Simulations for CO2 injection projects with Compositional Simulator

Abstract

Abstract The need for CO2 emissions reduction at a large scale globally implies that CO2 injection into the subsurface be undertaken in a greater variety of geological environments that has been the case previously. Often when the storage reservoirs are saline aquifers, exploration data for proposed injection sites are extremely sparse. The special behaviour of CO2-water/brine systems (mutual solubility and chemical reactivity) adds complex processes, such as dry-out, salting-out, chemical reactions to the dynamic model. Simulation in these situations is one of few means of assessing an injection site and testing various scenarios. The accurate description of physics and chemistry in numerical simulation tools is fundamental for understanding processes, as well as designing appropriate injection or mitigation strategies. We present simulations of CO2 injection into saline aquifers with a fully compositional code that has been expanded and enhanced to include specific phenomena, such as drying-out and salting-out. The examples illustrate the importance of pre-injection studies, as the wrong injection strategy may severely impact injectivity, putting the project in jeopardy. Introduction Selection, planning, construction and management of CO2 storage sites are still in its infancy. Only a few industrial scale projects are operating over the last 5 years, such as Sleipner (Norway), In Salah (Algeria) and Weyburn (Canada)1,2,3. New challenges exist also for CO2 Enhanced Oil Recovery projects. Although decades of experience exist in a few specific basins and geological environments, the need for CO2 emissions reduction at a large scale globally implies that CO2 injection into the subsurface be undertaken in new environments (offshore and in a greater variety of basin settings) and at volumes much larger than have ever been injected into the subsurface. In this paper, we focus on injection into saline aquifers, al-though the discussion and results presented herein may be useful for certain aspects of CO2-EOR. Special properties of CO2 (or gas mixtures in which it is the dominant component) enhance several processes when compared to injection or production of hydrocarbon gases. This is not only caused by the specific values CO2 properties exhibit such as density, viscosity, heat capacity, interfacial tension among others, but especially the strongly non-linear variation of these parameters with temperature and pressure4.For instance, in the range of 80 to 90 bar (at 35°C) the density and viscosity of CO2 increases by a factor greater than 2, while for methane, the change is of the order of a few percent. The mutual solubility between CO2 and brine affects the injection process and flow properties in three ways:CO2 dissolves in the brine increasing its density,CO2 dissolves in brine and reacts with water forming an acid and,H2O dissolves or vaporizes into CO2, removing water from the brine and increasing its salinity as salt concentration increases, leading to dry-out and salting-out. In this article, we first discuss in more detail the impact of mutual solubility for CO2 storage in saline aquifers. Numerical implementation of these concepts is presented. Simulations for a pre-injection study for a CO2 injection illustrate aspects that are needed for designing monitoring and injection strategies. Injection of CO2 in Saline Aquifers 'Saline aquifer' is understood here as a brine reservoir or geological formation with reservoir characteristics (porosity and permeability) with pores filled with brine. The term 'saline' expresses that CO2 storage is planned in reservoirs not intended to be used as fresh water resources, generally shallower than 1000 m, so that often 'deep' saline aquifer is added. The term 'saline formation' is also used for the same reason. Geological formations with these characteristics are rarely the target of exploration. Information derives mostly from regional geology based on surface mapping, few stratigraphic wells and large-scale seismic surveys. This type of work is mostly the domain of national and state geological surveys. Uncertainty due to sparse data coverage is much larger than for better known areas of hydrocarbon and mining exploration. Therefore, simulations play an important role to explore a range of possible scenarios and help constrain geologic risk of potential CO2 storage sites.