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Volume 32 Issue 4
Aug.  2021
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Hemin Yuan, Yun Wang, Xiangchun Wang. Seismic Methods for Exploration and Exploitation of Gas Hydrate. Journal of Earth Science, 2021, 32(4): 839-849. doi: 10.1007/s12583-021-1502-3
Citation: Hemin Yuan, Yun Wang, Xiangchun Wang. Seismic Methods for Exploration and Exploitation of Gas Hydrate. Journal of Earth Science, 2021, 32(4): 839-849. doi: 10.1007/s12583-021-1502-3

Seismic Methods for Exploration and Exploitation of Gas Hydrate

doi: 10.1007/s12583-021-1502-3
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  • Seismic and rock physics play important roles in gas hydrate exploration and production. To provide a clear cognition of the applications of geophysical methods on gas hydrate, this work presents a review of the seismic techniques, rock physics models, and production methods in gas hydrate exploration and exploitation. We first summarize the commonly used seismic techniques in identifying the gas hydrate formations and analyze the limitations and challenges of these techniques. Then, we outline the rock physics models linking the micro-scale physical properties and macro-scale seismic velocities of gas hydrate sediments, and generalize the common workflow, showing the frequently-used procedures of building models with detailed analysis of the potential uncertainties. Afterwards, we summarize the production techniques of gas hydrate and point out the problems regarding the petrophysical basis and abnormal seismic responses. In the end, considering the geological and engineering problems, we come up with several aspects of using geophysical techniques to solve the problems in gas hydrate exploration and production, hopefully to provide some important clues for future studies of gas hydrate.
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Seismic Methods for Exploration and Exploitation of Gas Hydrate

doi: 10.1007/s12583-021-1502-3

Abstract: Seismic and rock physics play important roles in gas hydrate exploration and production. To provide a clear cognition of the applications of geophysical methods on gas hydrate, this work presents a review of the seismic techniques, rock physics models, and production methods in gas hydrate exploration and exploitation. We first summarize the commonly used seismic techniques in identifying the gas hydrate formations and analyze the limitations and challenges of these techniques. Then, we outline the rock physics models linking the micro-scale physical properties and macro-scale seismic velocities of gas hydrate sediments, and generalize the common workflow, showing the frequently-used procedures of building models with detailed analysis of the potential uncertainties. Afterwards, we summarize the production techniques of gas hydrate and point out the problems regarding the petrophysical basis and abnormal seismic responses. In the end, considering the geological and engineering problems, we come up with several aspects of using geophysical techniques to solve the problems in gas hydrate exploration and production, hopefully to provide some important clues for future studies of gas hydrate.

Hemin Yuan, Yun Wang, Xiangchun Wang. Seismic Methods for Exploration and Exploitation of Gas Hydrate. Journal of Earth Science, 2021, 32(4): 839-849. doi: 10.1007/s12583-021-1502-3
Citation: Hemin Yuan, Yun Wang, Xiangchun Wang. Seismic Methods for Exploration and Exploitation of Gas Hydrate. Journal of Earth Science, 2021, 32(4): 839-849. doi: 10.1007/s12583-021-1502-3
  • Gas hydrate contains an amount of 2×1016 m3 of gas worldwide (Kvenvolden, 1988), which is almost twice of conventional hydrocarbon resources, making it a potential alternative energy source. Gas hydrate is mostly found in permafrost and continental shelves of oceans with relatively high pressure (8–30 MPa) and low temperature (10–20 ℃) (Sunjay et al., 2011). Among these reservoirs, nearly 99% are located at the ocean bottom (Ruppel, 2015; Dillon and Max, 2003). Several countries have launched national research programs to investigate gas hydrate, including Canada, USA, Japan, India, South Korea, Germany, and so on (Johnson, 2011).

    Gas hydrates can be detected through reflection seismic data. Owing to the high velocities of gas hydrate formation and the commonly presented gas formation below, gas hydrate formations are generally associated with special seismic marks, including bottom simulating reflectors (BSRs), polarity reversal, and blank zone (Liu et al., 2011; Thakur and Rajput, 2011). The magnitudes of BSR and polarity reversal are related to the acoustic impedance contrast between the gas hydrate formation and the formation beneath, while the blank zone is related to the properties within the gas hydrate formation (Chand et al., 2004; Sain et al., 2000). These seismic techniques have been widely used and confirmed to be effective in detecting gas hydrate formations in many cases. However, they are inadequate to invert the petrophysical parameters because of the complex micro-structures of the gas hydrate sediments.

