A novel thermal stimulation approach for natural gas hydrate exploitation —— the application of the self-entry energy compensation device in the Shenhu sea
Introduction
Natural gas hydrate (NGH) is a clathrate crystalline compound formed by gas and water at low temperature and high pressure, mainly occurring in marine sediments and terrestrial permafrost zones (Kvenvolden, 1988). The global NGH resource is approximate twice the total known volume of coal, oil, and natural gas globally (Xu and Li, 2015), which has become one of the significant strategic energy sources for the 21st century (Klauda and Sandler, 2005; Milkov, 2004). However, NGH resources are of poor grade and weakly aggregated, and the economic recoverability of the resources under the existing technology is poor (Wu et al., 2020). More than ten NGH tried tests have been conducted in five countries worldwide so far, including five in marine areas (Collett et al., 2011; Islam, 1991; Li et al., 2018; Mao et al., 2022; Yamamoto et al., 2014, 2019; Ye et al., 2020). However, in terms of current production capacity, there is still a gap of 2–3 orders of magnitude from the threshold of industrial production capacity (5 × 105 m3/d) (Sloan, 2003), and the average daily marine production capacity is generally 1 to 2 orders of magnitude higher than the average daily terrestrial permafrost production capacity (Wu et al., 2020).
Several prevalent methods have been proposed for NGH exploitation, including depressurization, thermal stimulation, inhibitor injection, CO2 replacement, and combinations of the above methods. Numerous trials and studies have shown that depressurization may be a relatively economical and effective production method (Aghajari et al., 2019). Nevertheless, the inefficient thermal recharge of NGH decomposition usually leads to low gas production efficiency in large-scale development by the depressurization method, which may trigger secondary NGH or ice generation, block methane transport channels, decrease permeability, and make the gas production rate drop abruptly (Cranganu, 2009; Liu et al., 2018). Much research has been conducted in recent years to alleviate these problems based on the depressurization method assisted by the thermal stimulation method, which has given many new ideas. Cranganu used the in-situ thermal stimulation by introducing a specially designed NGH heating apparatus into a horizontal borehole drilled into the reservoir for production, which is beneficial in reducing the cost (Cranganu, 2009). Liu et al. proposed a geothermal-assisted CO2 replacement; CO2 was first injected into the geothermal reservoir for heating and then returned to the NGH reservoir to promote decomposition (Liu et al., 2018). Then further proposed a novel recovery approach by delivering geothermal energy through dumpflooding to significantly lower the heat injection cost (Liu et al., 2020). Ning et al. used renewable solar energy to supply energy for the marine NGH using thermal stimulation, which may be an important auxiliary way in the future (Ning et al., 2010). Zhao et al. found that microwave exploitation is more suitable for formations with higher initial NGH saturation, but this method has not yet solved the problem of low heat utilization efficiency and is more suitable for local heating (Zhao et al., 2016). Liang et al. pointed out that electric heating-assisted production stimulation is better than hot water heating under depressurized production in vertical wells, but the increase of the electric heating power has a limited impact on the net energy gain (Liang et al., 2019). Li et al. proposed depressurization and backfilling with in-situ supplemental heat to solve the problems of NGH decomposition heat replenishment, reservoir structure stability, and improving reservoir permeability by injecting calcium oxide into the reservoir (Li et al., 2020). Ye et al. designed a heat transfer device using the geothermal gradient to achieve continuous heat recharge without external energy injection, but the heat transfer power is limited (Ye et al., 2022a). Unlike conventional hot water/steam injection methods, these new methods are innovative in heat sources. Although these new auxiliary thermal stimulation methods are still at the conceptual stage, it cannot be ruled out that they will significantly impact the industrialization of NGH once a technological breakthrough is achieved.
Indeed, thermal stimulation effectively avoids secondary NGH plugging in the wellbore and surrounding formations, but it fails to have sound economic applicability due to low heat transfer efficiency (Liu et al., 2019). The production test in the Mallik area of Canada in 2002 proved that the thermal stimulation method has little commercial value, but it can be used as an effective method to prevent wellbore and near-wellbore NGH plugging (Collett, 2019). The uneconomical reasons may be due to (i) heat loss is prominent during the production; (ii) the construction cost of the platform (wellbore) used for heat injection is still at a high level. Existing solutions to reduce heat loss are to usually reform the reservoir by hydraulic fracturing to form fracture networks around production wells, promoting the local permeability around production wells (Feng et al., 2019; Yu et al., 2019a). However, new platforms are rarely proposed to replace the conventional wellbore, except for some mechanical tool carts or roof pressure relief devices used in marine NGH surface exploitation (Li et al., 2020a; Xu et al., 2018). Therefore, there is an urgent need to research more economical, stable, and efficient ways to escape the dilemma.
