PETROPHYSICAL PROPERTIES

Given the distribution of pore pressure and water saturation as functions of time and distance from the water injectors, we predict the associated compressional (P) and shear (S) seismic velocities V_p, V_s and density $\rho$ throughout the reservoir at two stages of waterflood.

The Stanford Rock Physics and Borehole Geophysics Project (SRB) has conducted several lab measurements of dry and saturated rocks at various pressures, temperatures, and for variable pore-fluid saturants. Figure shows some of the SRB lab results for the Ottawa sand on which our reservoir study is based. The curve labeled ``GAS'' was obtained by measuring the bulk and shear moduli K and $\mu$ on an oven-dry sample of Ottawa sand, at a confining pressure P_c of 51 MPa, at a reasonable reservoir temperature of about 50 C. The fluid curves (water and oil) are calculated from the dry data by means of the Gassmann fluid substitution equation in order to predict the 100% fluid-saturated bulk and shear moduli for the Ottawa sand for both hot oil and hot water as injectants. This procedure is valid for predicting low frequency velocities such as those encountered by surface seismic reflection data.

Gassmann's Equation relates dry elastic moduli to fluid-saturated elastic moduli. For bulk moduli, Gassmann (1951) found that

 

$$K_s = \frac{K_m \cdot M}{1 + M} \;,$$ (4)

where M is a normalized ratio of mineral, fluid and dry bulk moduli such that

 

$$M = \left( \frac{K_d}{K_m-K_d} + \frac{K_f}{\phi (K_m-K_f)}\right) \;.$$ (5)

Equation (4) is a rearrangement of Equation 2.77 from Bourbié et al. (1987). Concerning shear moduli, Gassmann found no distinction between dry and saturated properties:

 

$$\mu_s = \mu_d \;.$$ (6)

In the above expressions, K_m is the bulk modulus of the pure mineral skeleton, K_f is the bulk modulus of the saturating fluid, and K_d is the bulk modulus of the dry rock under study. K_s is the Gassmann theoretical bulk modulus of the dry rock after complete saturation with wetting fluid. Similarly, $\mu_s$ is the Gassmann theoretical shear modulus predicted to be the same as the dry bulk shear modulus $\mu_d$.The underlying assumptions in Gassmann's Equation are:

• the rock satisfies linear isotropic elasticity,
• the rock skeleton is comprised of one single homogeneous mineral phase,
• the wetting fluid is homogeneous and completely saturates the porosity,
• the porosity $\phi$ is uniform throughout the sample, and
• the measurements are made at low frequency (quasistatic).

Figure shows the resulting P-wave velocity versus pore pressure paths for Ottawa sand under various types of reservoir production processes. For our study, we consider the ``waterflood'' process, labeled A, A'. Figure shows that there is a net increase in V_p of about 10% as water is injected to increase effective reservoir pore pressure, and hot oil is replaced with hot water. Given the pore pressure and water saturation data from the fluid flow simulation, we use the dry data in Figure and the Gassmann relation (4) to predict the partially saturated shear and bulk moduli as a function of distance from the water injection well. Given the predicted K_s(r,t) and $\mu_s(r,t)$, we calculate the P and S low frequency seismic velocities as:

 

$$V_p^{sat} = \sqrt{ \frac{K_s+ 4\mu_s /3}{\rho_s} } \;,$$ (7)

and

 

$$V_s^{sat} = \sqrt{ \frac{\mu_s}{\rho_s} } \;,$$ (8)

where the subscript ``s'' refers to a saturated quantity.

Using this technique, we calculate pore pressure, water saturation, P-wave velocity, S-wave velocity and density, as a function of distance from the well and injection duration time, in an Ottawa sand reservoir at 50 C under two phases of waterflood injection.

 

labdata
Figure 2 Petrophysical laboratory data, and saturated rock predictions for various reservoir production processes, in Ottawa sand. The dry rock data (lower curve) are taken from Han (1986).