    To overcome the limitations of seismic methods in characterizing the micro-scale properties of gas hydrate sediments, a number of rock physics measurements and modeling have been performed (Hu et al., 2014, 2013, 2010; Best et al., 2013; Wang et al., 2013; Hu and Ye, 2012; Priest et al., 2009, 2005; Westbrook et al., 2008; Sava and Hardage, 2006; Dai et al., 2004; Winters et al., 2004; Lee, 2002; Lee et al., 1996). These studies investigate the relations between acoustic velocities, attenuation, and petrophysical parameters of gas hydrate sediments, including mineral composition, porosity, pore structure, pore fluid type and saturation, and so on. Rock physics models link the micro-scale physical properties and macro-scale seismic responses, which can not only help improve the seismic characterization of gas hydrate reservoirs, but also provide important assistance in production monitoring.

    Production methods of gas hydrate mainly categorize into three schemes: thermal stimulation, depressurization, and inhibitor injection (Collett, 2002). Except for inhibitor injection, which uses chemical reaction to shift the pressure-temperature equilibrium boundary of gas hydrate, both thermal stimulation and depressurization techniques involve changing the temperature and pressure conditions of gas hydrate reservoir to promote its dissociation. In these processes, drastic changes can occur to the elastic properties of gas hydrate, which can be detected through seismic methods (Lee et al., 2011; Kowalsky et al., 2010). Hence, seismic methods can be utilized for gas hydrate production monitoring, which is of significant value not only for production, but also for environment protection purposes since huge blowout of gas hydrate can cause serious environment and climate problems (Boswell, 2009; Bains et al, 1999).

    We summarize the widely-used seismic techniques, rock physics models and production schemes of gas hydrate reservoirs. We also point out the limitations and challenges of these methods, as well as the potential risks of gas hydrate exploitation. In the end, we propose several potential research directions for improving the application of geophysical methods on exploration and exploitation of gas hydrate reservoirs.

  • Seismic reflection techniques are commonly used in characterizing gas hydrate reservoir and have been confirmed to be effective in many cases. The high velocities of gas hydrate formation can cause large impedance contrast between itself and the formation below, especially considering that the underlying formation usually contains free gas. This impedance contrast can lead to strong reflection amplitude and polarity reversal.

  • The mostly used seismic technique for mapping gas hydrates is the BSR (Sain et al., 2000). BSR is caused by the strong impedance contrast between the overlying and underlying formations, and is easy to identify on seismic reflection profile based on several features: it is usually parallel to the morphology of sea floor along isotherms; it has very strong amplitude but reversed polarity compared to the normal sea floor reflection, as shown in Fig. 1. Magnitude of the amplitude is related to the impedance contrast: larger contrast leads to stronger reflection, and thus larger reflection magnitude. Since BSR usually occurs at the boundary between the overlying gas hydrate formation and underlying water-saturated or gas-saturated formation, it is interpreted as the base of the gas hydrate stability zone (GHSZ).

    Figure 1.  The seismic marks of BSR and blank zone (Yang R et al., 2014).

    However, it should be mentioned that BSR does not always necessarily appear at the gas hydrate reservoirs, as it might occur in other scenarios. For instance, even without gas hydrate, if the underlying formation has very low impedance (e.g., caused by high gas saturation), BSR can still occur. Therefore, it is necessary to introduce more constraints from other techniques (e.g. geochemical and microbial) to confirm the presence of gas hydrate. Moreover, for some special gas hydrate reservoirs, e.g., fracture-filled or volcano-affiliated reservoirs, the reflections are usually not continuous and BSRs are not clear (Wang X J et al., 2018). Under such conditions, other exploration methods may provide assistance, e.g., electromagnetic data (Sha et al., 2015a).

  • Besides the BSR and polarity reversal, gas hydrate reservoir usually associates with a blank zone or dim amplitude zone (DAZ) on seismic profile. Blank zone is a seismic feature that marks a zone with weak reflectivity, indicating the absence of signals (Fig. 1). Different from BSR which indicates the bottom of gas hydrate formation, the blank zone is within the formation itself. Several explanations have been proposed to explain the presence of blank zone. The first one is the cementation effect of gas hydrate (Dillon et al., 1993). The solid gas hydrate cements the loose marine sediments, significantly increases the acoustic impedance, and also causes small impedance variations within gas hydrate formations, therefore, leading to the weak reflection inside the formation. A second explanation is about the homogeneity of sediments (Holbrook et al., 1996). The background marine sediments in the gas hydrate formation may be deposited homogeneously, resulting in minor changes of acoustic impedance within the formation, thus causing blank zone. The third explanation is about the strong attenuation associated with gas hydrate (You et al., 2014). Since in-situ gas hydrate is in solid state, it can cause scattering of the seismic energy, especially when it is distributed discontinuously, leading to intense attenuation. This attenuation finally results in the amplitude blanking on seismic profile. Although these interpretations explain the blank zone in different aspects, they all agree that the degree of blanking is related to the amount of gas hydrate in pore space: the higher gas hydrate saturation induces stronger blanking.

