Elsevier

Energy

Volume 52, 1 April 2013, Pages 308-319
Energy

Numerical simulation of gas production from hydrate deposits using a single vertical well by depressurization in the Qilian Mountain permafrost, Qinghai-Tibet Plateau, China

https://doi.org/10.1016/j.energy.2013.01.066Get rights and content

Abstract

In November 2008, gas hydrate samples were recovered during the scientific gas hydrate drilling project conducted in the Qilian Mountain permafrost located in the Qinghai-Tibet Plateau, China. This region is expected to become a strategic gas hydrate exploitation area in China. Based on the gas hydrate characteristics at the DK-3 drilling site located in this region, we used using Tough + Hydrate to numerically simulate the gas production potential of the gas hydrate deposits using a single vertical well by depressurization. The simulation results indicated that for a 1.5 MPa wellbore pressure, the average CH4 production rate of hydrate dissociation was approximately 188 ST m3/d, the reservoir average total CH4 production rate was approximately 539 ST m3/d, and the cumulative CH4 volume produced from the reservoir was approximately 35% and 39% larger than those for wellbore pressures of 1 MPa and 2.5 MPa. Moreover, we numerically simulated the spatial distribution evolution of temperature, hydrate saturation and gas saturation in the reservoir for a 1.5 MPa wellbore pressure; the simulation indicated that a large volume of free CH4 remained in the reservoir. During the dissociation time, the gas hydrate dissociation effective radius in the reservoir was less than 20 m, and the actual dissociated gas hydrates only accounted for 2.3% of the total gas hydrates in the simulated system. The results may suggest that the single vertical well by depressurization method is not optimal for the development of gas hydrate deposits in the Qilian Mountain permafrost. Other production strategies, such as a horizontal well design or the combination of depressurization and thermal stimulation, may be more economically feasible.

Highlights

► The in-situ testing date of gas hydrates in Qinghai-Tibet Plateau was used to simulate the depressurization process. ► The gas production potential of gas hydrate deposits with depressurization was simulated by using Tough + Hydrate. ► The gas production capacity and the water Production Rate were calculated under different condition.

Introduction

Natural gas hydrates (NGH) are solid compounds of natural gas molecules that are encaged within a crystal structure composed of water molecules. They resemble packed snow or ice [1] in physical appearance and burn easily; thus, they are also called “flammable ice”. As a type of potential and substantial future energy resource, the current estimates of the worldwide quantity of natural gas hydrates range between 1015 and 1018 m3 [2]. Even the most conservative estimates of the total quantity of gas trapped in hydrates may surpass by a factor of two the total fuel fossil reserves' energy content recoverable by conventional methods. In addition, gas hydrates have a number of industrial and technological applications. These applications include using hydrate as natural gas transport and storage materials [3], preconditioning of fuel gas mixture before combustion [4], [5], and CO2 capture technique in IGCC system [6] and in combustion effluent [7].

In nature, gas hydrates occur both in the permafrost regions and in the marine sediments in the oceans and deep lakes where pressure-temperature conditions are suitable and where sufficient methane is delivered to the zone of hydrate stability in the uppermost sediments [8]. Currently, there are mainly three methods to dissociate gas hydrates: depressurization [9], [10], [11], thermal stimulation [12], [13], [14] and inhibitor stimulation [15], which are all based on breaking the gas hydrate pressure-temperature equilibrium conditions. Moreover, there is also a novel method recovering methane from hydrate with gaseous carbon dioxide [16]. Of these, depressurization is thought to be the most effective method for hydrate dissociation due to its economic and environmental advantages. These years, the hydrate dissociation has become a hot point for research [17], [18], [19], [20]. Experiments and simulations have been undertaken for predicting the behavior of hydrate of hydrate dissociation in the sediment. Yoshihiro et al. [18] performed the experiments on the hydrate formation and dissociation in porous media through depressurization method. Li and Jiang et al. [19], [20] evaluated the gas production potential from hydrate deposits in the permafrost region by simulations, and analyzed the sensitivity of methane production rate from hydrate reservoir.

In recent years, breakthroughs have been made in the exploration of permafrost-associated gas hydrate accumulations within the circumarctic regions of the northern hemisphere, such as northern Alaska in the USA, the Mackenzie Delta-Beaufort Sea region in Canada, the West Siberian Basin in Russia, etc. [8], which all show evidence of gas hydrate deposits.

