Gas hydrate saturation in the Krishna–Godavari basin from P-wave velocity and electrical resistivity logs
Introduction
Gas hydrate is a solid substance consisting of an ice lattice in which hydrocarbon molecules (mainly methane) are imbedded. Gas hydrate occurs widespread in the KG Basin, eastern continental margin of India (Collett et al., 2008a, Ramana et al., 2009). Marine gas hydrate generally occurs within the top few hundred meters of sediments in continental margins worldwide (e.g. Kvenvolden et al., 1993). The inclusion of gas hydrate in marine sediments usually changes the physical properties of the bulk sediment subsequently. Gas hydrate can take on many forms, including small nodules, lenses, veins, fracture-filling, and pore-filling. In the simplest model, rising methane combines with the sediment pore fluid to form gas hydrate, partially replacing the pore fluid (i.e. pore-filling), but little change to the sediment structure or volume. More complex models involve gas hydrate crystal growth by displacement of the ambient sediment, in form of veins, fracture-fill, small nodules, or lenses.
The presence of gas hydrate in-pore space of marine sediments can therefore significantly affect the bulk physical properties of the sediments. Gas hydrates exhibit relatively high compressional wave velocity compared to pore-filling fluids such as water; therefore, the velocity of gas hydrate bearing sediments are usually elevated (Stoll et al., 1971, Tucholkeb et al., 1977). Seismic velocities can be obtained from multichannel seismic data, Logging-while-drilling (LWD) down-hole sonic velocity, or vertical seismic profile (VSP) measurements (Westbrook et al., 1994, Yuan et al., 1996, Paull et al., 1996). Numerous studies have attempted to relate seismic velocity to gas hydrate saturation, using a variety of approaches. Most methods can be classified as empirical porosity–velocity relations applied to effective porosity reduction models (e.g., Hyndman et al., 1993, Yuan et al., 1996), time-averaging approaches (e.g., Pearson et al., 1983, Lee et al., 1993), and first-principles-based rock-physics modeling approaches (e.g., Dvorkin and Nur, 1993, Carcione and Tinivella, 2000, Helgerud et al., 1999).
From the various physical-property down-hole logs, resistivity appears to be the most strongly affected by the presence of gas hydrate in the marine sediment. Its inclusion in the pore space of marine sediments can significantly affect the bulk physical properties of the sediment. The measurement of such properties can therefore be used to estimate gas hydrate saturation (e.g. Collett and Ladd, 2000, Yuan et al., 1996). Natural gas hydrate formation reduces the effective porosity and electric conduction, so that gas hydrate bearing sediment has high electrical resistivity. Down-hole resistivity logs have been used extensively to characterize the in situ properties of gas hydrate bearing sediments and estimation of gas hydrate saturations (e.g., Collett and Ladd, 2000, Collett, 2002, Guerin et al., 1999, Helgerud et al., 1999, Hyndman et al., 2001, Lee and Collett, 2005, Lee and Waite, 2008).
In this study gas hydrate saturation estimates are described using Archie’s (1942) law from the electrical resistivity log data and an effective medium modeling approach to predict the P-wave velocity for different amounts of gas hydrate saturation in the sediments.