  • AVO is a phenomenon that seismic amplitude changes with offset, and these changes are closely related to the lithology and fluid type. Hence, AVO is commonly used for reservoir characterization and fluid identification. Given the special properties of gas hydrate sediments compared to conventional reservoir rocks, it is believed that gas hydrate can generate obvious AVO responses. Currently, there are some researches regarding using AVO in the marine gas hydrate research. Xu and Chopra (2003) exhibited the potential of using AVO characteristics and attributes to identify gas hydrate formation in Mackenzie Delta, Canada. Chen et al. (2010) examined the usefulness of AVO analysis on the permafrost and marine gas hydrate through the BSR AVO inversion. Yang R et al. (2014) summarized the characteristics of gas hydrate AVO responses in terms of the gradient, intercept, and Poisson ratio. Based on sonic logs and rock physics analysis, Qian et al. (2014) integrated rock physics model, seismic inversion and AVO inversion to quantify the elastic parameters, hydrate saturation and distribution.

  • Seismic attributes are extracted from seismic data based on various transform and processing, and can be categorized into amplitude-related, frequency-related, phase-related, waveform-related, and attenuation-related attributes, etc. (Barnes, 2016; Brown, 1996), which can be used in lithology identification and reservoir characterization. Considering that the gas hydrate formation can cause amplitude and phase anomaly (BSR and polarity reversal related), energy anomaly (blank zone related), and traveltime anomaly (gas hydrate formation generally has high velocity), it is possible that gas hydrate formation can be identified through these attributes. Based on reflection strength, seismic amplitude reduction, and instantaneous frequency, Ojha and Sain (2009) tried to identify the gas hydrate and free-gas zones in Makran accretionary prism. Kim et al. (2015) applied reflection strength, instantaneous frequency and spectral decomposition to analyze the gas hydrate and overlying free gas zones in Ulleung Basin, East Sea. Kumar et al. (2019) used instantaneous amplitude, instantaneous frequency, instantaneous phase, root-mean-square (RMS) and sweetness amplitude attributes to determine the occurrence of gas hydrates in Mahanadi Offshore, India.

  • Due to the complex morphology of gas hydrate, different gas hydrate distribution and saturation may generate the similar seismic responses, and reflection seismic method cannot effectively distinguish them. Hence, OBS technique was developed. OBS places geophones on the ocean bottom and thus is not affected by the noise and disturbance, and also eliminates the influence of energy attenuation caused by the ocean, increasing resolution and signal-noise-ratio (Jaiswal et al., 2006). OBS allows imaging of substructure with more details due to the lower Fresnel zone. It can also record the horizontal, vertical, X-horizontal and Y-horizontal components of the seismic signals, making it effective in identifying the presence of gas hydrate and free gas through both P- and S-wave (Liu and Li, 2021). Based on OBS data, Wang et al. (2014) processed the PS wave data and conducted S-wave velocity inversion for gas hydrate. Satyavani et al. (2016) performed OBS data modeling to estimate gas hydrate saturation in Krishna-Godavari Basin. Song et al. (2018) conducted OBS data analysis to quantify the gas hydrate and free gas zones in the South Shetland Margin in Antarctica.

  • The presence of gas hydrate can also be inferred from other seismic features, including high velocities, acoustic masking, gas chimney, acoustic turbidities, pipes and pockmarks on seismic profiles (Lu et al., 2017; Yoo et al., 2013; Gay et al., 2012; Faure et al., 2006; Chand and Minshull, 2003). An example of these marks is shown in Fig. 2. All of these features are caused by the special properties of gas hydrate formation system, including the affiliated gas leaking and the free gas zone below. A combination of these different features can help confirm the presence of gas hydrate and reduce the prediction uncertainty.

    Figure 2.  Various seismic marks associated with gas hydrate (Yoo et al., 2013).

