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The top and bottom AVO response for the reservoir (Stø Formation) is determined by using the reflectivity and intercept-gradient analyses. The two-term Aki-Richards equation with robust correction is applied during the reflectivity and intercept-gradient analyses. The AVO class, intercept and gradient with different pore fluid of the two wells 7120/12-3 and 7120/12- 5 are shown in the Table 6.3.

In the well 7120/12-3, top of the reservoir (Stø Formation) shows the AVO class IV with negative intercept while the gradient is positive. The intercept for in-situ gas to 100% water saturation decreases from -0.17044 to -0.13117 while the gradient decreases from 0.31022 to 0.2343. However, the bottom of the reservoir has a positive intercept and a negative gradient. The intercept for 100% gas to 100% water saturated case decreases from 0.06024 to 0.04661 while the gradient also decreases from -0.10616 to -0.08478 (Fig. 6.4)

In the well 7120/12-5, top of the reservoir (Stø Formation) shows AVO class IV with intercept for in-situ water to 100% gas increases from -0.11826 to -0.15664 while gradient increases from 0.13367 to 0.15482. However, the bottom of the reservoir, the intercept for 100% water to 100% gas slightly increases from 0.0366 to 0.03936 while the gradient decreases from -0.05052 to -0.05553 (Fig. 6.5).

Table 6.3: AVO class, Intercept (A) and Gradient (B) of top and bottom of reservoir (Stø Formation) in the wells 7120/12-3 and 7120/12-5 at different fluid saturations.

Wells Fluid

Top of reservoir Bottom of reservoir Class Intercept (A) Gradient (B) Class Intercept (A) Gradient (B) 7120/12 -3 100% Gas IV -0.17044 0.31022 I 0.06024 -0.10616 50% Gas IV -0.15639 0.28121 I 0.05326 -0.09482 100% Water IV -0.13117 0.23437 I 0.04661 -0.08478 7120/12 -5 100% Water IV -0.11826 0.13367 I 0.03660 -0.05052 50% Gas IV -0.14806 0.14097 I 0.03769 -0.05496 100% Gas IV -0.15664 0.15482 I 0.03936 -0.05553

79 Figure 6.4: Response of angle dependent reflectivity with intercept-gradient plot in the well 7120/12-3 for three different fluid saturations (a) 100% gas (b) 50% gas (c) 100% water

80 Figure 6.5: Response of angle dependent reflectivity with intercept-gradient plot in the well 7120/12-5 for three different fluid saturations (a) 100% water (b) 50% gas (c) 100% gas

6.2 Discussion

In AVO modeling, the effect related to nature of fluid present in the reservoir has been demonstrated. As expected the AVO response changes with the type of fluid in the reservoir. Moreover, it results in changing the rocks physical properties such as density, Vp, Vs and Poisson’s ratio. The Stø Formation in two wells (7120/12-3 and 7120/12-5) has been analyzed by using five different gas saturation scenarios. The AVO response of reservoirs physical properties of Stø Formation is almost same for both the wells.

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√

Where K represents bulk modulus, μ represents shear modulus and ρ represents density. Equation 6.1 shows that Vp in a rock is dependent upon its bulk modulus, shear modulus and density. However, the bulk modulus and density of the rock is dependent on the nature of pore fluid. Water has a much higher density (1 g/cc) than gas. Similarly bulk modulus shows the same behavior as it decreases with increase in gas saturation. It is due to the fact that bulk modulus is the measure of incompressibility of rock. Since, gas is more compressible as compared to liquids so gas exhibits low bulk modulus. Therefore, the introduction of 10%gas into 100% water saturated rock shows a decrease in its Vp from 4433 m/s to 4258 m/s in the well 7120/12-3 (Fig. 6.6).

Figure 6.6: Changes in rock properties in the well 7120/12-3.

For higher gas saturations such as 50, 90 and 100%, increase in Vp is observed. The Vp increases notably from 4358 m/s at 10% gas saturation to 4410m/s at 100% gas saturation. This increase in Vp is due to decrease in bulk modulus and density as the gas saturation increases. However the rate of decrease in density is more than the decrease in bulk modulus. This results in slightly higher Vp values for increasing gas saturation.

Equation 6.2 shows that Vs depends on shear modulus and density of the rocks. Vs is independent of the type of fluid present in the reservoir as shear modulus is zero for the liquids. However, the density does affect the Vs. In case of 100% water saturation, density is 2.48 g/cc in the well 7120/12-3. But a significant decrease in density is observed for a gas saturation of 100%. It results in slightly higher Vs in the reservoir by increasing gas content. It rises from 2589 m/s at 10% gas saturation to 2628 m/s at 100% gas saturation (Fig. 6.6).

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The reservoir (Stø Formation) in the wells 7120/12-3 and 7120/12-5 expresses similar trend in Vp, density and Vs. Therefore, gas replacement for reservoir in well 7120/12-3 (Fig. 6.6) shows similar results as the reservoir in the well 7120/12-5 (Fig. 6.7).

Figure 6.7: Changes in rock properties in the well 7120/12-5.

The AVO response for the sand reservoir in well 7120/12-3 can be classified as class IV based on Rutherford and William (1989) gas sand classification. In the Figure 6.4, three cases are shown i.e. 100% gas, 50% gas and 100% water saturation. The density and Vp in rocks decrease with increase in gas saturation. Therefore, AI contrast exists between cap-rock and reservoir rock. In this case it is negative because presence of 100% gas in reservoir results in higher AI in cap rock than reservoir. It is shown by higher AI values at zero-offset for 100% gas saturated reservoir. However, increase in water saturation results in decrease in AI contrast because of higher density of water and higher Vp for water saturated reservoir. So a decrease in zero offset reflection coefficient can be seen from -0.17044 at 100% gas to - 0.13117 at 100% water saturation. Figure 6.4 also shows that AVO response from angles 0 to 45 degrees follows nearly the same trend for different gas saturations. The AVO response can also be presented in terms of AVO intercept versus Gradient as shown in the Figure 6.4. These crossplots also indicate an AVO class IV in the well 7120/12-3. Similar case can be observed with the reservoir in the well 7120/12-5 (Fig. 6.5) which also shows an AVO class IV and its behavior is similar as in case of reservoir in the well 7120/12-3.

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