Ludmila Adam

Gassmann's fluid substitution and shear modulus variability in carbonates at laboratory seismic and ultrasonic frequencies

Carbonates have become important targets for rock property research in recent years because they represent many of themajoroilandgasreservoirsintheworld.Someareundergoing enhanced oil recovery. Most laboratory studies to understand fluid and pressure effects on reservoir rocks have been performed on sandstones, but applying relations developed for sandstones to carbonates is problematic, at best.We measure in the laboratory nine carbonate samples from the same reservoir at seismic 3‐3000 Hz and ultrasonic 0.8 MHz frequencies. Samples are measured dry humidified and saturated with liquid butane and brine. Our carbonate samples showed typical changes in moduli as a function of porosity andfluid saturation. However, we explore the applicability of Gassmann’s theory on limestone and dolomite rocks in the context of shear- and bulk-modulus dispersion andGassmann’stheoryassumptions.Forourcarbonatesetat high differential pressures and seismic frequencies, the bulk modulus of rocks with high-aspect-ratio pores and dolomite mineralogy is predicted by Gassmann’s relation.We also explore in detail some of the assumptions of Gassmann’s relation,especiallyrock-framesensitivitytofluidsaturation.Our carbonate samples show rock shear-modulus change from dry to brine saturation conditions, and we investigate several rock-fluid mechanisms responsible for this change. To our knowledge, these are the first controlled laboratory experimentsoncarbonatesintheseismicfrequencyrange.

Seismic Wave Attenuation in Carbonates

[1]xa0The effect of pore fluids on seismic wave attenuation in carbonate rocks is important for interpreting remote sensing observations of carbonate reservoirs undergoing enhanced oil recovery. Here we measure the elastic moduli and attenuation in the laboratory for five carbonate samples with 20% to 30% porosity and permeability between 0.03 and 58.1 mdarcy. Contrary to most observations in sandstones, bulk compressibility losses dominate over shear wave losses for dry samples and samples fully saturated with either liquid butane or brine. This observation holds for four out of five samples at seismic (10–1000 Hz) and ultrasonic frequencies (0.8 MHz) and reservoir pressures. Attenuation modeled from the modulus data using Cole-Cole relations agrees in that the bulk losses are greater than the shear losses. On average, attenuation increases by 250% when brine substitutes a light hydrocarbon in these carbonate rocks. For some of our samples, attenuation is frequency-dependent, but in the typical exploration frequency range (10–100 Hz), attenuation is practically constant for the measured samples.

Elastic properties of carbonates from laboratory measurements at seismic and ultrasonic frequencies

Rocks saturated with a fluid can be described as viscoelastic materials. The velocity and elastic moduli of viscoelastic materials increase with frequency. Therefore, the elastic rock properties that we measure at high frequencies might not resemble the observations at lower frequencies. Laboratory measurements of velocity and elastic moduli are mostly performed at frequencies higher than those of surface seismic data, but with the stress-strain experimental procedure the moduli and velocity of laboratory samples can be measured at seismic frequencies.

Seismic Imaging And Anisotropy Analysis of 9C 3D-VSP Data At Weyburn Field, Saskatchewan, Canada

Three-dimensional nine-component vertical seismic profiles (VSP) were acquired during 9C 3D surface surveys as part of a CO2 monitoring project at Weyburn Field, Saskatchewan. The 9C 3D-VSP surveys were designed to provide waveform control, to estimate the anisotropic parameters of the medium and to produce a low-fold image of the reservoir. A polarization analysis of direct P-wave arrivals revealed a borehole deviation of 40m at a depth of 1397m and an apparent amplitude balancing problem between horizontal components of some receiver levels. An analysis of direct S-wave arrivals from orthogonal shear sources suggests a dominant periodic thinlayer anisotropy (VTI) combined with extensive-dilatancy anisotropy with vertical fractures oriented N-S. An image of the reservoir and Mississippian unconformity were produced using a two-component VSP-Kirchhoff migration of the upgoing P-wave data. The reservoir is observed up to approximately 200m away from the borehole. The 3DVSP and the surface 3D migrated images have a similar resolution at the reservoir. Neither of the images allow resolution of the two reservoir units.

CO2 sequestration in basalt: Carbonate mineralization and fluid substitution

Geological sequestration of carbon dioxide in deep reservoirs may provide a large-scale option for reducing the emissions of this gas into the atmosphere. The effectiveness of sequestration depends on the storage capacity and stability of the reservoir and risk of leakage into the overburden. Reservoir rocks can react with a CO2-water mixture, potentially resulting in the precipitation of minerals in the available matrix pore space and within pre-existing fractures. This induced mineralization may form internal seals that could help mitigate the leakage of CO2 into the overburden. For basaltic host rocks, carbonic acid partially dissolves minerals in the host rock, such as the calcium plagioclase mineral, freeing various cations (e.g., Ca2+ and Mg2+) for later precipitation as carbonate cements (Gislason et al., 2010).

Intrinsic P- And S-wave Attenuation of Carbonate Reservoir Rocks From Seismic, Sonic, to Ultrasonic Frequencies

New measurements of Pand 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.

Multi‐level 3D VSP travel time inversion in VTI media, Weyburn Field, Canada

A 12-level 3D vertical seismic profile (VSP) is acquired as part of Weyburn Project with of Reservoir Characterization Project (RCP) at Colorado School of Mines. Travel times from direct arrivals of compressional sources are used to invert for Thomsen’s parameters for a transversely isotropic with a vertical symmetry axis (VTI) media. Near offsets are used to estimate the vertical travel time and normal moveout (NMO) velocity, while the non-hyperbolic moveout of all offsets are fitted to estimate the horizontal velocity. and δ parameters are estimated from these velocities. A sensitivity and error analysis on the parameters that control the non-hyperbolic moveout equation is performed.

Moduli Dispersion And Attenuation In Limestones In the Laboratory

There is growing interest in using seismic attenuation for reservoir characterization. We measure limestone samples in the laboratory to analyze attenuation and elastic moduli as a function of frequency from 3 Hz to ultrasonic. We observe that there is a positive correlation between bulk modulus dispersion and permeability. We also measure different modes of attenuation and find that the attenuation in our fullysaturated samples is mostly due to bulk compressibility losses. We observe that attenuation significantly increases when brine replaces a light hydrocarbon.