  • Seismic methods can identify gas hydrate formations, but are inadequate to characterize the intrinsic petrophysical properties of gas hydrate bearing sediments without other constraints. Rock physics is a bridge linking the micro-scale rock properties with the macro-scale seismic responses, and thus can assist characterizing the properties of gas hydrate sediments.

    A number of studies have been performed investigating the petrophysical properties of gas hydrate sediments in the forms of both laboratory measurements and rock physics modeling (Wang et al., 2021; Hu et al., 2014, 2013, 2010; Best et al., 2013; Wang et al., 2013; Hu and Ye, 2012; Priest et al., 2009, 2005; Westbrook et al., 2008; Winters et al., 2004; Lee, 2002; Helgerud et al., 1999; Lee et al., 1996). In laboratory, Buffett and Zatespina (2000) generated the gas hydrate from dissolved gas and measured the velocities. Tohidi et al. (2001) made visual observations of gas hydrate formation and dissociation process by making synthetic samples with glass micro-models. Winters et al. (2004) measured and compared the physical properties of laboratory-generated and naturally-formed gas hydrates. Yun et al. (2005) measured both P-wave and S-wave velocities of uncemented gas-hydrate bearing sediments in laboratory. Priest et al. (2009, 2005) investigated the influence of gas hydrate morphology on velocities by using different laboratory conditions (excessive gas, excessive water, and dissolved gas). Hu et al. (2010) measured the acoustic velocities of gas hydrate bearing sediments and also investigated the gas hydrate distribution in pore space and the corresponding influence (Hu et al., 2014). Best et al. (2013) inspected the influences of gas hydrate morphology and water saturation on the attenuation of gas hydrate sediments through laboratory measurements.

  • Besides laboratory measurements, a bunch of models have been proposed to predict the velocities of gas hydrate bearing sediments. These models include the empirical models and theoretical models. Commonly used empirical models are Wyllie's time average equation, and weighted combination of Wyllie's equation and Wood's equation (Lee et al., 1996). Besides, Hyndman and Spence (1992) also developed an empirical model linking porosity and velocity of the sediments, which approximates the hydrate saturation by reducing the porosity. These models are generally simple and in some cases effective, since they are basically the statistical relations based on field data. However, the limitation of empirical models is the lack of broad applicability: they can work properly in one field, but may fail in others. Therefore, many studies have come up with theoretical models.

    Theoretical models of gas hydrates generally include three categories: the three-phase effective-medium theory (TPEM), the differential effective medium theory (DEM), and the Biot theory-related models, including three-phase Biot theory (TPB) and Biot-Gassmann theory modified by Lee (BGTL) (Chand et al., 2004). In TPEM theory, according to the gas hydrate morphology, Ecker et al. (1998) proposed the effective medium models for gas hydrate bearing sediments, in which he assumes three scenarios: gas hydrate distributes at grain contacts, coats the grains, and suspends in pore fluids. For the first two scenarios, a combination of the cementation model developed by Dvorkin et al. (1994, 1991) and modified Hashin-Shtrikman bounds (Dvorkin and Nur, 1996) can be applied to predict the elastic moduli, while for the third scenario, the elastic moduli of pore fluids can be calculated through mixing the water and hydrate through Reuss average or Wood's equation. For both the two scenarios, the moduli of the rock matrix can be approximated through Voigt-Reuss-Hill average (VRH) (Mavko et al., 2009). This model takes into account the different phases and morphologies of the components. However, the estimated velocities through the cementation model are generally higher and the prediction results with contact model are less sensitive to porosity variation than observed values. Besides, the model does not consider the anisotropic effects associated with particle alignment.

    In DEM theory, considering the complex factors of gas hydrate sediments (e.g., mineralogy, porosity, pore structure, pore fluids saturation, and clay particle anisotropy), Jakobsen et al. (2000) linked them through the models developed by Sheng (1990) and Hornby et al. (1994), and combined self-consistent approximation (SCA) and the differential effective-medium (DEM) theory to predict the elastic moduli. This model takes into account the pore structure, different phases and geometries of the components. However, it does not consider the pressure effects and neglects the weaker bonding and greater compliance at the edges of the individual domains of particle alignment (Bennett et al. 1991).

