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Collaborating Authors
Results
Oomoldic Carbonates: Pore Structure And Fluid Effects On Sonic Velocity
Baechle, Gregor T. (ExxonMobil Upstream Research Company) | Eberli, Gregor P. (University of Miami) | Boyd, Austin (Schlumberger Doll Research Center) | DeGrange, Jean Marie (Schlumberger Doll Research Center) | Al-Kharusi, Layaan (University of Miami)
Summary In order to model the effect of oil/gas production or CO2 injection at the seismic scale, we have to understand the effects of pore structure, pressure and fluid changes on velocity at the laboratory scale. To reach this goal, we measured carbonate rocks with a suite of miscible fluids, simulating the entire range of reservoir fluid moduli from light to heavy oils. In our experiments, compressional velocity (Vp) and shear wave velocity (Vs) are simultaneously measured at a frequency of 1MHz and under increasing effective stress from 3 MPa to 30 MPa. We observe large variations in velocities between 3200 m/s and 6500 m/s and a large scatter in the P-wave velocity-porosity relationship. The P-wave velocity shows up to 2000m/s difference at a given porosity. The velocity increases between 250 and 750m/s as pressure incresases from 3 to 30MPa. The bulk of the samples show increasing Vp/Vs ratios with pressurization, up to values between 1.7 and 1.84. The ratio of normalized bulk versus shear modulus ranges from 0.7 to 0.9. Twenty-one oomoldic carbonate samples with nearly spherical pores show a weak correlation between velocity and porosity under dry conditions. We attribute the weak correlation between velocity and porosity in rocks with similar pore geometry to variations in inter-crystalline porosity in the rock frame. This finding questions the assumption that spherical pores have a dominant effect on velocity. Four oomoldic samples were chosen for fluid substitution and saturated "in-situ" with seven different pore fluids. Significant effects of fluid changes on velocity are observed. A linear correlation exists between bulk modulus and fluid modulus (r2 > 0.97). In contrast, shear modulus changes correlated with the viscosity of the fluids: the lower the fluid viscosity, the lower the shear modulus. Our results question common hypotheses for modeling pore-structure effects on acoustic properties in carbonates; (a) P-wave velocity is controlled by the percentage of spherical porosity, and (b) the P-wave velocity in oomoldic rocks is insensitive to fluid and pressure changes because of high stiffness of the rock frame. These findings imply that one has to be cautious in relating rock-physics model parameters to volumetric dominant pore types. Introduction Three main factors influence directly the elastic moduli of the rock: rock framework, pore fluid and pore space. Indirect factors, such as changes in temperature and pressure have the potential to modify the effect of the direct factors on elastic moduli. Carbonate rocks display complex pore structures with a wide range of pore sizes and pore shapes. Although the pore shape is the most significant rock property that affects the elastic property of the rock (Wang, 2001), pore shape is not easy to quantify. Anselmetti and Eberli (1993) observed a relationship between pore types and velocity, where rock samples containing moldic and intraparticle porosity have a higher velocity than samples containing micro-moldic porosity and microporosity. Oomoldic porosity is a common end member pore type in carbonate rocks, in which the porosity in the rock consists almost entirely of near spherical macropores.
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- Geology > Rock Type > Sedimentary Rock > Carbonate Rock (0.90)
- Geophysics > Seismic Surveying > Seismic Processing (0.34)
- Geophysics > Seismic Surveying > Seismic Modeling > Velocity Modeling (0.34)
- Energy > Oil & Gas > Upstream (1.00)
- Materials > Chemicals > Commodity Chemicals > Petrochemicals (0.32)
Summary Elastic moduli of water saturated sedimentary rocks have in some cases been found to be lower than what would be expected from Gassmann-substitution of moduli for rocks in the dry state, Such water weakening of elastic moduli of carbonate sedimentary rocks may be discussed by effective medium modeling. In the present case we use the isoframe model, which is based on upper Hashin- Shtrikman bounds for mixtures of a stiff carbonate frame and a suspension of carbonate particles in fluid. The proportion of carbonate in the frame is given as the iso-frame value ranging from zero to one. We model ultrasonic compressional wave and shear wave data for dry and water saturated samples from a range of geological settings. Our modeling indicates that water weakening is related to permeability. Samples with permeability up to 1 mD have relatively high water weakening possibly as a consequence of fluid interaction with the relatively frequent crystal contacts in the low-permeability samples. For samples with permeability above 100 mD we rather find a stiffening of water saturated samples. This may be a dispersion effect, as high permeability and high frequency may cause the water in the water saturated samples to move out of phase with the solid during propagation of the sonic wave and thus cause a stiffening effect. Introduction Several studies indicate that shear modulus for carbonate sedimentary rocks may be different in the water saturated state from in the dry state (e.g. Adam et al. 2006). This is in discordance with the widely used Gassmann''s equations, where shear modulus becomes independent of saturation provided the frequency in the saturated case is low enough for the pore-fluid not to move relative to the solid rock (Gassmann 1951, Berryman 1999). In this pseudostatic fluid saturated (undrained) state, the pore pressure builds up in proportion to the applied deforming stress. From studies of sandstone, Khazanehdari and Sothcott (2003) find weakening of shear modulus as well as of bulk modulus, and Assefa et al. (2003) suggest that the weakening is mainly caused by fluid-matrix interaction at grain contacts. The observed saturation effects may also be assigned to dispersion and to be controlled by fluid mobility (Batzle et al. 2006). This would indicate that in the published studies the applied frequency is high enough for the pore fluid to be out of phase with the solid. Fluid mobility is the ratio between permeability and fluid viscosity, so when we compare water saturated sedimentary rocks, fluid viscosity is constant and permeability comes into focus. Based on a large set of carbonate core data we show how the weakening of elastic moduli is related to rock texture and permeability. In order to see the effect of water also on other elastic moduli, we use the iso-frame model (Fabricius et al. 2007) to obtain stiffness measures (IF values) independently for four elastic moduli based directly on core data: Compressional wave modulus for water saturated and for dry samples, shear modulus for water saturated and for dry samples as calculated from compressional wave velocity, shear velocity and density.