[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 reservoirs pose significant challenges for reservoir modelling and flow prediction due to heterogeneities in rock properties, limits to seismic resolution and limited constraints on subsurface data. Hence, a systematic and streamlined approach is needed to construct geological models and to quickly evaluate key sensitivities in the flow models. This paper discusses results from a reservoir analogue study of a Middle Jurassic carbonate ramp in the High Atlas Mountains of Morocco that has stratigraphic and structural similarities to selected Middle East reservoirs. For this purpose, high-resolution geological models were constructed from the integration of sedimentological, diagenetic and structural studies in the area. The models are approximately 1200×1250 m in size, and only faults (no fractures) with offsets greater than 1 m are included. Novel methods have been applied to test the response of flow simulations to the presence or absence of specific geological features, including proxies for hardgrounds, stylolites, patch reefs, and mollusc banks, as a way to guide the level of detail that is suitable for modelling objectives. Our general conclusion from the study is that the continuity of any geological feature with extreme permeability (high or low) has the most significant impact on flow.

Modelling and simulation of a Jurassic carbonate ramp outcrop, Amellago, High Atlas Mountains, Morocco

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

The effect of pore structure complexity and saturation history on the variations of acoustic velocity as function of brine and oil saturation in carbonates

Our study shows that time-lapse changes in the amplitude of the seismic reflection at an oil–water contact (OWC) and/or produced OWC can be used to estimate directly the displacement efficiency of water displacing oil, E D , without the need of a rock and fluid physics model. From this value, it is possible to determine the remaining oil saturation if required. A preliminary application is performed using several published literature examples, which are reinterpreted to assess the average E D and ensure that the theory is consistent with expectations. Next, a North Sea field model with a known E D is used to create fluid-flow predictions and the corresponding synthetic time-lapse seismic data. Application to these data again confirms the basic principles of the method and defines the accuracy when applied to 4D seismic data. Finally, an observed 4D seismic dataset from a producing field in the North Sea is analysed. The results suggest a displacement efficiency of between 21 and 65% with an accuracy of 3% due to data non-repeatability (with a NRMS of between 11 and 13%). Given an average irreducible water saturation of 0.32, this calculates the remaining oil saturations at between 24 and 53% for this field. A prerequisite for use of the proposed OWC approach is that a discrete contact be interpreted on either the 3D or 4D seismic datasets. Therefore, successful application of this technique requires moderate- to high-quality seismic data and a fairly thick reservoir sequence without significant structural complexity.

Quantifying remaining oil saturation using time-lapse seismic amplitude changes at fluid contacts

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.

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

4D Seismic Monitors Fluid Changes Over Time in a Carbonate Reservoir

In preparation for a large vibroseis 3D seismic survey over the Bu Hasa oil field, 2D and 3D field tests were carried out to determine optimum source energy distribution. Several versions of the 2D and 3D data were acquired with different source geometries and processed up to stack, migration and inversion (for 3D). The 2D test showed that in this area final data quality does not depend on vibroseis sweep length (i.e. source energy). A subsequent 3D test showed that final data quality is marginally dependent on dense source arrays, but is enhanced considerably using a finer areal distribution of source points. This could be used to improve the data quality and at the same time to reduce the cost of the subsequent production 3D survey.

Erik Kleiss

Papers

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

4D Seismic Monitors Fluid Changes Over Time in a Carbonate Reservoir

The Impact of Energy Distribution On Data Quality And Cost of 3D Seismic Survey Over Bu Hasa Onshore Field In Abu Dhabi, UAE