    In TPB theory, based on previous work of Leclaire et al. (1994), Carcione and Tinivella (2000) developed a three phase Biot model by integrating the potential and kinetic energies related to the grain and hydrate contacts to stiffen the rock frame, and Gei and Carcione (2003) further extended the theory by including the effects of partial saturation, pore pressure, and dissipation mechanisms. This model relates the elastic velocities with porosity, saturation, dry rock moduli, fluids and grain properties, but it requires calibration of the involved empirical parameters before application. Ba et al. (2017) also discussed the rock anelasticity caused by patchy saturation and heterogeneity. By assuming that the gas hydrate sediment's VP/VS ratio is proportional to the VP/VS ratio of the matrix and porosity, Lee et al. (2002) proposed the Biot-Gassmann theory modified by Lee (BGTL), which is appropriate for sediments with relatively low porosity.

    Most of recently developed models to predict gas hydrate elastic velocities are based on the three types above. Based on DEM theory, Liu X X et al. (2018) considered two scenarios of assuming gas hydrate as matrix components and as pore fluids, and performed the modeling to predict its elastic velocities in porous media. Combing SCA and DEM models, Singhroha et al. (2019) developed a rock physics model linking velocities with gas hydrate saturation, and combined it with multicomponent OBS data to predict gas hydrate saturation in Vestnesa Ridge, Western Svalbard Margin. To deal with the anisotrophy of gas hydrate sediments caused by clay platelets and fracture-filling morphology, Ghosh and Ojha (2021) also applied a combination model of SCA and DEM models to simulate the gas hydrate properties. Wang et al. (2021) also took into account the two scenarios above and simulated the gas hydrate velocities using TPEM theory. Similarly, based on TPEM theory, Guo et al. (2021) combined Krief theory and BISQ theory to predict the frequency-dependent P-wave velocity of the gas hydrate in porous media.

    Moreover, some researchers have also performed rock physics modeling to simulate the elastic wave attenuation and dispersion in gas hydrate bearing sediments. Toms et al. (2006) conducted a comparative review describing the mechanisms of theoretical models to simulate the wave attenuation and dispersion in partially saturated rocks. Best et al. (2013) applied the squirt-flow mechanism to explain the observed attenuation in gas hydrate measurements. By comparing the P-wave attenuation models of wave-induced flows, Sun et al. (2015) pointed out that wave-induced fluid flow is one of the main reasons for P-wave attenuation and dispersion in seismic frequencies. Zhang Z J et al. (2014) took into account both intrinsic and scattering attenuations in the seismic interpretation of gas hydrates field data and found that scattering attenuation is more sensitive to faults and fractures than intrinsic attenuation. Applying BISQ model (Dvorkin and Nur, 1993) on the analysis of velocity dispersion and attenuation of elastic waves in marine gas hydrate bearing sediments, Zhang et al. (2016) found that attenuation increases clearly with gas hydrate concentration, but seems to be independent of porosity and clay content. Marín-Moreno et al. (2017) took into account both the factors of gas-bubble damping and squirt-flow mechanism, and discovered the different behaviors of gas hydrate attenuation at different frequencies.

  • Based on the analysis of the above models, we find that most of the models follow a general workflow (Fig. 3): (1) build the rock matrix; (2) generate the rock frame by including the pores in the matrix; (3) form the pore fluids mixture by mixing different pore fluids; (4) saturate the dry rock with the pore fluids mixture.

    Figure 3.  Workflow of establishing rock physics models for gas hydrate sediments.

    In the first step of building rock matrix, two scenarios are involved: single mineral and multiple minerals. In the single mineral case, the rock matrix moduli are the moduli of the mineral, while in the case of multiple minerals, a commonly used model is the VRH average, which simply averages the Voigt bound and Reuss bound of the minerals to form the matrix. Besides, if the minerals include clay particles with anisotropic geometry, a self-consistent approach (SCA) or DEM model can be applied to deal with it. When including pores into the rock matrix, given the pore structure (pore aspect ratios), the DEM model is commonly used to calculate the moduli of dry frame. Afterwards, the effective modulus of pore fluids can be calculated. This step also involves two scenarios. If the pores are well connected, the pore fluids are fully mixed and pore pressure can easily reach equilibrium, they are in iso-stress condition and thus the modulus can be estimated with Wood's equation. This is the most common scenario in pore fluids mixing. However, in some cases when the pore connectivity is poor and the pore fluids are in patchy saturation, it is possible that the pore fluids are under iso-strain condition, which is more appropriate to mix with the Voigt model (Mavko et al., 2009). For instance, Fabricius et al. (2007) used the Voigt model to estimate the effective modulus of pore fluids in simulating chalks with high porosity but low permeability. In the last step of fluid saturation, the mostly used method is Gassmann equation (Mavko et al., 2009), because it predicts the saturated rock moduli at low frequency and thus the prediction results are comparable to seismic velocities. Nevertheless, it should be mentioned that the above workflow just shows a common scheme of building rock physics models for gas hydrate sediments, and it does not cover all the possible scenarios. Other models can be applied partly or entirely to replace the steps of the workflow. For instance, in the case that the pore space is fully saturated with solid gas hydrate, Hertz-Mindlin model (Mavko et al., 2009) can be used for the prediction, and even in a simpler way, VRH model can be used to complete the modeling.

