saturations of hydrate and those estimated using the Archie equation (see page 269) is achieved.


8.2.5 Qingdao Experiment


In 2012 a very interesting experiment on hydrate formation and dissolution was presented by Hu and Ye of the Qingdao Institute of Marine Geology in China. Tey measured P- and S-wave velocities as the hydrate concentration in two rock samples was gradually increased from zero to 70%. Te sediment samples were first immersed in pure water, then loaded into a high-pressure vessel, and injected with methane. Te temperature was kept at 2°C for hydrate formation. To simulate the effect of hydrate dissolution, the temperature was gradually increased to room temperature. For an unconsolidated sand sample, they found that


the P-wave velocity increased from approximately 1,600 m/s for zero hydrate concentration to approximately 3,600 m/s for 70% hydrate concentration. Te corresponding values for the S-wave velocity were 600 and 1,600 m/s, respectively. Tis means that the Vp/Vs-ratio decreases from 2.7 for no hydrate to 2.25 at 70% hydrate concentration. For comparison, it is interesting to note that Helgerud (2001) measured a Vp/Vs-ratio of approximately


Rock Physics


Rock physics provides the con nec tions between geophysical (elastic and electromagnetic) prop erties of the rock measured at the surface of the earth, within the borehole or in the laboratory, with the intrinsic properties of rocks, such as mineralogy, porosity, pore shapes, pore fluids, pore pressures, permeability, electrical resistivity, viscosity, stresses and overall architecture such as laminations and fractures. Tese parameters affect how seismic and electromagnetic waves/fields physically travel through the rocks. Establishing relationships between geophysical expression and physical rock properties therefore requires knowledge about the elastic/ electromagnetic properties of the pore fluid and rock frame, and models for rock-fluid interactions. Equations that attempt to describe the relationships


between seismic velocities and lithology, porosity, pore fluid, etc. are either theoretical or empirical. Zhijing Wang summarises the tension between theoretical


and empirical approaches this way: most direct measurements are carried out either in the laboratory or inside a borehole, whereas most theoretical calculations are based on the Gassmann equation (Gassmann, 1951) because of its simplicity and ease of use. Direct laboratory measurements are carried out in controlled, simulated reservoir environments and provide accurate effects of pore fluids on seismic properties. Direct borehole measurements, however, are often affected by uncontrollable factors such as stress concentration, hole washout, mud invasion/filtration, and saturation conditions. In both laboratory and borehole measurements, the wave frequencies are higher than seismic frequencies. Jan Dewar summarises that the crux of all this is that there


266


Figure 8.11: P-wave velocity versus porosity assuming that the methane hydrate is a part of the rock frame (black line), of the pore fluid (green line). The red line shows a rock which is 100% water saturated (no hydrate), the blue solid line represents 1% patch gas saturation, and the dark blue line represents 1% homogenous gas saturation.


1.9 for pure hydrate. A linear extrapolation of the measured Vp/ Vs-ratio from Hu and Ye’s experiment yields a Vp/Vs-ratio of approximately 2.0.


are a great number of relationships between seismic velocities (and constituent elastic properties) and rock parameters, all valid to some degree but not valid always, and many that do not illuminate the physical principles involved. Te trick is to try to gain a fundamental understanding so that, on a practical basis, the different relationships can be evaluated for their applicability to solving specific problems. Teoretical rock physics modelling can also be applied


to understand the physical properties of natural gas hydrate systems and accumulations. A major goal is to establish linkages between gas hydrate concentration/saturation in sediments and the measureable physical properties, like P- and S-wave velocities and electrical resistivity. Modelling the elastic properties of sediments as a function of gas hydrate content can be achieved in various ways, such as effective medium modelling or three-phase Biot theory. Te effective medium theory can incorporate the effects of cement and grains. In the case of gas hydrate, the hydrate can form in various ways. Te effective seismic velocities vary quite strongly depending on which formation scenario is used. To model gas hydrate systems via rock physics one needs


to define the elastic/electromagnetic properties of the system in terms of (i) elastic/electromagnetic properties of the (unconsolidated) sediments that host the hydrates, (ii) elastic/ electromagnetic properties of the embedded gas hydrates, (iii) the concentration of hydrates in the sediments, and (iv) geometrical details of the distribution of hydrates within their host sediments. Te inverse modelling problem is to infer hydrate concentration from geophysical measurements. Acoustic well-logs with full waveform show a pronounced decrease of wave amplitude in hydrate-bearing zones.


Helgerud (2001)


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