DE-FC26-06NT43029

Goal
The goal of this project is to conduct carbon dioxide (CO₂) injection tests at the Citronelle oilfield in Mobile County, AL. The project will introduce CO₂-enhanced oil recovery (CO₂-EOR) for tertiary recovery from Alabama?s uniquely structured sandstone reservoirs, providing oilfield operators and CO₂ producers with improved estimates of oil yields from EOR, and the capacity of the depleted reservoirs to sequester CO₂. The research will improve the reliability of computer simulations of oil yield from CO₂-EOR, calculations of sequestration capacity, and the rate at which CO₂ can be introduced into underground formations. The simulations of Citronelle field will be integrated with computer visualizations of the migration of oil, water, and CO₂ with the results made accessible to reservoir engineers, geologists, utility planners, and researchers studying carbon sequestration.

Performers
University of Alabama at Birmingham, Birmingham, AL
Alabama Agricultural and Mechanical University, Normal, AL
Denbury Resources Inc., Plano, TX
Geological Survey of Alabama, Tuscaloosa, AL
Southern Company, Birmingham, AL
University of Alabama, Tuscaloosa, AL
University of North Carolina at Charlotte, Charlotte, NC

Background
Carbon dioxide-enhanced oil recovery is a well-established method for increasing oil recovery from the Permian Basin oilfields of Texas and New Mexico and from the Williston Basin in North Dakota and Montana. Denbury Resources has also been successful in applying the technique in Mississippi oilfields. Typically, ten percent of the original oil present in a reservoir at the start of production can be recovered using CO₂-EOR. A recent study by Advanced Resources International of Arlington, VA, estimated that 64 million additional barrels of oil could be recovered from the Citronelle field using this technology. When oil production is complete, the reservoir and adjacent formations can provide sites for storage of CO₂ produced from the combustion of fossil fuels in power plants and other industrial processes that generate large quantities of CO₂. Southern Company is evaluating the capacity of such reservoirs as possible sites for permanent sequestration of CO₂ that has been separated from coal and natural gas combustion products produced by its power plants.

The Citronelle field is an ideal site for CO₂-EOR and sequestration from both reservoir engineering and geological perspectives. The field is mature and waterflooded, with existing infrastructure, including deep wells, and consists of fluvial-deltaic sandstone reservoirs in a simple structural dome. Presence of the regionally extensive Ferry Lake Anhydrite seal, four-way structural closure, and lack of faulting make the field naturally stable with respect to CO₂ storage. However, the geology of the heterogeneous siliciclastic rocks in this field is very different from fields where CO₂-EOR has been applied commercially, such as in the carbonate strata of the Permian and Williston basins. The proposed demonstration will introduce CO₂-EOR for tertiary recovery from Alabama?s uniquely structured energy resources and thus realize benefits to the nation from additional petroleum production.

Impact
A successful demonstration of the technology in this project could lead to new commercial CO₂-EOR and sequestration efforts across the nation (including reservoir types where CO₂-EOR has thus far not been applied) offering a potential two-for-one solution to the United States' energy security and environmental concerns.

Accomplishments
The principal focus (January 1, 2007 to August 31, 2008) was on selection and preparation of the test site and thorough studies of its geology, environmental conditions (air, soil, and vegetation), seismic imaging, and produced fluids to establish the background conditions prior to CO₂ injection. The petrology, sedimentology, and stratigraphy of the Rodessa Formation in the vicinity of the test site were determined and documented at an unprecedented level of detail. Realistic and informative reservoir simulations and visualizations were performed. The environmental and ecological background conditions surrounding the site are well documented. Seismic signals have been recorded under both baseline water flood conditions and during CO₂ injection. The minimum miscibility pressure and absence of precipitation from oil in the presence of CO₂ were established and an economic analysis identified the optimum CO₂ slug size for water-alternating-gas oil recovery under specified CO₂ cost and oil price constraints. All indications are that the pilot test will provide an unequivocal demonstration of CO₂-enhanced oil recovery and essential data and simulations on which to base a commercial CO₂ flood in the Citronelle Field. The CO₂ storage capacity of depleted oil reservoirs and saline formations in Citronelle Dome was estimated by static calculations to be between 0.5 and 2 billion short tons of CO₂, sufficient to sequester the CO₂ from a nearby 1500 MW (electric) coal-fired power plant for 35 years.

Carbon dioxide injection began for Field Test No. 1 (the first of two CO₂-EOR pilot tests in the heterogeneous sandstone reservoirs of the Citronelle field) at the end of November 2009. After resolution of some initial difficulties in pumping, injection of CO₂ continued without significant interruption at an average rate of 31 tons/day from January 27 to September 25, 2010, in good agreement with the average of 35 tons/day anticipated by reservoir simulations. Injection of the 7500 tons of CO₂ allocated for the first test is complete. Incremental oil recovery is predicted to be 11,500 stock tank barrels, an increase of 60 percent over the yield expected from conventional secondary oil recovery by water flood.