  • Since in-situ gas hydrates are in solid state which cannot be produced directly through boreholes, the production needs to dissociate gas hydrate into free gas and water and then produce the gas in boreholes. These production methods mainly include three types: thermal stimulation, depressurization, and inhibitor injection (Lee et al., 2011; Collett, 2002).

    Thermal stimulation method involves injecting hot water into the gas hydrate formation to increase reservoir temperature (Fig. 4a). Gas hydrate can dissociate when the reservoir conditions cross the pressure and temperature boundary of the GHSZ. Hence, as the temperature of gas hydrate formation increases, gas hydrate will follow the path 1 in Fig. 5 and turns to gas and water, which can then be produced. Depressurization technique works in the water or gas formation beneath the gas hydrate formation. It is implemented through removing the gas or water in the beneath zone, which can lead to the pore pressure drop (Fig. 4b). As the pressure in the gas zone drops, the gas hydrate at the bottom of GHSZ will follow the path 2 in Fig. 5 and dissociate. Inhibitor injection method does not require changing in-situ environmental conditions (pressure and temperature). It injects the inhibitor (e.g., brine or methanol) into the reservoir and works on the gas hydrate itself through chemical mechanism (Fig. 4c). Generally, the injected inhibitor can shift the thermodynamic equilibrium curve to lower temperature and higher pressure (path 3 in Fig. 5), so that the in-situ conditions are out of the boundary. Hence, the gas hydrate will become unstable and dissociate into gas and water, which can then be produced.

    Figure 4.  Production methods of gas hydrate. (a) Thermal stimulation method; (b) depressurization method; (c) inhibitor injection method.

    Figure 5.  Schematic mechanisms of the production methods. ①, ②, and ③ are corresponding to the thermal stimulation, depressurization, and inhibitor injection methods, respectively. It should be mentioned that this figure just shows the primary mechanism of each production technique, while the true production may involve multiple mechanisms, which is discussed in the following section.

    All the production methods will make the gas hydrate dissociate from solid state to gas state, which can cause significant changes to the gas hydrate elastic properties. Bai et al. (2016) revealed that VP of gas hydrate drops about 41% when only 10% of the gas hydrate dissociates into gas. Moreover, the changes of pressure and temperature of the reservoir can also affect the elastic properties of the gas hydrate bearing sediments. According to Priest et al. (2005), the gas hydrate sediments' VP can increase up to 35% and VS can increase up to 44% when pressure increases from 250 to 2 000 kPa. Based on laboratory measurements by Wang et al. (2008), the VP of gas hydrate decreases over 13% when temperature rises from -5 to 3 ℃. These properties changes can be detected through time-lapse seismic methods, providing the possibility of seismic monitoring for gas hydrate production.

  • Although seismic methods have been studied widely to investigate the gas hydrate reservoirs, there are still many problems and challenges. The BSRs are generally considered as marks of the gas hydrate formations. However, they do not necessarily indicate the gas hydrate reservoirs. Theoretically, BSRs occur due to the huge impedance contrast between the overlying high-impedance formation and underlying low-impedance formation. Studies have revealed that BSRs are more likely to relate with the presence of underneath gas formation (Haacke et al., 2007; Wu et al., 2007; Hyndman et al., 2001; Pecher et al., 1998; Singh et al., 1993). The presence of gas significantly reduces the impedance of underlying formation, causing the large impedance contrast. Moreover, due to the complex geological conditions, the gas hydrate and free gas formations can also exist even without exhibiting BSR (Holbrook et al., 1996). Hence, it is worth considering to combine BSR and other exploration techniques (e.g., electromagnetic, geochemical, and microbial) to confirm the gas hydrates presence.

