Enhancement of gas production from methane hydrate reservoirs by the combination of hydraulic fracturing and depressurization method
Introduction
Methane hydrate (MH) is crystalline solid composed of water and methane gas [1], [2]. Natural MH occurs mainly in permafrost regions and deep oceanic sediments where the necessary conditions of low temperature and high temperature exist for hydrate stability [3], [4], [5]. The amount of global hydrate-bound methane is estimated to be on the scale of 1015–1018 ST m3, exceeding all known convectional oil and gas resource [6]. Therefore, methane hydrate is considered as one of the most promising alternative energy resources to address the world’s energy demand and future climate change. Over the last few decades, methane hydrate has become a hot topic of research, and most of the research effort has been focused on extracting gas from methane hydrate. Examples are the off-shore production tests at the South China Sea in 2017 [7], [8] and at the Eastern Nankai Trough in 2013 and 2017 [9], [10]. Other than these, many systems for comprehensive methane hydrate utilization process have also been proposed, and one of the most representative systems is the low CO2 emission power generation method proposed by Maruyama et al. [11]. It is expected that the natural MH can be efficiently explored and economically exploited in the near future.
Gas production from methane hydrate reservoirs is a complex process, involving multiphase (gas, liquid, ice and hydrate) flow, heat transfer and endothermic reaction. At present, several methods have been proposed to extract gas from MH, such as depressurization, thermal stimulation, inhibitor injection and gas replacement [12]. Among these methods, the depressurization method, which is done by decreasing the system pressure below the thermodynamic balance conditions, is accepted as a promising strategy of gas production from methane hydrate reservoirs because of its simple implementation, high energy efficiency, and high productivity [6], [13].
Depressurization-induced gas production from field-scale MH has been widely investigated in recent years. Yang et al. [3] investigated gas production behaviors by depressurization under different well pressure conditions, and found that more gas could be produced at the conditions of lower well pressure. Tang et al. [14] investigated the control mechanisms of gas production from field-scale hydrate reservoirs, and found that the reservoir permeability was the key factor controlling hydrate dissociation and gas production processes. Konno et al. and co-workers [15], [16], [17] investigated the effect of reservoir permeability and temperature on gas production, and pointed out that the propagation of pressure reduction was highly affected by the reservoir permeability, and the gas recovery factor was mainly dependent on the reservoir temperature. However, for most naturally occurring MH reservoirs with potential developing value, the initial effective permeability is relatively low because of pore filling by solid hydrate, although their absolute permeability is high [18], [19], [20]. This would greatly restrict the advance of low-pressure front and the transport of fluids within the reservoir, resulting in low gas production rate. In addition, the recovery factor and gas production rate in long-term production are limited by the lack of sensible heat, in particular in extracting gas from MH reservoirs in permafrost [15], [16]. Therefore, low gas production efficiency is the major barrier to achieve commercial-scale gas production from MH reservoirs by depressurization [21]. To improve gas production efficiency, thermal stimulation method, including well heating and hot water injection, has been proposed to assist depressurization, but the effect is limited [22], [23], [24]. For example, for well heating method, heating production well can only affect the dissociation reaction in the vicinity of production well, and for hot water injection method, the injected hot water cannot transfer farther from the well due to the low reservoir permeability. In our previous study [25], we investigated the gas production behaviors from layered MH reservoirs and found that the use of horizontal well can dramatically increase the gas production rate. However, this method would significantly increase the operating costs.
Hydraulic fracturing is a reservoir stimulation technique in which rock is fractured by a pressurized liquid. The purpose of hydraulic fracturing is to create artificial fractures in the deep-rock formations through which fluids (e.g., gas, water, and oil) will flow more freely. This method is widely used in the petroleum industry in order to increase oil or gas productivity. In recent years, the feasibility of creating artificial fractures in hydrate-bearing sediments by hydraulic fracturing has been investigated. Ito et al. [26] carried out a laboratory study of hydraulic fracturing behavior in hydrate-free sand and mud layers mimicking MH-bearing sand. They found that the fluid injection induced a fracture-like structure at the interface between sand and mud layers. Konno et al. [21] investigated hydraulic fracturing behavior in MH-bearing sand, and the result showed that the permeability was increased after fracturing and was maintained even after re-confining and closing the fractures. Too et al. [27] examined the susceptibility of MH-bearing sand with high saturation to fracture, and artificial fractures were achieved using hydraulic fracturing in a penny-shaped crack. The studies mentioned above demonstrated the feasibility of creating artificial fractures in MH sediments by hydraulic fracturing. The creation of artificial fractures can form a high-permeability fractured zone in MH sediments, which can be utilized to accelerate the pressure reduction during depressurization process and thereby enlarge the hydrate dissociation zone and induce more hydrate to dissociate. This means that the production performance in MH reservoirs by depressurization can be improved after hydraulic fracturing. Therefore, the use of hydraulic fracturing to increase reservoir permeability could be a promising way to improve gas production efficiency from MH reservoir. However, it is still unclear how the fractured zone induced by fracturing affects hydrate dissociation during depressurization and how well hydraulic fracturing affects gas production efficiency.
In this study, the combination of hydraulic fracturing and depressurization method is proposed to improve gas production efficiency from MH reservoirs. In order to evaluate the efficacy of this method, we construct a fractured MH reservoir model and investigate by means of numerical simulation the hydrate dissociation and gas production behaviors, as well as the factors affecting them. Numerical simulations are performed on two types of MH reservoir: high-temperature reservoir and low-temperature reservoir, which are corresponding to the naturally occurring MH reservoirs in deep oceanic sediments and permafrost regions, respectively. It is hoped that this study may provide beneficial reference for commercial exploitation and utilization of this energy resource in the future.
Section snippets
Production scheme
In this study, the production scheme includes two main stages: hydraulic fracturing stage and gas production stage, as shown in Fig. 1. Hydraulic fracturing stage can create a high-permeability zone within the hydrate-bearing layer (HBL) by inducing artificial fractures. Due to lack of reliable data from field-scale hydraulic fracturing, the detailed stimulation process and method is not discussed in this work, and we just assume that a vertical well is drilled for hydraulic fracturing, and a
Results and discussions
In this section, the gas production behaviors from the hypothetical fractured MH reservoir during a production period of 8 years are investigated. We consider two fractured reservoir cases in the numerical simulation: high-temperature reservoir and low-temperature reservoir. For all the cases, two permeability values, k = 1 D and k = 5 D, are used in the fractured zone, respectively. In addition, the factors affecting gas production are analysed in each case.
Conclusions
In this study, the combination of hydraulic fracturing and depressurization method has been proposed to improve gas production efficiency from MH reservoirs. In order to evaluate the efficacy of this method, we investigate by means of numerical simulation the gas production behaviors from two hypothetical fractured MH reservoirs with different initial temperature conditions. In addition, the factors affecting hydrate dissociation and gas production are analysed in each case. Based on the
Conflict of interest statement
We declare that no conflict of interest exits in the submission of this revised manuscript, and the manuscript is approved by all authors for publication.
Acknowledgments
This work was supported by the JST-CREST Project (No. JPMJCR13C4: Breakthrough on Multi-Scale Interfacial Transport Phenomena in Oceanic Methane Hydrate Reservoir and Application to Large-Scale Methane Production).
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