Carbonate rocks commonly contain a variety of pore types that can vary in size over several orders of magnitude. Traditional pore-type classifications describe these pore structures but are inadequate for correlations to the rock's physical properties. We introduce a digital image analysis (DIA) method that produces quantitative pore-space parameters, which can be linked to physical properties in carbonates, in particular sonic velocity and permeability. The DIA parameters, derived from thin sections, capture two-dimensional pore size (DomSize), roundness (), aspect ratio (AR), and pore network complexity (PoA). Comparing these DIA parameters to porosity, permeability, and P-wave velocity shows that, in addition to porosity, the combined effect of microporosity, the pore network complexity, and pore size of the macropores is most influential for the acoustic behavior. Combining these parameters with porosity improves the coefficient of determination (R2) velocity estimates from 0.542 to 0.840. The analysis shows that samples with large simple pores and a small amount of microporosity display higher acoustic velocity at a given porosity than samples with small, complicated pores. Estimates of permeability from porosity alone are very ineffective (R2 = 0.143) but can be improved when pore geometry information PoA (R2 = 0.415) and DomSize (R2 = 0.383) are incorporated. Furthermore, results from the correlation of DIA parameters to acoustic data reveal that (1) intergrain and/or intercrystalline and separate-vug porosity cannot always be separated using sonic logs, (2) P-wave velocity is not solely controlled by the percentage of spherical porosity, and (3) quantitative pore geometry characteristics can be estimated from acoustic data and used to improve permeability estimates.

Quantification of pore structure and its effect on sonic velocity and permeability in carbonates

Multicomponent seismic recording (measurement with vertical‐ and horizontal‐component geophones and possibly a hydrophone or microphone) captures the seismic wavefield more completely than conventional single‐element techniques. In the last several years, multicomponent surveying has developed rapidly, allowing creation of converted‐wave or P‐S images. These make use of downgoing P‐waves that convert on reflection at their deepest point of penetration to upcoming S‐waves. Survey design for acquiring P‐S data is similar to that for P‐waves, but must take into account subsurface VP/VS values and the asymmetric P‐S ray path. P‐S surveys use conventional sources, but require several times more recording channels per receiving location. Some special processes for P‐S analysis include anisotropic rotations, S‐wave receiver statics, asymmetric and anisotropic binning, nonhyperbolic velocity analysis and NMO correction, P‐S to P‐P time transformation, P‐S dip moveout, prestack migration with two velocities and wa...

Converted-wave seismic exploration: Methods

[1] A comprehensive suite of geophysical logs was collected in the SAFOD Pilot Hole from a depth of 775 m to 2150 m in highly fractured Salinian granite. The Pilot Hole intersected numerous macroscopic fractures and faults with extremely varied orientations. Despite the highly variable orientation of the fractures and faults, the fast polarization direction of the shear waves is very consistent with the direction of maximum horizontal compression determined from wellbore breakouts and drilling induced tensile fractures. At least three major shear zones were intersected by the borehole that are characterized by anomalously low velocity and resistivity, anomalously high shear velocity anisotropy and an absence of stress-induced wellbore breakouts (which suggests anomalously low differential stress). We argue that the physical mechanism responsible for the seismic velocity anisotropy observed in the Pilot Hole is the preferential closure of fractures in response to an anisotropic stress state.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2003GL019020

Stress-Induced Seismic Velocity Anisotropy And Physical Properties In The Safod Pilot Hole In Parkfield, Ca.

Parametric inversion is used to advantage in a technique and apparatus for borehole logging to determine properties of anisotropic formations, and a dispersion function that varies with frequency is used in the modeling of the formations. An embodiment of the method includes the following steps: providing a logging device that is moveable through the borehole; exciting a sonic source at a transmitter location on the logging device to establish flexural waves in the surrounding formations; measuring at each of a plurality of receiver locations on the logging device, which are spaced at a respective plurality of distances from the transmitter location, orthogonal wave components of split flexural waves that have travelled through the formations; computing, for each of the plurality of distances and for multiple frequencies, model orthogonal wave components which would result from the superposition of model split-flexural waves having selected wave parameters including respective fast and slow model slownesses which are variable functions of frequency and model polarizations; determining an error value which depends on the differences, at each of the plurality of receiver locations, between measured wave components and the model composite waves; modifying the model parameters; iteratively repeating the computing, determining, and modifying steps to reduce the error; and storing the ultimately modified model parameters as being indicative of properties of the formations.

Dipole shear anisotropy logging

We show both experimentally and theoretically how stress concentrations affect the velocity field around a borehole, and how the velocity field influences dipole anisotropy measurements. At low frequencies the dipole mode is sensitive to the far-field stresses primarily, so standard sonic log interpretation correctly yields the direction of maximum stress. At higher frequencies, the dipole mode is sensitive to near-field stress concentrations such that the fast polarization direction is aligned with the direction of minimum tectonic stress. These effects combine to produce a crossover in the dipole dispersion curves measured in the fast and slow directions. With broad-band dipole data, the dispersion crossover can be used as an indicator of stress-induced anisotropy dominating over weak intrinsic anisotropy.

Effects of borehole stress concentrations on dipole anisotropy measurements

Crossed dipole acoustic logs represent a new class of measurement of subsurface formations which can provide detailed information on anisotropy not previously available. As with any new class of data, departures from previous conceptions may occur. In this case, the detail inherent in sonic-scale measurements provide information on anisotropy at sub-formation scale whereas seismic investigations of anisotropy are normally restricted to sequence-scale. When borehole conditions allow formation properties to be measured, the tool provides detailed information interpretable in terms of fracturing. Crossed dipole acoustic logs were acquired using the multisource/multireceiver technique, and Alford rotation (Alford, 1986; Thomsen, 1988 and Esmersoy, 1994) was employed to resolve observed flexural or shear-wave mode splitting. The data show indications of sensitivity to natural fracturing and, secondarily, to borehole ovality. The azimuth of the leading shear-wave polarization, qS1, compare favorably with independent data on fracture azimuth. Thomsen anisotropy parameter {gamma} as high as 10--20% is observed in certain anisotropic intervals. Such intensities, and their spatial occurrence on log data, are new information on anisotropy.

Case Studies of the Dipole Shear Anisotropy Log

Recent Progress On Formation Resistivity Measurement Through Casing

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 (r 2 > 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.

Oomoldic carbonates: Pore structure and fluid effects on sonic velocity

Forward Modeling of Fracture-induced Sonic Anisotropy Using a Combination of Bore-hole Image And Sonic Logs

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Case Studies of the Dipole Shear Anisotropy Log

Recent Progress On Formation Resistivity Measurement Through Casing

Recent Progress On Formation Resistivity Measurement Through Casing

Oomoldic carbonates: Pore structure and fluid effects on sonic velocity

Forward Modeling of Fracture-induced Sonic Anisotropy Using a Combination of Bore-hole Image And Sonic Logs