    In above section, we mention three possible reasons of blank zone. Actually, there is one more possible reason for the blank zone in the thin interbed gas hydrate formations commonly existing in South China Sea (SCS) (Zhang et al., 2007), which is the tuning effect of thin interbed. Tuning effect can cause destructive interference between seismic reflection signals, leading to damping amplitude and blank zone (Wang Y et al., 2018). Even for simple rhythmic thin interbed composed of only two lithology medium, the reflection characteristics change obviously with layer thickness. The blank reflection can appear even when the layer thickness is very small (left parts in Figs. 6b and 6c) (Wang Y et al., 2018; Li, 2008; Ellison et al., 2004). Hence, it is indispensible to consider this possibility of blank zone.

    Figure 6.  (a) The rithmetic thin interbed model with layers of difference thickness. The reflections of the thin interbed with (b) water formation above and (c) unconsolidated formation above (Wang et al., 2018).

    Current seismic methods for gas hydrate reservoir characterization are mainly based on P-wave, which, however, have limitations. P-wave generally has low vertical resolution compared to S-wave. Besides, using P-wave data alone has challenges of non-uniqueness of inversion. For instance, it is inadequate to determine the saturation of gas hydrate, because multiple scenarios may cause the similar BSR responses (e.g., the scenario of overlying high gas hydrate saturation formation with underlying water saturated formation and the scenario of overlying low gas hydrate saturation formation with underlying gas saturated formation) (Song et al., 2002). In addition, it is difficult to determine the gas hydrate morphology in pores through P-wave data alone, since different gas hydrate morphologies can lead to the similar elastic moduli.

    A compensation for P-wave data is to introduce S-wave data, which has several advantages. First, S-wave has higher resolution than P-wave data and can facilitate the imaging process, thus leading to more detailed characterization of the geological structure. In particular, considering that gas hydrates are generally located at shallow zones beneath sea bottom where S-wave signals are still strong without much attenuation, the S-wave data can provide high quality information for imaging the structure. Besides, S-wave velocity has distinct trend from P-wave velocity when gas hydrate distribution or saturation changes. For instance, in the case that gas hydrate is suspended in pore fluids, the P-wave velocity increases, while S-wave velocity is barely affected. Hence, integrating the S-wave and P-wave velocities will provide a more robust method to characterize the gas hydrate morphology and saturation in sediments. Thirdly, given that S-wave and P-wave data have different responses to the pore fluids, the corresponding BSRs also show distinct characteristics. Thus, the introduction of S-wave data can also help determine if the BSRs are caused by the lithology change or gas hydrate presence.

    The various rock physics models of gas hydrate bearing sediments can work in different scenarios depending on the assumptions. Although previous studies have investigated pressure and temperature effect on other rocks (Qi et al., 2020; Motra and Stutz, 2018; Yuan et al., 2018, 2016), many models of gas hydrate ignore these two important factors. Despite that these two factors are in narrow ranges within the gas hydrate formation, they are closely related to the gas hydrate production. As presented in above section, two major categories of gas hydrate production are related with pressure and temperature change, and even the inhibitor injection production scheme also involves a certain degree of pressure and temperature change, since the injection pressure is larger than the pore pressure and the chemical reaction may cause temperature change. Hence, it is necessary to investigate the physical properties of gas hydrate at various pressure and temperature conditions and conduct rock physics modeling, which can help assess the in-situ conditions of gas hydrate reservoir and assist the production monitoring.

    In addition, current rock physics models generally neglect one important type of gas hydrate formation––the fracture-filled gas hydrate reservoir, which is also an important type of gas hydrate in SCS (Liu X Q et al., 2018; Sha et al., 2015b; Zhang G X et al., 2015, 2014; Yang S X et al., 2014). Different from the pore-filling reservoirs, fracture-filling gas hydrate mainly distributes along the sea bottom fracture zone (Liang et al., 2017). On the other hand, however, since the fractures are generally vertical or oblique, the gas hydrate distribution is discontinuous, unable to generate clear BSR on seismic profile. Moreover, fractures also have significant influences on the P-wave and S-wave velocities (e.g., anisotropy) (Fang et al., 2017), and the in-filled solid gas hydrate makes the impact even stronger. For this type of reservoir, one possible simulation method is the empirical model (e.g., weighted average formula by Lee et al., 1996). However, empirical model requires huge amounts of data as reference, and usually lacks wide applicability. Currently, there is a handful of theoretical model available that can characterize this fracture-type gas hydrate. Qu et al. (2017) combined DEM model and Biot theory to characterize the elastic properties of fracture-filled gas hydrates. Ghosh and Ojha (2021) combined SCA and DEM theories to simulate the fracture-filled gas hydrate bearing sediments and estimated the saturation based on well logs.

