Are seismic velocity time-lapse changes due to fluid substitution or matrix dissolution? A CO2 sequestration study at Pohokura Field, New Zealand

Abstract:

Time-lapse seismic signatures can be used to quantify fluid saturation and pressure changes in a reservoir undergoing CO2 sequestration. However, the injection of CO₂ acidifies the water, which may dissolve and/or precipitate minerals. Understanding the impact on the rock frame from field seismic time-lapse changes remains an outstanding challenge. Here, we study the effects of carbonate-CO₂-water reactions on the physical and elastic properties of rock samples with variable volumes of carbonate cementation. The effects of fluid substitution alone (brine to CO₂) and those due to the combination of fluid substitution and mineral dissolution on time-lapse seismic signatures are studied by combining laboratory data, geophysical well-log data and 1-D seismic modeling.

Nine rocks from Pohokura Field (New Zealand) are reacted with carbonic acid. The elastic properties are measured using a high-density laser-ultrasonic setup. We observe that P-wave velocity changes up to -19% and correlate with sandstone grain size. Coarse-grained sandstones show greater changes in elastic wave velocities due to dissolution than fine-grained sandstones. To put this in perspective, this velocity change is comparable to the effect of fluid substitution from brine to CO₂. This can potentially create an ambiguity in the interpretation of the physical processes responsible for time-lapse signatures in a CO₂injection scenario.

The laboratory information is applied onto well-log data to model changes in elastic properties of sandstones at the well-log scale. Well-logs and core petrographic analyses are used to find an elastic model that best describes the observed elastic waves velocities in the cemented reservoir sandstones. The Constant-cement rock physics model is found to predict the elastic behaviour of the cemented sandstones. A possible late-time sequestration scenario is that both mineral dissolution and fluid substitution occur in the reservoir. 1-D synthetic seismograms show that seismic amplitudes can change up to 126% in such a scenario. Our work shows that it is important to consider that time-lapse seismic signatures in carbonate-cemented reservoirs can result not only from fluid and pressure changes but also potentially from chemical reaction between CO₂-water mixtures and carbonate cemented sandstones.