Pore Fluid Properties

Pore Fluid Properties

1. Fluids & Saturation

   • Rocks saturated with less compressible fluids show higher Vp & AI
   • Hence, rocks with less compressible fluids have higher Vp/Vs ratios 4-D & AVO are dependent on both fluid & rock properties
   • Fluid effect on rocks with stiff pores (hard rocks) are smaller than rocks with compliant pores (soft rocks)
   • Vs & SI are much less affected by pore fluids because fluids have no rigidity

2. Porosity, Pore Shape & Clay content

   • Rock velocity & impedance decrease with increasing porosity
   • Pore shape has greater effect on seismic properties of rock. Pore shape varies in wide ranges and is hard to quantify
   • Effect of clay content ion seisimic properties depends further on clay type & position of clay particles in rock . Main effect of clay is due to density effect

3. Pressure

   • 2 distinct pressures exist in a reservoir
     • Overburden pressure (Po)/ confining pressure : pressure exerted by the total overburden rock
     • Reservoir pressure (Pr) / fluid pressure / pore pressure: pressure exerted by the fluid mass
     • Net overburden pressure=Po-Pr. Seismic velocities and impedance increases non-linearly as net overburden pressure increases

4. Temperature

   • With increasing temperature, seismic velocities & impedance decreases slightly in gas/water-saturated rocks, but decrease significantly in consolidated sands with heavy oils

5. Lithology

   • Vs trend: Sst > Dolomite > Lst > Unconsolidated sand
   • Vp trend: Dolomite > Lst > Sst > unconsolidated sand
   • Vp/Vs trend: Lst > Dolomite > Deeply buried unconsolidated sand > Sst
   • Shales have wide ranges of velocities & impedances, but always have higher Vp/Vs ratios than reservoir sands

6. Density

   • Empirical relationships relate increasing bulk density with increasing bulk density
     • Gardner:

7. Vp versus Vs

   • Deriving Vs from available Vp: Castagna empirical relation for water-saturated clastic silicate rocks
     • Mudrock line: 1.36 + 1.16Vs (km/s)
   • Gassmann equation
   • Greenberg & Castagna : and interative model that combines Gassmann equation & Voigt-Reuss-Hill velocity average to estimate Vs in porous rocks. Required input parameters: lithology, saturation, porosity, Vs

8. Vp/Vs ratio

   • Lithology indicator
     • Vp/Vs can different shales (high) from reservoir sand, and carbonates (limestone from dolomite)
   • Direct hydrocarbon detection, especially with AVO techniques
     • Because Vs insensitive to fluid changes while Vp is, resulting in Vp/Vs changes

9. Anisotropy & Shale Properties

   • 2 types of anisotropy
     • Induced anisotropy
       • Caused by stress anisotropy & fracturing. Stress anisotropy preferentially aign pores, grains crakcs & fractres
     • Intrinsic anisotropy
       • Caused by preferential alignment of elongate grains / pores & fine lamination
       • Normally in the form of transverse isotropy (ex: shales)

10. Texture

    • Seismic velocities & impedance will be lower when there is: good grain-to-grain contact, cementation, poorly sorted sand, good sorting, well-rounded grains

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