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# Sensitivity of vertical seismic profiling for monitoring CO$$_2$$ storage in a low porosity reservoir – An example from the St-Lawrence Lowlands, Canada

Abstract

We have performed a series of rock physics measurements under various simulate confining and pore pressure and temperature states to test the seismic response of two tight reservoir samples of the sedimentary basin of the St. Lawrence Lowlands to different CO$$_2$$ phases. Results show that the seismic velocity and amplitude can be used to detect the CO$$_2$$ phase transition. Laboratory measurements were used to calibrate a stochastic geological model that was used to generate synthetic seimograms reproducing the response to CO$$_2$$ injection in the reservoir formations. The modeled CO$$_2$$ injection scenario included 15 years of injection followed by 35 years of CO$$_2$$ migration. Synthetic time-lapse seismograms were produced after 5, 15 and 50 years form the start of injection. Results show that substitution of brine by CO$$_2$$ is responsible of a time-delay in the seismic traces despite very low reservoir permeabilities and porosities. A comparison between a classical blocky model and our stochastic model shows that the blocky model leads to a misinterpretation of the CO$$_2$$ effect on the seismic response.
Keywords: CO$$_2$$, ultrasonic measurements, seismic modeling, time-lapse VSP, low porosity sandstones, CO$$_2$$ injection modeling

# Introduction

Measurements, monitoring and verification (MMV) of geological CO$$_2$$ storage are essential components for ensuring storage integrity and social acceptance of Carbon Capture and Storage (CCS). Satisfying these societal requirements is necessary to allow the deployment of the MMV technologies at a scale sufficient to reduce the rate of increase of anthropogenically produced CO$$_2$$. Also, in a carbon market context, appraisal and verification of stored CO$$_2$$ should be integrated components of CCS projects. As such, monitoring programs of CO$$_2$$ injection should ultimately allow for the quantitative estimation of CO$$_2$$ saturation throughout the reservoir and watch for any migration of carbon dioxide into surrounding geological formations. Geophysical methods are challenged in this respect, and multi-method approaches should be favored. Gravity monitoring could be helpful for mass balance estimation, especially if downhole gravimeters can be positioned close to the reservoir (Dodds et al., 2013). Electrical methods can also play an important role due to the very low sensitivity of electrical properties to pressure effects and high sensitivity to pore fluid conductivity (Schön, 2004; Schmidt-Hattenberger et al., 2013).
To date, active source seismic methods remain the principal geophysical method of all monitoring programs. Indeed, seismic methods were shown to be efficient for MMV due to their high resolution and their sensitivity to porosity and fluid saturation (White, 2013; Lumley, 2010; Lumley et al., 2010; Carcione et al., 2006). Nevertheless, ambiguity in seismic data interpretation occurs due competing pressure and saturation effects, consecutive to relatively small changes in seismic properties at mid to large CO$$_2$$ saturations. Despite these issues it is likely that active-source seismic methods will always play a central part in CO$$_2$$ monitoring programs due to their high resolution compared to other geophysical methods and consequently responsible operators will need to properly understand the seismic properties of carbon dioxide.
Since 2008, the INRS CO$$_2$$ research chair granted by the Ministère du Développement durable, de l’Environnement et des Parcs du Québec, studies the feasibility of geological storage of CO$$_2$$ in the province of Quebec, Canada. As part of the research program of the Chair, an assessement of the performance of seismic monitoring is carried out for in the particular context of the St. Lawrence Lowlands.
The Cambrian-Ordovician sedimentary basin of the St. Lawrence Platform in southern Quebec has been identified as the most prospective basin in the province of Quebec for CO$$_2$$ storage (Malo et al., 2012). The Becancour region is located on the south shore of the St. Lawrence River, midway between Montreeal and Quebec City (Fig. \ref{fig:map}). The Bécancour region’s deep saline aquifers were selected as a potential target for injection of CO$$_2$$ in a future pilot project, based on seismic reflection and well log data available from gas exploration, and based on the proximity of an industrial zone emitting up to 1 Mt of CO$$_2$$ per year. The success of the storage depends on the capability to monitor movements of the injected gas into the subsurface. As in all current CCS projects, seismic methods are an important component of the monitoring program at Bécancour. In such projects, prior estimation of elastic property changes in response to the injection of CO$$_2$$ is crucial to perform proper monitoring and subsequent interpretation of the time-lapse seismic data. There is thus a mandatory need to understand how injected CO$$_2$$ influence seismic response and how seismic methods could get a reliable quantitative estimates of injected CO$$_2$$ (White, 2013a).
Ca me plait pas The aim of this study is to better understand the dependance of the seismic properties on the porosity, mineralogy and pore fluid of the Bécancour reservoir through both laboratory measurements and numerical modeling. In this contribution, we presents first a set of laboratory measurements of the $$P$$- and $$S$$-wave velocity and amplitude made on two sandstone samples of the target reservoir, fully saturated with CO$$_2$$ at different temperatures and pressures. The results of this experiments are then used to build a geolo