ReviewGeomechanics involved in gas hydrate recovery☆
Introduction
Gas hydrate, a kind of clathrates, is a non-stoichiometric crystalline compound in which gas molecules fit in cavities composed of hydrogen-bonded water molecules. It is widespread in the permafrost and deep oceans where the necessary conditions of low temperature and high pressure exist for their formation and stability [1], [2]. As the most extensive distribution of organic carbon (over 50%) in the world, gas hydrate is closely involved in environment and energy domains [3]. In particular, the carbon reserve of gas hydrate estimated is more than twice the total carbon amount of the proved conventional fossil fuel worldwide, approximately 18000Gt [4]. Besides, gas hydrate owns high energy density, that is, 1m3 of gas hydrate could produce as much as 164 SCM (standard cubic meters) gas [5]. Due to these advantages, gas hydrate is considered as a promising substitute of conventional energy [6].
Since the Soviet Union exploited hydrate resources in 1969, the United States, Japan, China and other countries have also carried out pilot recovery. Three main methods are applied: depressurization, thermal stimulation and chemical injection [7], [8]. All the three methods are based on decomposing hydrate solids into gas and water for the recovery. The depressurization method consists of reducing pressure lower than the hydration pressure at the prevailing temperature. Temperature is raised above the hydration temperature at the prevailing pressure in the thermal stimulation. Chemical injection invokes chemical inhibitors to shift the P–T equilibrium for the hydrate decomposition [7], [8], [9]. Of these possible methods, the depressurization method appears to be the most effective and economical one [10]. The thermal stimulation or chemical injection is usually combined with depressurization to prevent the secondary formation of hydrate during the production [11].
Certain geomechanical issues, however, seriously limit the long-term gas production during the pilot recovery. Sand production is recognized as the most critical one (Table 1). The hydrate-bearing reservoirs are fine-grained and poorly consolidated, and sand is easily produced and subsequently blocks the wellbore. Besides, the solid hydrate decomposes into gas and water during the exploitation, reducing the strength of reservoirs. The occurrence of large quantities of gas increases pore pressure dramatically and thus decreases the effective stress. These factors also favor the sand production in the gas hydrate-bearing reservoir [12]. In addition to sand production, long-term production of gas hydrate may lead to a significant subsidence, which will lead to wellbore instability and even cause production accidents. These geomechanical issues should be considered primarily for the commercial exploitation of gas hydrate.
Compared with conventional oil and gas production, the exploitation of gas hydrate is characterized by a phase transition process [15], [16], [17], [18], [19], [20]. The temperature, pore pressure, and stress fields are all disturbed when hydrate transits into water and gas. The decomposition of gas hydrate is an endothermic process, leading to a variation in temperature. The gas and water produced by the phase transition cause a multiphase flow in the reservoir. Meantime, the transport properties (e.g., permeability) vary dramatically with the loss of hydrate solid skeleton, which makes the transport phenomenon more complicated. The decomposition of hydrate also leads to the deformation or even the collapse of the reservoir because of 1) variation of pore pressure, 2) loss of solid skeleton, and 3) temperature change. In addition, the physical properties of hydrate-bearing reservoirs also vary during the phase transition process. For instance, the specific heat of water is about twice that of gas hydrate. The loss of solid skeleton increases the permeability and also reduces the strength of reservoir rock.
This paper seeks to summarize the state-of-art of knowledge on the geomechanics behaviors of the gas hydrate-bearing reservoir. After the introduction, investigations on the evolution of physical fields (i.e., temperature, pore pressure and stress) during the production processes are reviewed. Experiments and numerical simulations on the THMC coupling process during the exploitation of gas hydrate are discussed in the following section. Then, several typical geomechanical issues (i.e., sand production, subsidence) during the exploitation and their influences on the well integrity are discussed. The final part is the summary.
Section snippets
Thermal properties
Compared with conventional reservoirs, phase transition is the most prominent feature during the exploitation of gas hydrate. Phase transition refers to the conversion between solid hydrate and gas/water during the formation and decomposition of gas hydrate. Because the decomposition of hydrate is an endothermic reaction, the temperature field is disturbed dramatically during the hydrate recovery. For instance, when applying the depressurization method for decomposing the gas hydrate, the
THMC Coupling in the Hydrate-bearing Reservoirs during the Gas Recovery
Numerous experiments and simulations are conducted to investigate the thermal–hydrological–mechanical–chemical (THMC) coupling process during the exploitation of gas hydrate. A series of experiments showed that the decomposition rate of hydrate and gas production rate depend on the formation characteristics and the initial temperature and pressure [70], [71], [72], [73], [74], [75], [76]. With size effect eliminated [77], Li et al. [78], [79], [80] conducted the depressurization experiment in
Geomechanical Issues during the Exploitation of Gas Hydrate
As mentioned above, the wellbore integrity is a key factor should be ensured in the commercial exploitation of gas hydrate. By definition, well integrity is the application of technical, operational, and organizational methods to reduce the risk of fluids being leaked out of control throughout the life cycle of the well in the reservoir [104], [105]. The main risk lies in geomechanical issues, such as sand production and subsidence. The exploitation of gas hydrate is often accompanied by sand
Conclusions
The current paper reviews the physical properties of gas hydrate-bearing sediments and summarizes the state-of-art of knowledge on the geomechanical responses during the exploitation. Compared to conventional oil and gas reservoirs, gas hydrate-bearing reservoir is shallow and unconsolidated. Hydrate filling in sediments plays a role of a load-bearing component, decreasing the permeability while increasing the stiffness and strength. Besides, the phase transition involved in the exploitation
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Supported by the National Natural Science Foundation of China (51809275) and the Science Foundation of China University of Petroleum, Beijing (2462018BJC002).