About 90% NGHs are present in the seabed clayey silt or silt sediments (You et al., 2019). In 2017, the first depressurization production in the Shenhu seas proved that the NGH sediments contained in the seabed clayey silt are also technically recoverable (Li et al., 2018) (Fig. 1). The depth of the marine reservoirs on the northern continental slope of the Central South China Sea and Okinawa Trough in the East China Sea is mainly 25–365 m, most of which are stored in the shallow deep sea (Ning et al., 2020). The torpedo anchor is a critical foundation of offshore floating structures, suitable for clay, silt, and sandy (F E N Brandão et al., 2006). Its installation process mainly involves the anchor body being suspended at a certain height and then freely released in the designated sea area, penetrating the seabed through the kinetic energy obtained by free fall in the water (Fig. 2). There is no restriction for the application in ultra-deepwater and no need to require special subsea equipment or large support vessels (Hasanloo et al., 2012; Hossain et al., 2015). The current studies mainly explore the factors influencing torpedo anchors' bearing capacity and penetration depth and have not applied its penetration principle in other fields. Thus, based on the installation principle of the torpedo anchor, a novel self-entry energy compensation device (ECD) was proposed in this study. Generally, the temperature in the exploitation area decreases due to the energy absorption by the NGH decomposition, which in turn reduces the efficiency of the NGH decomposition rate. The ECD can solve the problems of the Joule-Thomson effect and seawater freezing caused by heat absorption due to massive NGHs decomposed in the conventional depressurization mode.
In this study, after the introduction, the operating principle and critical technologies of the ECD were described in Section 2. Then, research methods and the penetration and exploitation models of the ECD were reported in Section 3, taking the W17 station in the Shenhu sea. Immediately after, the ECD penetration depth law and the applicability of the combined depressurization + thermal stimulation (DP + TS) method in this area were preliminary evaluated in Section 4. Finally, the paper ended with a summary and suggestions in Section 5.
Section snippets
Operation principle
There are mainly two ways based on the DP + TS method on drilling methods. The first is to drill a coaxial wellbore (Feng et al., 2019), stop the production well (depressurization), and inject hot water/steam. The second is to drill two wellbores (Yu et al., 2019a, 2019b, ): production wells (depressurization) and injection wells (hot water/steam), and then the two wellbores work at the same time. The operation model of the ECD is similar to the second one, yet the major difference is that the
Analysis methods and model construction
In this section, the geological exploration data of the first South China Sea exploitation was drawn to establish the penetration and exploitation models by ABAQUS and CMG STARS. The penetration law of the ECD and the applicability of the DP + TS model in this area were explored and evaluated, respectively.
Model verification
Fig. 9-a shows the comparison of the model with Kim's numerical simulation results using the same anchor body and soil parameters as the Medeiros field test (Kim et al., 2015; Medeiros, 2002). The motion trend of the torpedo anchor is similar to Kim's numerical results, and the penetration depth is consistent with the field test result, indicating that the numerical method in this study can effectively simulate the penetration behaviour and accurately calculate the penetration depth.
Fig. 9-b
Conclusion
In this study, a novel ECD was innovatively proposed to supply energy to the low-temperature area, where the temperature in the exploitation area around the production well will decrease due to the energy absorption by the NGH decomposition. The principle and critical technologies of the ECD were described and analyzed in detail. Aiming at the reservoir data from the W17 station in the Shenhu sea, the penetration and exploitation models were developed to explore the penetration law of the ECD
Credit author statement
Hongyu Ye: Data curation, Writing- Original draft preparation, Methodology, Writing- Reviewing and Editing. Xuezhen Wu: Conceptualization, Writing- Reviewing and Editing. Dayong Li: Software, Supervision. Yujing Jiang: Supervision. Bin Gong: Writing- Reviewing and Editing.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This study has been partially funded by National Natural Science Foundation of China (No. 41907251, No. 52179098, No. 520095077), Natural Science Foundation of Fujian Province (No. 2019J05030), Natural Science Foundation of Shandong Province (No. ZR2019ZD14).
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2023, Gas Science and EngineeringApplication of the enlarged wellbore diameter to gas production enhancement from natural gas hydrates by complex structure well in the shenhu sea area
2023, EnergyCitation Excerpt :The previous studies have demonstrated that enlarging the diameter of vertical wells by a highly permeable well wall can effectively promote exploration efficiency [24] and explored the effect of using a large-diameter casing on the NGH exploitation in vertical wells [25], showing that the approach may be applicable to poorer reservoir conditions. In addition, Wu and Ye et al. proposed a new device similar to a torpedo anchor, which has a larger diameter in the reservoir section than a single vertical well, effectively improving productivity [26,27]. Comparative studies on the influence of wellbore diameters, especially the research on horizontal wells or multi-branch horizontal wells, are rarely reported.