    One important target for gas hydrate exploration is to determine the saturation for source potential evaluation, which is challenging and cannot implement without proper rock physics models that link seismic responses with the gas hydrate saturation in sediments. However, due to the complex morphology of gas hydrate, different saturation scenarios may cause the same elastic responses and thus the prediction results contain uncertainties. To reduce the uncertainty and improve the saturation estimation, more sophisticated models are needed. These models should be able to distinguish the gas hydrate morphology in the sediments and predict the saturation. One possible method is to incorporate both VP and VS in the modeling, since these two velocities have different responses to gas hydrate morphology and saturation as mentioned above. Moreover, the true reservoir might contain multiple patterns of gas hydrate morphology and the saturation may be inhomogeneously distributed. Characterizing the physical properties of this type of reservoir is challenging, which requires more constraint as input (e.g., P-wave, S-wave, converted wave, well log data, and core data, etc.). Combining Krief model (Krief et al., 1990) and BISQ theory, Guo et al. proposed a rock physics model to characterize the seismic PP and PS waves dispersion and attenuation. Besides, since gas hydrate formation can cause intense attenuation to the acoustic waves as revealed by Priest et al. (2006) and Best et al. (2013), the conventional seismic attenuation can be used to map the gas hydrate distribution and concentration, which has been verified in other rocks (Pang et al., 2019). In summary, rock physics modeling should take into account the in-situ factors of gas hydrate sediments, including the porosity, pore structure, saturation, mineral composition, pressure and temperature, so that it can improve the gas hydrate reservoir exploration and production monitoring.

    The production of gas hydrate involves changes of pressure and temperature in the thermal stimulation and depressurization methods. Actually, the pressure and temperature effects are coupled. For instance, in the depressurization process, the gas hydrate dissociates into gas and water, which is an endothermic process and thus the reservoir temperature will inevitably decrease. Hence, for production monitoring, the influences of pressure and temperature should both be addressed. Qin et al. (2020) has addressed these problems in the first gas hydrate depressurization production test in SCS.

    For all these production strategies, one important issue is to determine the remaining gas hydrate in the formation, which can be solved through geophysical methods. Once the gas hydrate dissociates, the formation velocities drops drastically, which can be detected through seismic data. Besides, an integration of P-wave and S-wave data enables more effective production monitoring. It can determine how much gas hydrate dissociates and how much remains based on the seismic velocity change caused by production, in which the rock physics models can help quantify the relations between seismic velocities and gas hydrate saturation. Moreover, reservoir monitoring is also important for environmental issues. The production may cause seafloor subsidence and landslide, which can lead to further environment or climate problems. Seismic monitoring can help determine the high-risk areas, providing early warning for further production.

  • We summarize the seismic techniques, rock physics models, and production schemes of gas hydrate reservoir, and also point out the limitations, challenges, and potential problems of these methods. In light of the problems, we propose several potential research directions which may provide implications for future studies.

    BSR, blank zone, and polarity reversal are widely used in gas hydrate sediment detection. However, they are not solely caused by gas hydrate, and can be caused by other scenarios. To reduce the prediction uncertainty, it is necessary to integrate other information to confirm the presence of gas hydrate. Due to the special properties of gas hydrate bearing sediments, they can also be detected through other seismic techniques, including seismic attributes, AVO, and OBS.

    Multiple rock physics models are available for characterizing the petrophysical properties of gas hydrate. Despite empirical models that are based on statistical relations, most theoretical models follow the workflow of calculating the moduli of rock matrix, frame, pore fluids modulus, and saturated modulus, in each step of which different models and corresponding uncertainties are involved. Moreover, to facilitate production monitoring, one important aspect in rock physics modeling is to consider the pressure and temperature factors.

    Production methods mainly include thermal, depressurization, and injecting inhibitor techniques. Most of them involve the changes of temperature and pressure conditions, which can lead to huge changes in the elastic properties of gas hydrate. These changes can be detected and captured by geophysical methods, which, in turn, can be utilized for production monitoring.

    Several potential research directions are proposed, which include introducing S-wave data, improving rock physics measurement and models, and using seismic attenuation to map gas hydrate. Moreover, given the thin interbed gas hydrate formations in SCS, one important research is to estimate the gas hydrate saturation and velocities in thin interbed. These researches can help improve the seismic characterization and production monitoring of gas hydrate, and may provide significant implications for future research.

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