In total 10 of the NGHP Expedition 01 gas hydrate sites from the KG Basin are used for the gas hydrate saturation estimates. Specifically we show results from Sites NGHP-01-03, NGHP-01-05 and NGHP-01-07, as representative examples. Site NGHP-01-03, in 1076 m of water, is in the southern portion of the KG Basin, Site NGHP-01-05, within 945 m of water, is located in the central part of basin. Site NGHP-01-07 is situated on the eastern most part of the basin at grater water depth around 1285 m (Fig. 1). We assume that the gas hydrate primarily replaces a portion of the sediment pore fluid, since no large pieces (“massive”) of gas hydrate were recovered at these site and dispersed gas hydrate has been inferred (Collett et al., 2008a). The gas hydrate saturation versus resistivity may be quite different if there are massive gas hydrates that displace the sediment (e.g., Mathews, 1986). The problem for estimating gas hydrate saturation is how to separate the effects on the down-hole log resistivity data of the resistive hydrate and of the unusually low salinity in situ pore fluids. In situ pore fluid salinity measurements from direct down-hole sampling are not generally available. To obtain the effect of the gas hydrate on the measured resistivity, the in situ pore fluid salinity was inferred from the measurement of the interstitial properties of the pore fluid of the recovered core. Should core-derived values of pore-water salinity be unavailable, we invoke Arps’ (1953) law to derive the in situ pore-water resistivity as function of down-hole temperature using regionally defined seafloor temperatures and thermal gradients, and seawater resistivity at the seafloor as input parameters (Collett et al., 2008a, Shankar et al., submitted for publication).
In the final part of this study we combine the observation of bottom-simulating reflectors (BSR) in 2D and 3D seismic data with the gas hydrate saturations defined from log-parameters to calculate a total volume of gas hydrate in the KG basin.
Section snippets
Log data and methods
The down-hole logging program during NGHP Expedition 01 was specifically designed to assess the presence and saturation of gas hydrates on the continental margin of India (Collett et al., 2008a). Several LWD and wire-line logging devices were deployed, as described below. Not all tool strings were run in each hole. During NGHP Expedition 01, LWD data were acquired at five sites drilled in the KG Basin on the eastern continental margin of India.
Gas hydrate saturation (Sh) estimation from logs
There is an extensive body of literature on the use of well logs for estimating gas hydrate saturations (e.g., Collett et al., 1984, Collett et al., 1999, Collett, 2001, Collett and Lee, 2004, Kleinberg et al., 2003, Kleinberg et al., 2005, Lee and Collett, 2008, Mathews, 1986, Guerin et al., 1999, Hyndman et al., 1999). The most commonly used logs for gas hydrate saturation estimates include resistivity and sonic logs. Below, we discuss the gas hydrate saturation estimates based on resistivity
Regional gas hydrate volume assessment
In this exercise we attempted to estimate the total amount of gas hydrate in the study area of the KG basin, which can be used later as key ingredient in other studies related e.g. to carbon cycles and climate change. We base the calculations on the assumption that we only find structure-I gas hydrate, as the drilling showed that 99.9% of the gas recovered (in voids and from gas hydrate samples) is biogenic methane (Collett et al., 2008a).
The first step in this calculation is the definition of
Discussions
The presented gas hydrate saturation estimates are based on several assumptions, where the most fundamental assumption is the mode of gas hydrate occurrence as pore-filling medium (effectively reducing porosity, without adding stiffness to the overall sediment matrix or cementing the individual sediment constituents) so that Archie’s (1942) relationship and an effective medium model can be invoked in the calculations. From the 10 drill sites selected, Sites NGHP-01-05 and NGHP-01-07 have a
Conclusions
We have used available logging data from the India NGHP Expedition 01 to derive estimates of gas hydrate saturation at 10 sites within the KG basin (NGHP-01-02, -03, -04, -05, -06, -07, -11, -14, -15, -16). We also used available 2D and 3D seismic data to calculate the total amount of gas hydrate present in the study area.
Gas Hydrate saturations were estimated using electrical resistivity and P-wave velocity logs. Resistivity logs were analysed using Archie’s (1942) relation, combined with
Acknowledgements
Authors would like to thank DGH and ONGC for their fruitful collaboration, especially by making the 2D and 3D seismic data set available for extended studies on gas hydrate in the KG Basin. We further would like to acknowledge the entire onboard team of scientists and crew-members of the India NGHP Expedition 01, who acquired the log and core data used in this analysis. In addition, U Shankar is grateful to the Department of Science and Technology, Govt. of India, New Delhi for the BOYSCAST
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