Limitations to Storage Pressure in Finite Saline Aquifers and the Effect of CO2 Solubility on Storage Pressure

Abstract

Abstract CO2 storage in aquifers is an attractive option for reducing CO2 emissions into the atmosphere. The successful example of CO2 storage in the Sleipner injection project proves that large volumes of CO2 can be stored in a water-bearing subsurface formation. In general, most of the injected CO2 will displace water and will accumulate as free gas at the crest of the aquifer. On the other hand, CO2 is also soluble in water. Therefore, the aquifer or depleted reservoir will eventually be saturated with CO2. The present paper explores various aspects of the solution process, based on numerical simulation studies for a Norwegian type of open aquifer storage location and other types of storage structures. The case studies indicate that only in the long term (> 1000 yrs), solubility may offer a storage mechanism for most of the CO2 stored in depleted fields. This is due to the accumulation of CO2 in the gas phase, which has a limited contact area with the water phase. However, if injection strategies focus on exposing the injected CO2 to as much "fresh water" as possible (i.e. through extended and dispersed migration paths) the amount of CO2 in the free gas phase can be reduced significantly and within a shorter time. It is commonly believed that all CO2 will eventually disappear because it will dissolve in water. We agree with the fact that all CO2 will dissolve if there is enough water, but when a lot of material (CO2) is added to the system, this will affect fluid volumes and pressures in the total storage system. By studying a practical, real storage location, we try to find out some of the pressure effects of CO2 solubility on the total storage capacity of the selected storage site. Introduction Following the Sleipner success, European research studies and pilots such as the K12-B site injection project in the Netherlands have focused on CO2 storage in trap structures, preferably in depleted fields where seal integrity is well proven. However, most depleted (gas) fields in Europe have insufficient storage capacity to achieve a significant reduction of CO2 emissions. A typical power station produces in the order of 10 million tons/year of CO2 (2x800 MW). As the average life of a power plant is approximately 40 years, some 400 million tons would have to be stored for each power plant, far more than can be accommodated in even the largest depleted oil or gas field in Europe. On the other hand, CO2 is also soluble in water. Therefore, the aquifer or depleted reservoir will eventually be saturated with CO2. This paper explores various aspects of the solution process, based on numerical simulation studies for storage locations that are similar to the Sleipner field. A recent IPCC publication[1] quotes many papers that rather naively claim that CO2 simply disappears as it is gradually absorbed by water. We will have to answer the question: will CO2 really disappear when it dissolves in the formation water? CO2 solubility Introduction The SIMED II™ software (Australian Petroleum Cooperative Research Centre, APCRC) has been used for both model studies. TNO has a co-operation agreement with the owners of the software to perform additional development work on the software. As part of this agreement, the solubility of CO2 in water has been incorporated into the SIMED II simulator, hereby assuming that equilibrium thermodynamics can be used to describe the physical constraints.