(733a) Design of Multiple Brine Producer/Injector Configurations to Increase Carbon Dioxide Injectivity in Saline Formations

Carbon dioxide injection into subsurface geological horizons is one of the more promising carbon storage technologies. Mt. Simon sandstone formation appears to be an excellent candidate of carbon dioxide sequestration for the following reasons:

1. Mt. Simon formation is an extensively high capacity storage formation. Accordingly, the injection potential of the reservoir is high.  
2. There exists an un-permeable formation, Eau Clair shale, above Mt. Simon sandstone formation which serves as a seal to prevent the injected carbon dioxide from migrating into upper horizons.
3. Mt. Simon formation was once used to dispose industry waste water. The existing brine inside the formation can be produced to create more space for carbon dioxide to be injected.

The goal of the study discussed here is to examine the relationship between the carbon dioxide injectivity and the injector type and brine production by using numerical reservoir simulation techniques, and finally generate a solution for optimal injector type and arrangement of injector and brine producer to inject as much CO₂as possible while producing minimum amount of brine.   

The main challenges offered by the study area are:

1. The extensively large nature of the reservoir and the large variation of the key storage and transport characteristics of Mt. Simon sandstone formation.
2. As brine producers are used to reduce the injection pressure limit, CO₂ breakthrough may happen at the production well prematurely, which potentially can reduce the efficiency of the CO₂ injection operation.

In parallel to the aforementioned goals and challenges, a mathematical model is built allowing the numerical simulation of the study area.  Instead of simulating the entire reservoir, the model studies a small unit of the reservoir. The small unit has the same reservoir properties range to represent one small part of the entire reservoir. In order to reduce the uncertainties from the various reservoir properties, the analysis is achieved by implementing the Monte Carlo simulation protocol. Reservoir property arrays are generated within the property ranges applicable to the study area. The model does multiple simulation runs with the reservoir property arrays. The output of the model describes the injectivity of the formation within the P90, P50 and P10 confidence intervals for expected cumulative injection.

The analysis also considers the effect of different injector types on the injectivity values. The main injector types studied in the model includes horizontal, vertical and multi-lateral injectors. Different types of brine production patterns considered in the study are:

1. Four-spot pattern, with a producer- injector ratio of 0.5;
2. Five-Spot pattern, with a producer- injector  ratio of 1;
3. Seven-spot pattern, with a producer- injector ratio of 2, and
4. Nine –spots pattern, with a producer –injector ratio of 3.

An injection efficiency (IE) term is defined , which is calculated as the ratio of cumulative carbon dioxide injection to the cumulative brine production to evaluate the enhancement effectiveness of a brine production pattern in a carbon dioxide injection scenario. The model outputs the value of IE after multiple Monte Carlo Simulation runs and describes the result for the P90, P50 and P10 IE confidence intervals.  

The core simulator used in the study is CMG GEM^®, which uses a compositional formulation to consider the thermodynamic behavior of CO₂-brine system under operating reservoir conditions.