Acoustic velocities from core samples and sonic logs from an offshore carbonate oil field were analyzed to examine the validity of an analytical carbonate rock physics model (Xu et al., 2007) that was developed by modifying the original Xu‐White (1995) model. The model assumes idealized ellipsoidal pores of different aspect ratios (short axis/long axis of the ellipsoidal cavity). It uses three pore types to abstractly represent the acoustic and pressure response of carbonate reservoir rocks: "crack‐like" pores with a low aspect ratio, reference pores that serve as the background trend, and "stiff" pores with a high aspect ratio. We examined the validity of the model by simultaneously fitting Vp and Vs data by adjusting the volumetric fraction of the three pore types. The results showed that this simplified model could be used to fit the sonic data to within 2% over large reservoir intervals that include different carbonate rocks with porosity varying from below 10% to above 30%. The model describes the nonlinear variation of Vp/Vs ratio with porosity. Our favorable relation between inversion results and thin‐section determined petrophysical pore systems may enable us to use such inversions to predict petrophysical pore types and facies for carbonate reservoir rocks.
Vertical seismic profiling (VSP) data can complement surface seismic data where higher resolution is desired or necessary to fully understand a reservoir. VSP data typically provide greater resolution because of the inherently smaller Fresnel zone and because the bandwidth is usually wider than comparable surface seismic data. High fidelity images are required for effective reservoir monitoring. Also, special situations exist, such as attempting to image beneath salt or volcanics, where VSP data can potentially yield superior results because the seismic energy travels through the distorting overburden only once.
Carbonates are very complex, increasing the difficulty of developing rock physics models to describe geophysical responses. However, since carbonates make up such a large fraction of the hydrocarbon reserves in the world, it is important to develop such models. In this paper, we discuss a rock physics model that is applicable to clastic and carbonate rocks, and across a range of grain sizes. It incorporates known or relatively easily measurable basic parameters. We demonstrate the impact of several physical effects on geophysical responses.
New measurements of P‐ and S‐wave velocity dispersion in carbonate reservoir rocks from seismic (<100Hz) to sonic (∼10kHz) and ultrasonic (∼1MHz) frequencies were analyzed to derive the frequency‐domain intrinsic attenuation spectrum. Three rock samples were analyzed, all with porosity in the same range: one sample had high permeability and two had low permeability. We used the standard linear solid model to describe the twin relationship between velocity dispersion and attenuation. The analysis led to the following observations: (1) P‐wave attenuation (1/Qp) and S‐wave attenuation (1/Qs) are similar in each of the frequency bands(seismic, sonic, ultrasonic): 1/Qp ∼ 1/Qs; (2) The attenuation spectrum in each frequency band has an associated characteristic relaxation distance; (3) For a given carbonate reservoir rock, attenuation in the ultrasonic frequency band can be "anomalously" high (Q∼1) but still be "normal" (Q∼10–100) in the seismic frequency band.
More than four months of continuously recorded micro-earthquake data acquired at Cold Lake, Canada, was analyzed using advanced algorithms for microearthquake location and subsurface tomography. Robust determination of the spatial, temporal, and magnitude distribution of seismicity is the first step toward understanding the relationship between the stress perturbations caused by the cyclic steam stimulation (CSS) process and seismicity. Acquisition geometry was constrained because the receivers were located in a single vertical borehole. Despite this constraint, we were successful in improving event locations by use of the double-difference method, which highlights several tight event clusters. The deep cluster at a depth of [Formula: see text], just above the oil reservoir, shows very high seismicity during the CSS processes. A second cluster is observed at shallower depths in the successive steam cycle. This suggests that repeated steaming causes the deformation to spread to shallower depths. The number of events, however, decreases in the second steam cycle. Even though some of the largest events occur below the Clearwater reservoir, we observed few events in the reservoir itself, indicating that the reservoir may be an aseismic region. The size and distribution of seismicity during the first cycle agrees with a Mohr's circle analysis using simple geomechanical modeling.
Unconventional resources such as shale gas are becoming increasingly important exploration and production targets. To understand the geophysical responses of shale-gas plays, we use a rock physics relationship, which is constrained with geology and formation-evaluation analysis, to calculate effective properties such as impedance and [Formula: see text]. Numerical studies suggest that in-situ rock para-meters such as mineral composition (e.g., clay, quartz, and calcite) and TOC, as well as the interaction among them, can significantly influence the geophysical responses of the organic-rich rocks, thus providing the basis for the geophysical characterization of shale-gas plays.
Over four months of continuously recorded micro‐earthquake data acquired at Cold Lake, Canada was analyzed using advanced algorithms for micro‐earthquake location and subsurface imaging. Robust determination of the spatial, temporal and magnitude distribution of seismicity is the first step towards understanding the relationship between the stress perturbations caused by the Cyclic Steam Stimulation (CSS) process and seismicity. The acquisition geometry was constrained because the receivers were only located in a single vertical borehole. Despite this constraint, we were successful in improving event locations, which highlight several tight clusters of events. The clusters migrate to shallower depths with successive steam cycles. This shows that repeated steaming causes the deformation to spread to shallower depths, thus increasing the risk for fluid incursion in the overburden shales. Though some of the largest events occur in or below the Clearwater reservoir, we observe very few events in the reservoir itself. The size and distribution of seismicity agree to the first order with predictions from simple geomechanical modeling, but are also probably controlled by lithologic variations.
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