Laboratory Experiments and Reservoir Simulation Studies in Support of CO2 Injection Project in Mattoon Field, Illinois, USA
Authors: Sim S.S.K. , Udegbunam E.O. , Haggerty D.J. , Baroni J.J. , Baroni M. 
Published: March 2010

Abstract
This paper describes the results of rock and fluid property measurements and of the reservoir simulations associated with the demonstration of CO2-assisted oil recovery in the Cypress Sandstone reservoirs at Mattoon Field, Illinois. This work provided technical support for the field project.
Results from core flood tests indicate that oil recovery from immiscible displacement of reservoir crude oil with carbon dioxide will increase with displacement pressure. Miscible displacement of oil with CO2 from the Cypress Sandstone reservoirs at Mattoon field is not possible. As shown by the slim-tube experiments, the minimum miscibility pressure (MMP) of Mattoon oil is close to the formation parting pressure of the Cypress Sandstone at Mattoon field. Nevertheless, phase behaviour experiments show that dissolved CO2 significantly enhanced oil recovery through oil swelling and viscosity reduction at pressures below miscibility conditions.
Numerical simulations of CO2 injection into various reservoirs within the Sandstone were performed. A straight CO2injection program and a water alternating gas (WAG) injection program were simulated and compared in both the A-sandstone of the Pinnell Unit and E-sandstone interval of the Sawyer Unit. In the Pinnell Unit, the simulated results show an inefficient displacement of reservoir oil by CO2 and that neither of the two methods will be economically feasible because of the poor interwell communication and limited areal extent of the producing interval. This result is supported by the low oil recovery from the field CO2 injectivity tests in the Pinnell Unit. On the contrary, simulated results show that a significant amount of additional oil can be produced from the Sawyer Unit. The wateralternating- gas (WAG) injection program yielded more oil than water injection alone. The simulated results also indicate strategically- placed new wells will enhance the recovery of additional oil.

Introduction
This work provided technical support for a CO2-assisted oil recovery demonstration project funded by a cost-shared agreement (Cooperative Agreement number DE-FC22-93BCI4955) between the United States Department of Energy and American Oil Recovery, Inc. (AOR). The Mattoon Field, which is the site of the project (Figure 1), was discovered in June 1940. Development included approximately 420 producers and 90 dry holes drilled on a 4.05 ha (10 acre) well spacing. The principal reservoirs are found in the Cypress, Rosiclare and Aux Vases Sandstones of the Mississippian System (Figure 2). Secondary recovery was initiated in 1952 utilizing Pennsylvanian brine as well as treated municipal sewage water(1).
The target intervals for additional oil recovery with CO2 are the Chesterian Cypress Sandstone reservoirs in the Mattoon field. In general, Cypress Sandstone reservoirs are the most prolific in Illinois(2). Recent studies(3,4,5) show that Cypress reservoir sandstones consist of four to five stacked layers having varying reservoir quality and often separated by thin shaly sandstone or calcareous beds. Because of the variations in reservoir quality, some layers are not as efficiently swept as others during water flooding. Such layers will contain more bypassed mobile oil than others. Furthermore, Cypress Sandstone reservoirs typically have high immobile oil saturation.