Time-Lapse Seismic Interpretation In CO₂-EOR And Storage

With the continuous improvement of social industry,the production of conventional oil and gas reservoirs could not meet its demands.And it is difficult and costly to develop the unconventional oil and gas.The tertiary oil recovery and exploiting potentialities of old oil reservoir has become the key to stable production and increase production.As the greenhouse gas,CO₂ has been proven to be the most excellent displacement agent in large scale tertiary oil recovery.Injecting CO₂ into the microscopic pores into underground rock can be safely stored over thousands of years,which makes CCUS?Carbon Capture,Utilization,and Storage?technology the most effective way to reduce carbon emission and rejuvenate old even abandoned fields.Time-lapse seismic monitoring is a valid method to monitor the effciency of CO₂-EOR,sweep range,CO₂ distribution range underground and storage safety.The prediction of the pressure and fluid saturation changes after CO₂ injection into the reservoir is the key to quantitatively determine the oil displacement effect and storage safety.However,the results are not well when the conventional seismic attributes are applied to recognize reservoir and fluid characteristics.To complement the deficiencies,the seismic texture analysis based on the gray level co-occurrence matrix is introduced in this paper,and applied to analyze the time-lapse seismic data of the CO₂-EOR and storage area from SINOPEC Shengli Oilfield,China.The CO₂ migration pathways,directions,and distribution underground have been identified after calculating the texture of the time-lapse seismic volume,comparing,selecting the texture energy from 14 texture attributes and acquiring the difference.The key point of time-lapse seismic texture analysis is the selection of texture computation parameters and the consistency of the result.The same parameters are used to calculate twice seismic texture of caprock and reservoir,and we minimized the caprock texture difference of time-lapse data,acquired a correction factor which also applied to the reservoir and analyzed the CO₂ migration and distribution according to the reservoir correction.Case studies indicate that reservoir texture after calibration has less interference information than the background,higher contrast between the main faults and background,higher resolution,the visuality of reservoir characteristics has been enhanced obviously,and CO₂ migration pathways,swept range and distribution are more clearly.The main faults of the reservoir,the migration pathways and distribution of CO₂ are clearly displayed which is better to explain the increased or stable production of oil wells in the study area,especially the rapid increase in production wells far away from CO₂ injection wells.After the qualitative analysis of CO₂ occurrence underground,we improved Landr?formula,using Landr?'s petrophysical model to the Smith&Gidlow AVO simplified equation,and established a petrophysical model and a two-layer model for fluid saturation and pressure changes during the injection of CO₂ into reservoir.Through the forward and inversion of the two-layer model,Smith&Gidlow simplified equation with less error in inversion has been selected.The gradient and intercept of caprock and reservoir have been acquired from pre-stack time-lapse seismic data and corrected by caprock consistency principle.Combining petrophysics model,Smith&Gidlow simplified equation,gradients and intercepts,the effective pressure change caused by CO₂ injection can be inverted.Case indicates that the pore pressure around the injection well has been replenished due to the CO₂ injection,and combined with the production wells production change.It is concluded that the variation of production well output in Gao89 area is closely related to the variation of effective pressure after CO₂ injection into reservoir.The accuracy of time-lapse seismic texture attribute analysis also has proved the inversion results.