farther away from the borehole. Thereby, hydrate dissociation at the position farther away from borehole will also be suppressed at the subsequent experimental moments.
3.3 Comparison of the present experimental study with published simulation works
As mentioned in introduction, this study can provide basic experimental data for verification of some numerical simulation models that used for the investigation of hydrate dissociation around wellbore. Therefore, differences between results of the present experimental investigation and the previous simulations should be explored in detail. Comparison of the experimental results with those obtained by two numerical simulation models in published works has been conducted in this section, and the comparison results are demonstrated as Figure 11.
As what we can see in Figure 11, there are some differences between results of two numerical simulations and this experimental investigation. Among them, the most significant difference is the width of the dissociation transition area. In both numerical simulations in Figure 11, width of the dissociation transition area is significantly compressed and is relatively constant throughout the simulation. For the simulation based on the model given by Freij-Ayob et al. (2007), width of the dissociation transition area is almost always maintained at about 1.15rb during the whole simulation. Similarly, for the simulation based on the model given by Ning et al. (2013), in the whole simulation, width of the dissociation transition area is maintained at about 1.31rb. However, width of the dissociation transition area at any experimental moments is obviously wider than that in both numerical simulation. Moreover, width of the dissociation transition area becomes wider and wider as the experiment continues. At 4.0h, 12.0h and 24.0h, width of the dissociation transition area in the present experiment is 3.36rb, 5.71rb and 8.68rb. At 48.0h, the position where gas hydrates begin to dissociate is no longer in sediment, so width of the dissociation transition area at this experimental moment is not discussed. Yet, it can be inferred from Figure 11d that the width of the dissociation transition area is undoubtedly greater than 8.68rb at the end of the experiment.
From Figure 11, we can also find that the positions where gas hydrates start to dissociate are almost the same in two numerical simulations, as well as in the experiment. However, we have already known that width of the dissociation transition area is obviously different from each other between the present experiment and previous numerical simulations. Thereby, range of the completely dissociation area obtained by experiment is naturally different from that obtained by previous simulations. As previously mentioned, in experiment, gas hydrates at borehole wall don’t completely dissociate until 12.0 hours after the experiment starts, and the final range of the completely dissociation area is only 1.34rb. However, for both numerical simulations in Figure 11, gas hydrates at the borehole wall have completely dissociated at the beginning of simulation. Moreover, as two simulations continue, range of the completely dissociation area will gradually expand. For simulation based on a model developed by Freij-Ayob et al. (2007), range of the completely dissociation area is 2.75rb, 4.15rb, 6.32rb, and 8.25rb respectively at 4.0h, 12.0h, 24h, and 48h. Likewise, for simulation based on a model developed by Ning et al. (2013), range of the completely dissociation area is 2.14rb, 3.56rb, 5.63rb, and 7.65rb respectively at 4.0h, 12.0h, 24h, and 48h.
Actually, the difference between the experimental results and the simulation results is mainly attributed to the inappropriate understanding of the dissociation mode in numerical modeling. In numerical simulations, it is generally assumed that hydrate dissociation gradually advances outward from the borehole wall in the form of "thin piston", and the "thin piston" is exactly the dissociation transition area. Besides, It is also believed that natural gas hydrate in dissociation transition area can rapidly dissociate in numerical modeling, so that the dissociation transition area can move outward to the next position. Therefore, as observed in Figure 11, width of the dissociation transition area is basically unchanged throughout the simulation, and the completely dissociation area can appear rapidly and widen continuously. In fact, this is not exactly the case. In drilling operation, natural gas hydrates in the near-wellbore region dissociate outward in the form of a gradually widening dissociation transition area, not the form of "thin piston". Inaccurate simulation of hydrate dissociation affects the accuracy of borehole stability prediction. Thereby, in numerical modeling of hydrate dissociation around wellbore, not only more conditions need to be considered, but also the dissociation mode needs to be further modified.
3.4 Hydrate dissociation for sediments with different hydrate saturation
Borehole stability can be influenced by hydrate saturation through affecting hydrate dissociation around wellbore. Therefore, investigations on hydrate dissociation in hydrate-bearing sediments with different hydrate saturations need to be conducted.
In this section, effect of hydrate saturation on hydrate dissociation in the near-wellbore region was investigated, and the experimental result was displayed in Figure 12. As observed in Figure 12, for all hydrate saturations studied herein, hydrates at any position of the sediment have begun to dissociate or have completely dissociated at the end of experiment. It can also be seen from Figure 12 that dissociation of natural gas hydrates in sediments weakens nonlinearly with the increase of hydrate saturation. In range of low hydrate saturation (Sh≤0.24), hydrate dissociation weakens so obviously with the increase of hydrate saturation. If range of the completely dissociation area is used to describe it, final range of the completely dissociation area narrows rapidly within range of low hydrate saturation as the hydrate saturation increases. When hydrate saturation is only 0.08, gas hydrates in sediments dissociate rapidly in experiment, and final range of the completely dissociation area reaches 4.28rb. However, when hydrate saturation has increases to 0.24, final range of the completely dissociation area has decreased to 1.34rb, which is 2.94rb narrower than that when the hydrate saturation is 0.08. Notably, when hydrate saturation exceeds 0.24, the phenomenon that hydrate dissociation weakens as hydrate saturation increases has become less obvious. When hydrate saturation increases from 0.24 to 0.48, final range of the completely dissociation area only decreases from 1.34rb to 0.53rb, and the decline is only 0.81rb. We can boldly infer that if hydrate saturation continues to increase, final width of the completely dissociation area will be narrower than 0.53rb.
