Mechanistic Studies of CO2 Sequestration
Authors: Schembre-McCabe Josephina M. , Kamath Jairam , Gurton Randal M. 
Published: February 2016

Abstract
Geologic sequestration of carbon dioxide in aquifers or in hydrocarbon reservoirs offer a promising alternative to reduce the amount of CO2 released to the atmosphere. Most prior work has focused on CO2 containment. However, target reservoirs can have low permeability and are often finite, and the ability to properly model the injection stage is of significant economic concern.
We conducted thorough mechanistic studies of the injection stage using a detailed compositional simulator. Issues that were probed included effects of mobile and immobile oil saturations, damage due to geochemical reactions, stimulation, and presence of neighboring injectors.
Total mobility plots signal effects of relative permeability curves on injectivity trends during the injection phase and help identify injectivity-damaging curves.
Finally, most studies on CO2 injection for disposal purposes assume that the reservoir has an infinite capacity for CO2 storage, condition that is unlikely to be met by real-world projects. The low compressibility of water limits the amount of CO2 that can be stored in aquifers when no withdrawal of fluid is planned Finite storage size, either due to finite reservoir size or presence of interfering injectors, limits injectivity and it can have big impact on well counts needed, project economics, and total storage capacity. We also demonstrate that mechanisms present during injection stage in sequestration are very different from those in traditional Enhanced Oil Recovery operations due to its storage nature.

Introduction
The petroleum industry has been using carbon dioxide injection in enhanced oil recovery processes (EOR) since the 1970s. There has been recent interest in geologic sequestration of carbon dioxide in oil and gas reservoir to reduce the amount of CO2 released to the atmosphere.
Geosequestration is the capture and long term storage such that anthropogenic CO2 is removed from the atmosphere for a significant, perhaps geologic, period of time. Because the objective of CO2 sequestration is to maximize the amount of CO2 injected and ensure that the CO2 remains safely confined, the design of the CO2 injection changes from traditional CO2 injection design used for EOR processes. Thus, variables that measure the success of these operations are different from those used in traditional gas injection.
The two main value drivers of geosequestration projects are injectivity and containment. Over the last years, studies have been mostly concerned with containment or secure trapping of CO2 which assess reliability of these projects. However, well count, development plan and operational limitations are important economic drivers.
The focus of this paper is CO2 injectivity and the parameters that affect it. Injectivity, I, is defined as the ratio of a well volumetric flow rate, q, to a characteristic pressure drop or flow potential (?p),
I=q/?p Eq. 1
Injectivity is a measure of the ability of placing a fluid into a geological formation and it affects directly well counting; therefore, it is considered a key factor in any economic evaluation of a CO2 storage project.
Because of the pressure term in the definition of injectivity (Eq. 1), injectivity is linked to the storage capacity of geological structure or reservoir. The pressure required to place CO2 into the formation gradually increases as the volume of CO2 builds up. As reservoir pressure increases, the injectivity is diminished. Moreover, maximum pressure applied during injection is limited by the maximum acceptable pressure increase, without reactivating existing faults and/or creating new fissures.
In the first part of the paper, we use an infinite storage system to look at the effects of relative permeability, presence of residual oil saturations (in order to simulate injection into a depleted oil reservoir), damage due to geochemical reactions and well stimulation. Most sequestration projects will have multiple injectors causing finite boundary effects, and the second part investigates the impact of this on injectivity.