A rock physics modelling approach for time-lapse monitoring and characterization of fluid–rock interactions in hydrocarbon reservoirs

Abstract

One of the research gaps is to understand the development of seismic characteristics of gas-saturated rock along with the change in rock properties because of chemical reactions. We suggest a method to explain the change in elastic properties brought on by CO₂ injection in a rock by capturing the physico-chemical interactions observed in the laboratory in a theory of rock physics. To explain the laboratory-measured physical characteristics and velocity of a dynamic rock–fluid system, we include a time-dependent component in the existing cemented-sand model. We demonstrate theoretically the rate of change of elastic moduli of the dry frame by incorporating the measured rate of change of cement due to chemical dissolution. We adapt the theory such that it can be applied to the field data and calibrate the theory using water-saturated well log data from the Ankleshwar field, an established oil field in the Cambay basin, western India. Theoretical time-lapse logs of velocity and density are then produced using the theory over a range of CO₂ saturations, assuming cementing material in grain contacts and geochemical interactions comparable to those observed in the laboratory rock. Then, using theoretical logs, corresponding time-lapse synthetic seismic data are produced for different saturation. These data clearly demonstrate that, for a uniform model, velocity decreases by up to 18% as CO₂ saturation increases from 0% to 20% (ignoring the chemical effect), and that, for a specific saturation, say 20%, chemical effects result in a 17% decrease in velocity from the present to the end of 60 years. However, for the patchy model, velocity decreases maximum by 14% and 16% due to varying saturation and chemical reaction. Moreover, for a particular saturation of CO₂, say 20%, velocity differs by 16% for different types of models. This research contributes to making strategy for CO₂-sequestration in a designated field.