Are changes in time‐lapse seismic data due to fluid substitution or rock dissolution? A CO₂ sequestration feasibility study at the Pohokura Field, New Zealand

Abstract

Seismic methods are commonly used to monitor the subsurface when carbon dioxide (CO2) is injected into a reservoir. Besides fluid saturation and pressure changes, CO2–water mixtures may cause rock alteration. In this petrophysical study, we compare the elastic property changes due to fluid replacement and those due to mineral dissolution for carbonate‐cemented sandstones at the Pohokura Field, New Zealand. We quantify the effects of fluid substitution from fully brine to fully supercritical CO2 saturation and carbonate cement dissolution on the seismic signatures of the reservoir rocks by combining laboratory results, petrographic analyses, and geophysical well log data. We conclude that elastic property changes due to mineral dissolution are significantly greater than those due to fluid substitution alone. The northern part of the Pohokura Field has coarser‐grained sandstones, which experience the largest changes in wave speeds. Our hypothesis is that these changes result from carbonate cement dissolution in the presence of CO2–water–sandstone reactions. If time‐lapse seismic data were to be acquired in this field, the northern area could show P‐wave velocity reductions of up to 20% and a 131.7% increase in seismic amplitude from a brine‐saturated rock to an altered, fully CO2‐saturated rock. In comparison, the southern part of the field, where sandstones are mostly fine‐grained, we expect a P‐wave velocity decrease of 6% if such dissolution process took place. Finally, we show that the elastic properties of the reservoir rocks can be described with the constant‐cement model. The model is used to predict that the dissolution process reduces the volume of grain contact cement, on average, from 2.5% to 1.75% of the total rock mineral volume. Our analysis suggests that changes to the rock frame, which includes carbonate minerals, cannot be ignored for a CO2 injection scenario.

Chemo‐mechanical Alterations Induced From CO2 Injection in Carbonate‐Cemented Sandstone: An Experimental Study at 71 °C and 29 MPa

“…Combined with image analysis (scanning electron microscope and thin section), previous studies have shown that the magnitude of these changes is strongly dependent on the initial pore structure and the microstructural relationship between cement and framework grains by affecting the reactive surface area under similar reservoir conditions (Hangx et al, ; Rinehart, Dewers, et al, ; Vanorio et al, ; Vialle & Vanorio, ). Furthermore, modeling suggests that the underlying mechanism of mechanical alteration is due to the volume reduction of grain contact cement (Sim & Adam, ; Zheng et al, ).…”

Dissolution and secondary mineral precipitation in basalts due to reactions with carbonic acid

“…A core extracted 2 years post‐injection showed evidence of secondary carbonate mineral precipitation in the form of ankerite throughout the rock vesicles [ McGrail et al , ]. Reservoir characterization and monitoring of the CO 2 plume in the field is required to understand changes in the rock physical properties due to rock‐fluid interactions [ Sullivan et al , ; Khatiwada et al , ; Sim and Adam , ].…”

Imaging CO2$_2$ reinjection into basalts at the CarbFix2 reinjection reservoir (Hellisheiði, Iceland) with body‐wave seismic interferometry

“…These surveys often rely on the assumption that the velocity change is only caused by fluid substitution. However, CO$$_2$$ can react with or dissolve the host rock, causing changes in the seismic velocity of the bulk rock (Lumley, 2010; Sim & Adam, 2016).…”