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This study aims to investigate the feasibility of CO2-EOR monitoring by full waveform inversion (FWI) of time-lapse VSP data in an onshore CO2-EOR site in Abu Dhabi. CO2-EOR monitoring using conventional time-lapse surface seismic in onshore oil fields in Abu Dhabi is often technically challenging for two main reasons. The first is that elastic property change in response to pore fluid substitution is relatively small because the elastic modulus of the reservoir rock frame is far larger than that of the pore fluids. The second is the low repeatability of time-lapse survey data due to high amplitude surface-related noise which varies temporally. However, seismic monitoring with FWI of time-lapse borehole seismic data may offer a solution for these issues. FWI is capable of detecting small velocity changes such as those associated with pore fluid substitution. Furthermore, borehole seismic surveys may provide more highly repeatable, higher quality data compared to surface seismic surveys because borehole seismic data is less affected by surface-related noise. This study consists of two parts, a field data analysis and a synthetic study. In the field data analysis, we studied the resolution and repeatability of FWI results at field-data quality, including the presence of actual noise using time-lapse VSP data. VSP data was acquired at the very early stage of EOR and there was no CO2 injection in the time between the two time-lapse VSP surveys. As a result, a high-resolution P-wave velocity model, consistent with a sonic log, was obtained. The P-wave velocity model also revealed excellent repeatability between the two survey data sets. In the synthetic study, time-lapse FWI was performed using synthetic VSP data representing pre- and post- CO2 injection periods. The results of the synthetic study showed that even in the presence of realistic 4D noise, which was estimated in the field data analysis, FWI successfully delineated the distribution of velocity changes caused by CO2 injection when the cross-sectional area of the injection-induced velocity changes were larger than the resolution of the FWI results. With these results, we demonstrated that FWI using time-lapse VSP data was applicable for CO2-EOR monitoring in the field as long as the criteria were met. This conclusion encourages the application of FWI using time-lapse VSP data for CO2-EOR monitoring in onshore Abu Dhabi.
This study aims to investigate the feasibility of CO2-EOR monitoring by full waveform inversion (FWI) of time-lapse VSP data in an onshore CO2-EOR site in Abu Dhabi. CO2-EOR monitoring using conventional time-lapse surface seismic in onshore oil fields in Abu Dhabi is often technically challenging for two main reasons. The first is that elastic property change in response to pore fluid substitution is relatively small because the elastic modulus of the reservoir rock frame is far larger than that of the pore fluids. The second is the low repeatability of time-lapse survey data due to high amplitude surface-related noise which varies temporally. However, seismic monitoring with FWI of time-lapse borehole seismic data may offer a solution for these issues. FWI is capable of detecting small velocity changes such as those associated with pore fluid substitution. Furthermore, borehole seismic surveys may provide more highly repeatable, higher quality data compared to surface seismic surveys because borehole seismic data is less affected by surface-related noise. This study consists of two parts, a field data analysis and a synthetic study. In the field data analysis, we studied the resolution and repeatability of FWI results at field-data quality, including the presence of actual noise using time-lapse VSP data. VSP data was acquired at the very early stage of EOR and there was no CO2 injection in the time between the two time-lapse VSP surveys. As a result, a high-resolution P-wave velocity model, consistent with a sonic log, was obtained. The P-wave velocity model also revealed excellent repeatability between the two survey data sets. In the synthetic study, time-lapse FWI was performed using synthetic VSP data representing pre- and post- CO2 injection periods. The results of the synthetic study showed that even in the presence of realistic 4D noise, which was estimated in the field data analysis, FWI successfully delineated the distribution of velocity changes caused by CO2 injection when the cross-sectional area of the injection-induced velocity changes were larger than the resolution of the FWI results. With these results, we demonstrated that FWI using time-lapse VSP data was applicable for CO2-EOR monitoring in the field as long as the criteria were met. This conclusion encourages the application of FWI using time-lapse VSP data for CO2-EOR monitoring in onshore Abu Dhabi.
Time-lapse seismic plays a vital role in monitoring the dynamic behavior of reservoirs during carbon dioxide enhanced oil recovery (CO2-EOR). However, its application to carbonate fields onshore Abu Dhabi presents several challenges, necessitating appropriate datasets and analysis tools. For proper understanding of fluid behaviors in a carbonate field onshore Abu Dhabi, the time-lapse vertical seismic profiling (VSP) datasets were acquired with an optimum signal-to-noise ratio. Firstly, pre-processing was performed on baseline and monitoring surveys, resulting in improvement of data quality and repeatability in a quantitative manner. We then applied full waveform inversion (FWI) to estimate time-lapse velocity changes induced by CO2-EOR. After the parameter optimization, the inverted velocity showed updates in the correct direction while minimizing the data residual. To stabilize and optimize the time-lapse FWI application, three different approaches were performed: a parallel approach, a common model approach, and a target-oriented approach. As a result, the time-lapse velocity changes near the injection well were clearly recognizable in the reservoir of interest. This has suggested that time-lapse property changes associated with CO2-EOR operation can be captured through time-lapse FWI. Therefore, time-lapse borehole seismic imaging in conjunction with the applied FWI scheme can be an effective tool for precise monitoring of fluid behavior in a carbonate reservoir during CO2-EOR operations.
When implementing CO2 WAG EOR, fluid distribution and pore pressure in the reservoir vary. The interpretation of seismic reservoir monitoring for WAG-EOR often includes uncertainty in the changes obtained by time-lapse seismic surveys because these changes are the integration of effects by several factors. The objective of this study is to estimate the impact of each factor on elastic property and investigate a dominant factor through a rock physics study at a CO2 WAG site in Abu Dhabi. To estimate the effect of pore-pressure change on elastic velocity, an empirical relation between elastic velocity and confining pressure was used. Sonic velocity using core plug samples of the reservoir was measured under multiple confining pressures to construct empirical relations. As for the effect of fluid saturation changes on elastic velocity, we estimated it through Gassmann's equation. Realistic changes in pore pressure and saturations during the WAG operations were based on reservoir simulation results. Finally, based on the input petrophysical data and the relations from rock physics analyses, elastic property changes in WAG-EOR are estimated. Density and P-wave velocity (Vp) changes caused by the fluid substitution from oil to water are more significant than those by the substitution from oil to CO2. Pore pressure changes have less of an impact on Vp variations than does fluid substitution. Based on the estimation of elastic property changes associated with WAG operations, density changes are affected mainly by the increase in water saturation because of the density difference between water, oil and CO2. Vp changes seem to be consistent with the increase of gas (CO2) saturation. Even in the upper zone of the reservoir where oil is swept by both injected water and CO2, the effect of CO2 is prominent as well-known low gas saturation effects. Vp is significantly affected by the small amount of CO2 which was observed in the comparison of Vp with and without 10% of CO2 saturation in the rock. P-impedance changes are consistent with Vp changes because of the larger changes of Vp than the density changes. From these observations, the changes obtained by seismic reservoir monitoring would mainly represent the effect of CO2 saturation changes. We quantitatively looked into the impact that changes in pore pressure and saturation had changes on elastic properties such as density and elastic velocity. Furthermore, it was determined which elements predominately control changes in elastic properties when these changes occur simultaneously in WAG-EOR operations. These findings play an important role in the interpretation of seismic reservoir monitoring for WAG-EOR in Abu Dhabi.
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