[1] The 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.

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Seismic Wave Attenuation in Carbonates

Carbonate reservoir rocks of different pore structure have quite distinct and detectable 4D seismic signatures. Using three defined pore structure types (PST), these seismic signatures are correlated with the permeability patterns of carbonate rocks. In both the high-permeability PST3 and intermediate-permeability PST2 regions, gas injection is preferred than water injection for effective 4D seismic monitoring. In the low-permeability PST1 region where lies most of the bypassed oil, both gas and water injection options result in similar 4D seismic signal magnitude. Fluid saturation changes under in-situ reservoir conditions can be better detected by offset domain 4D seismic analysis.

Effects of pore structure on 4D seismic signals in carbonate reservoirs

Seismic tomography has been developed and applied for decades in seismological applications and for basic research purposes. During the last decade, large-scale applications in the oil and gas industry became standard as tomostatics and velocity modelling for pre-stack depth migration. In this paper, I take a snapshot of some current industrial applications, quantifying practical aspects by yardsticks such as data and model size and I try to draw a road map for the current decade.

Yardsticks for industrial tomography

VSP results successfully demonstrate that high resolution VSP data can be collected and are repeatable in the Abu Dhabi carbonate environment. Comparisons between shallow marine air gun and land vibrator VSP images show similar quality results, suggesting that land and marine surveys can potentially be combined in transition areas. Walkaway VSP repeatability tests with a marine air gun and land vibrator show that VSP images can be repeated with significantly lower background noise compared to surface seismic. Background noise is the non-repeatable signal that remains after subtracting two VSP images collected at different times, when no changes in the subsurface have occurred. With fixed geophones and an optimized 4D VSP processing flow background noise levels below 1% of the original VSP amplitudes were achieved. Both reflected p-wave signals and converted shear wave signals were found to be repeatable. The positive results from this VSP repeatability pilot and a 4D feasibility study indicate that time-lapse 3D-VSP’s could provide another means for monitoring reservoir saturation changes in Abu Dhabi Carbonate fields.

Time-Lapse VSP Test Results Demonstrate Good Repeatability For Both Land-Vibrator And Marine-Air Gun VSP Surveys In Middle East Carbonates

This thesis discusses the impacts of fabric heterogeneity, fluids and fluid saturations, effective pressures, and frequency of investigation on the elastic and viscoelastic properties of calcite-rich limestone and chalk formations. Carbonate reservoirs have been analyzed either with empirical relations and analogs from siliciclastic reservoirs or using simplistic models. However, under the varying parameters mentioned above, their seismic response can be very different. The primary reason is because these rocks of biochemical origins readily undergo textural changes and support heterogeneous distribution of fluid flow and elastic properties. Thus, many current rock physics models are unable to predict the time-lapse elastic response in these reservoirs. I have measured elastic properties of calcite rich rocks in the seismic frequency range of 2 to 2000 Hz and at the ultrasonic frequency of 800 kHz. The samples selected for this study represent the typical heterogeneities found in carbonate formations. These measurements covering a large frequency range provide an understanding of the dispersion and attenuation mechanisms during seismic wave propagation in the subsurface. I find that a heterogeneous formation shows significant velocity dispersion and attenuations when saturated with brine, and even more on saturation with CO2. I also show that the shear modulus of carbonate rocks changes significantly (from 8% for brine saturation to 70% for CO2 saturation) upon fluid saturation with polar fluids. I evaluated rock physics models, such as Gassmann’s and with uniform and patchy fluid substitution, and Hashin-Shtrikman to predict saturated elastic properties in carbonates. Fluid sensitivity is directly related to the initial stiffness of the rock instead of porosity, as normally assumed. The Gassmann model can predict elastic properties for uniform saturations - mostly in homogenous rocks. Heterogeneous rocks, however, are better modeled using a patchy fluid saturation model. The results of this study provide valuable information on modeling the elastic response of saturated carbonate rocks currently lacking in fluid substitution models. It also provides data for reservoir simulation models to incorporate heterogeneity effects for realistic rock property variation to honor the textural complexities in carbonate reservoirs instead of using the simplified Gassmann model.

Impact of Texture Heterogeneity on Elastic and Viscoelastic Properties of Carbonates

Preliminary results from a time-lapse 4D pilot study shows that saturation changes over time can be monitored in an onshore carbonate reservoir with the 4D surface seismic method. The 4D results were successful in showing that 4D responses can be observed in reservoir zones where sufficient saturation changes had occurred. The results of the 4D pilot showed that 3D seismic surveys can be repeated onshore with acceptable background noise levels, if done in the correct manner. Model predictions were found to be in agreement with the 4D results. 4D responses were found in the main reservoir layer where saturation changes are occurring and not in the water leg where saturation changes have not occurred. The 4D responses are in good agreement with available well control by greater than 80 percent. Detailed 4D validation showed the 4D results to match available production logging tool (PLT) measurements surprisingly well. The results also suggest that pressure changes are contributing to some of the observed 4D responses. Based on the results of this pilot, it has been shown that 4D can monitor saturation changes in time in this carbonate reservoir. The 4D results are helping to assess sweep efficiency and identify potential bypassed oil reserves.

Abi Modavi

Papers

Successful Pilot Onshore Abu Dhabi Shows That 4D Can Monitor Fluid Changes In a Giant Middle East Carbonate Field