Figure 13 schematically illustrates the reason why hydrate dissociation weakens with the increase of hydrate saturation in drilling operation. We all know that the heat required for completely dissociating the specific amount of gas hydrates is constant. As observed in Figure 13, for all cases, heat Q is assumed to be transferred into the cube infinitesimal element with side length dr in the same time interval dt. If hydrate saturation is high (see Figure 13a, assuming Shhigh), the heat Q provided by drilling fluid can only make gas hydrates in sediment with width of dr' in infinitesimal element completely dissociate. However, if hydrate saturation is low (see Figure 13b, assuming Shlow), the heat Q can make gas hydrates in sediment with width of dr'' in infinitesimal element completely dissociate. Relationship between dr'' and dr' can be expressed by Eqs. (5):
(5)
Since Shhigh is assumed to be higher than Shlow, dr'' is always wider than dr'. And, the greater the difference between Shhigh and Shlow, the wider dr'' is than dr'.
3.5 Effect of stabilizer concentration on hydrate dissociation
As an environmentally friendly additive for drilling fluid, soybean lecithin is a by-product in the process of refining soybean oil and will not have a serious impact on the marine environment. So, soybean lecithin is a hydrate stabilizer worthy of recommendation. In the present study, effect of soybean lecithin concentration on hydrate dissociation has also been investigated. Figure 14 displays the final width of the completely dissociation area and dissociation transition area when the stabilizer concentration in the drilling fluid is different. As we can see from Figure 14, width of the completely dissociation area decreases as the stabilizer concentration increases until it reaches 0. Width of the completely dissociation area is 6.42rb when there is no soybean lecithin in drilling fluid. However, when the stabilizer concentration reaches 0.60wt%, not only width of the completely dissociation area, but also width of the dissociation transition area are both 0. In other words, when the concentration of soybean lecithin is higher than 0.60wt%, dissociation of natural gas hydrates in the near-wellbore region around wellbore can be completely prevented during drilling operation. All these indicate that, addition of soybean lecithin in drilling fluid will have a better inhibitory effect on dissociation of gas hydrates around wellbore in drilling operation.
Previous studies have shown that soybean lecithin does not affect the stability of hydrate by changing the thermodynamic equilibrium conditions (Chen et al. 2007). In fact, just as displayed in Figure 15, soybean lecithin inhibits hydrate dissociation by forming the mesh membrane on hydrate surface to limit mass transfer. Transfer of water and methane molecules from the hydrate surface to the fluid in pores is free if there is no soybean lecithin in drilling fluid (see Figure 15a). That is to say, the mass transfer resistance can almost be ignored when the concentration of soybean lecithin is 0. With the increase in concentration of soybean lecithin, the mass transfer resistance gradually increases due to the formation of mesh membrane on the hydrate surface. When its concentration is not very high, adjacent soybean lecithin molecules form local mesh membrane on hydrate surface (see Figure 15b). In this case, some of the water and methane molecules produced by hydrate dissociation are blocked on the hydrate surface by the mesh membrane, further dissociation of gas hydrates is inhibited to some extent. However, if the concentration of soybean lecithin is high enough, the mesh membrane formed by soybean lecithin can completely cover the hydrate surface (see Figure 15c). Transfer of almost all water and methane molecules from the hydrate surface to the fluid in pores is blocked, then no hydrate dissociation can continue to occur in the subsequent experiment.
3.6 Prevention of uncontrollable hydrate dissociation and accompanying methane leakage
As mentioned above, methane leakage caused by hydrate dissociation in drilling operation poses the threat to the marine environment and the marine organisms, as well as the drilling safety. To prevent the marine environmental issues such as borehole collapse or methane leakage while drilling in hydrate reservoir, reducing hydrate dissociation around wellbore is the key. Adding environmental friendly hydrate stabilizer (such as soybean lecithin) to drilling fluid is an environmental and effective measure. Nevertheless, the stabilizer concentration in drilling fluid needs to be designed in advance according to acceptable hydrate dissociation or methane leakage. In terms of the experimental conditions herein, if it is required that hydrate dissociation and methane leakage cannot occur in drilling operation, concentration of soybean lecithin needs to be higher than 0.60wt%. However, it is unrealistic not to allow hydrate dissociation and methane leakage during drilling operation. According to Figure 14, If hydrates in sediment with a width of 0.5rb around wellbore is allowed to completely dissociate, the concentration of soybean lecithin needs to be at least 0.39wt%. Similarly, through Figure 14, we can determine the lower limit of soybean lecithin concentration corresponding to any acceptable width of completely dissociation area required by marine environmental protection.