The Qinghai-Tibet Plateau permafrost in China has a vast expanse with a total area of up to 150 × 104 km2. In recent years, a series of geological, geophysical, and geochemical investigations were conducted in the Qinghai-Tibet Plateau permafrost that confirmed the favorable occurrence conditions and prospecting potential for gas hydrate deposits in this region [21] of which the Qilian Mountain permafrost is the most important target area [22]. In 2008 and 2009, the scientific gas hydrate drilling project in the Qilian Mountain permafrost was implemented in China, as shown in Fig. 1(a) [19]. To date, a total of four test wells have been completed with well depths of 182.23 m, 635.20 m, 765.01 m and 466.65 m in DK-1, DK-2, DK-3 and DK-4. For all four wells, the thicknesses of the frozen layers range from 95 m to 115 m with an average thickness of 105 m. In the DK-1, DK-2 and DK-3 wells, gas hydrates were observed directly within several intervals, and gas hydrate samples were obtained successfully. Gas hydrates were also observed in the DK-4 well, but no samples were recovered. In all four wells, a series of gas hydrate-associated anomalies were observed [23]. The gas hydrate-bearing core sample recovered from DK-3 and its burning phenomenon are shown in Fig. 1(b) [23].

Gas hydrates in the Qilian Mountain permafrost are characterized by a relatively thin permafrost layer, shallow buried depth, complex gas components and coal-bed methane origin [19]. On the surfaces of the broken sections of the drill core, ice-like gas hydrate crystals can be observed directly. These gas hydrate crystals occur in the fissures of siltstones, mudstones and oil shale in the form of thin layers, flakes and lumps, or in the pores of sandstones in the disseminated form [23]. Temperature conditions are very crucial to the existence of gas hydrates in the Qilian Mountain permafrost. The average annual permafrost ground temperature ranges from 272.15 to 270.15 K [21]. According to the field measurements, the thermal gradients within the frozen layer and below the frozen layer are 0.011–0.033 K/m and 0.028–0.051 K/m [21], [24], which favor the stable formation of gas hydrates.

Three hydrate-bearing layers have been discovered at the DK-3 drilling site at the depths of 133–156 m, 225–240 m and 366–396 m [23]. Li et al. [19] studied the gas production potential of the first hydrate-bearing layer when using a single horizontal well either by the huff and puff method. The gas and water production was the significant parameter during the hydrate dissociation. Li [19], [25] studied the gas production rate and the gas-to-water ratio when the hydrate dissociation. And Jiang [20] mainly analyzed the methane production rate from hydrate reservoir as well. On the other hand, depressurization is thought to be the most economic and environmental advantages method for hydrate dissociation. Therefore, in this study, we will investigate the third layer hydrate at the DK-3 drilling site in the Qilian Mountain permafrost using a single vertical well by depressurization to evaluate the gas production potential of the gas hydrate deposits in this region.

Section snippets

Modeling code

The parallel version of Tough + Hydrate (pT + H) was used to conduct the numerical simulations in this study. This code is a widely used gas hydrate simulator and is developed from TOUGH2 family of codes at the Lawrence Berkeley National Laboratory. It is exclusively used in the simulation of hydrate formation and decomposition under conditions typical of common natural CH4 hydrate deposit (i.e. in the permafrost and in deep ocean sediments). It can model the non-isothermal gas release, phase

CH4 production rate

Fig. 3 shows the CH4 production rate evolution of hydrate dissociation (QR) for the three different wellbore pressure cases (1 MPa, 1.5 MPa and 2.5 MPa) over time. As shown in the figure, the QR evolution for the three cases shows nearly the same trend, which is a rapid decline at the early stage of gas production, followed by a decreasing rate of decline. Among these cases, the hydrate dissociation time for Case 1 is the shortest, lasting only approximately 1715 days. This is mainly because

Conclusions

In this study, numerical simulations and quantitative analyzes on gas and water production characteristics have been conducted for gas hydrate deposits at the Qilian Mountain permafrost DK-3 drilling site in the Qinghai-Tibet Plateau, China using a single vertical well by depressurization. Based on the simulation results, the following conclusions can be drawn:

  • 1.

    During the hydrate dissociation process, QR, QPT and QPG all reduce significantly. The majority of the gas production is from the output

Acknowledgment

This study has been supported by State Key Development Program for Basic Research of China (Grant No. 2009CB219507), National Natural Science of China (Grant No. 51006017), National High Technology Research and Development Program of China (Grant No. 2006AA09209-5) and Natural Science Foundation of China (Grant No. 50736001).

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