Investigation of CO2 Storage Security Increase by Brine Alternative CO2 Injection WAG_CCS
Authors: Jazayeri Noushabadi Mahmoudreza , Brisset Anne , Thibeau Sylvain 
Published: November 2018

Abstract
Carbon Capture, Utilization and Storage (CCUS) accounts for around 14% of the cumulative emissions reductions needed through 2050 (IEA, 2016) in its 2°C scenario. Deep saline aquifers were recognized as the largest potential storage resource available worldwide for CO2 storage into geological formations. Securing the geological storage of CO2 is mandatory with this kind of project. Indeed, under specific conditions, the resulting pressure build-up of a CO2 injection into an aquifer can possibly lead to leak into shallow geological aquifers or atmosphere through preferential pathways such as geological faults and wells. The brine extraction is envisaged to decrease the reservoir pressure build-up while injecting CO2. In this study, an investigation was made to use a part of this extracted brine to increase the CO2 storage security by accelerating both residual and solubility trapping mechanisms through the deployment of water (W) alternative CO2 (G) injection (WAG_CCS) at field scale. If this alternative CO2 injection process gives interesting results, then this approach will also lead to the reduction of the duration of post-injection site monitoring. In addition, the WAG_CCS process may help increasing the sweep efficiency of CO2 by controlling the mobility ratio and consequently improving the storage capacity.
Several WAG_CCS pattern models were simulated with Eclipse software to investigate the impact of the method. A real geological model of an aquifer (Sleipner model, public data) was used for the simulations. As simulation base case, the CO2 is injected into the aquifer through one injection well for a period of 25 years followed by a 3500 years post injection simulation. Several other injection scenarios are simulated where water (W) is extracted from the same formation and partly reinjected alternatively with CO2 (G). The injection period schedules are as follow: 3months(G)-3months(W) to 1year(G)-1year(W). The mobile gas volume (structural trapping) and residual gas volume and dissolved gas volume (solubility trapping) are compared for all simulated cases.
An experimental design screening was implemented in order to investigate the impact of several parameters such as well numbers, permeability, critical gas saturation…
The results of this study gave answers to the WAG_CCS process efficiency in CO2 geological storage. It can be concluded that it can (1) be efficient under realistic geological conditions; (2) speed up the capillary trapping mechanism; (3) accelerate the dissolution trapping mechanism; (4) control the CO2 mobility and increase the sweep efficiency of CO2; and (5) help to manage project risks.
The water extraction from an aquifer during the CO2 storage is a subject which was already studied and proposed in several publications but the utilization of the extracted water is still a research subject. Extracted water desalinization, reinjection in depleted formations, surface dissolution of CO2 within the extracted water before injection are some of investigated subjects.