Abstract

This paper describes the results of rock and fluid property measurements and the results of integrated reservoir modeling associated with Phase I of a demonstration of CO 2-assisted oil recovery in the Cypress Sandstone reservoirs at Mattoon Field, Illinois. The objectives of the project are to characterize the Cypress Sandstone through an integrated geologic and engineering approach; to identify facies-defined waterflood subunits: to design and implement an enhanced oil recovery program utilizing CO2; and to transfer technology to oil and gas operators.

Although results from slim-tube displacement experiments show that implementation of a miscible flood may not be possible, phase behavior experiments show that the effects of dissolved CO2 on oil swelling and viscosity reduction are significant even at pressures below miscibility conditions. Results from core flood tests indicate that oil recovery from immiscible displacement of reservoir elude oil with carbon dioxide will increase with displacement pressure.

Reservoir simulation studies and injectivity tests on the Pinnell Unit of the project area indicate poor inter-well communication and limited areal extent of the producing interval this unit. Inefficient displacement of reservoir elude by the injected CO2 is also indicated from production tests.

Injectivity tests on the Sawyer Unit of the project area show good inter-well communication. Results from analyses of core samples from a newly drilled infill well within this unit confirm the isolation of the target E-sandstone interval from the extensively waterflooded lower B and C sandstone intervals. Reservoir simulation results indicate that a significant amount of additional oil can be recovered by the implementation of a water alternating gas (WAG) injection program utilizing eight or more oil producers.

Introduction

The Mattoon Field, located near the city of Mattoon, Illinois (Fig. I), was discovered in June, 1940. Development included approximately 420 producers and 90 dry holes drilled on a ten-acre Spacing. The principal reservoirs are found in the Cypress, Rosiclare and Aux Vases Sandstones of the Mississippian System (Fig. 2). Secondary recovery was initiated in 1952 utilizing Pennsylvanian brine as well as treated municipal sewage water (1).

Seventy percent, or 164 of the 236 total oil leases are currently being managed by 59 operators. As of March 1993, the cumulative oil production from Mattoon Field was 21 million barrels.

This project is supported by a cost-shared agreement (Cooperative Agreement number DE-FC22- 93BC14955) between the United States Department of Energy and American Oil Recovery, Inc. (AOR), and is part of a DOE program designed to recover oil remaining in Class 1 fluvial deltaic reservoirs. The Cypress Sandstone reservoirs in Mattoon Field were selected as the site for demonstration of the utilization of CO2 for improved oil recovery in a marginal field.

The objectives of the AOR project are to characterize the Cypress Sandstone reservoirs; to identify and map facies-defined waterflood subunits (FDWS); to design and implement an improved oil recovery program utilizing carbon dioxide in the field; and to transfer the technology to oil and gas operators.

In the first phase of the project, computer-generated geological and reservoir simulation models were developed to assist in the selection of sites for the demonstration and implementation of a CO2 displacement program in Phase II.

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