A parametric study of WAG recovery, using the MASTER 3.0 reservoir simulator, showed that a properly designed WAG recovers as much oil as continuous CO₂ injection. Using the simulation results, three-dimensional animations were programmed showing the evolution of fluid saturations in Donovan Sands 14-1 and 16-2 during two CO₂ injections of 7500 tons each, followed by water. The animations nicely capture the mobilization of oil by CO₂, development of the oil bank, the role of water in mobilizing the bank, and the residual oil left unrecovered.

Shear-wave velocities were measured to depths of 12,500 feet using the Refraction Microtremor technique. Wireless geophones were placed along two straight paths spanning 30,100 and 25,600 feet south and southwest, respectively, of the injection well. Shear-wave velocities recorded before and during CO₂ injection suggested a 10% increase in stress associated with CO₂ injection in the geologic layers above the injection zone. An interesting systematic increase in the dependence of shear-wave velocity on depth below ~4500 feet has been observed during injection of the CO₂ and water.

A pressure-transient test in the injector that was conducted in November and December 2011 provided strong evidence for the presence of a 600 to 1000 foot vertical fracture intersecting the injection well. The presence of a fracture was completely unexpected because the injection well had never been intentionally hydraulically fractured and the hydrocarbon-bearing sands at Citronelle are free of natural fractures. Although the direction of the fracture cannot be determined from the results of the pressure-transient test, a fracture in the direction of maximum horizontal compressive stress in the southeastern U.S. would explain the early excessive breakthrough of CO₂ and loss of oil production from the fourth producer in the five-spot (Well B-19-11). The avoidance or management of hydraulic fracturing will be an important component of the reservoir management plan during a commercial CO₂ flood.

Work conducted since November 2011?following attainment of a solution to the erosion-corrosion problem in the power-oil pumps?has been focused on understanding the loss of production at Well B-19-11, and the loss in injectivity to water following the injection of CO₂. Both of these issues have a significant bearing on the design of a commercial CO₂ flood at Citronelle.

An injection profile test run in the injector in January 2012 showed that the loss in injectivity to water (following CO₂ injection) is due to reduced injectivity in only one of the two target sands. Injectivity to water decreased from approximately 140 to only 20 bbl water/day in the upper sand (Sand 14-1) and remained approximately the same, at 31 to 39 bbl water/day, in the lower sand (Sand 16-2). The ability to mitigate formation damage due to interaction of CO₂, water, minerals, and brine solutes will be a critical factor in the decision to conduct WAG recovery at Citronelle, should CO₂ supply be limited.

Approximately 10,000 bbl of incremental oil assignable to EOR have been recovered from three of the wells in the inverted five-spot as of the end of April 2012. Although this has been offset by the loss in production from Well B-19-11, the trend of net incremental oil production indicates that it will become positive again during the next quarter.

Oil produced from three wells in the test pattern (B-19-7, B-19-8, and B-19-9) was gathered, along with production from five other wells to the north and east, at Tank Battery B-19-8. Produced oil from well B-19-11 in the test pattern goes to Tank Battery B-19-11, along with production from three other wells to the west and south. The record of oil production is discussed in a recent Quarterly Progress Report (October 30, 2012, pp. 19?27). Here it is noted that, with the start of surfactant treatment in July 2012, the decline in production experienced at Tank Battery B-19-8 from March to July 2012 has been reversed, with oil production remaining steady at 38 bbl oil/day in August, September, and October 2012. Production at Tank Battery B-19-11 remains on the declining path established following breakthrough of CO₂ at Well B-19-11, evidently through a fracture, as shown by Eric Carlson in the January 30, 2012 Quarterly Progress Report (pp. 37?44). Production at Tank Battery B-19-11 has not shown significant response to any changes in conditions at the injector, including the return from CO₂ to water injection at the end of September 2010.

In mid-October, the Denbury group in Citronelle began an effort to increase the pressure and water injection rates on other wells. The higher pressure resulted in some line leaks, requiring that those lines be shut in and a pump shut down to prevent blowing out other lines. The pressure at an injector, B-19-10 #2, during this period was unstable at first, then settled down at a value 100 psi lower than before. Surfactant treatment was stopped shortly thereafter, on November 7, 2012. The reduction in pressure provides an explanation for the low injection rates observed from October 20 to November 7, 2012.

Current Status (January 2013)
The following tests to determine how to improve injectivity in Citronelle Field will be performed:

• Surfactant injection test to unequivically determine if capillary pressure effects are responsible for the low injectivity to water in Sand 14-1, to be followed by an injection profile test to distinguish its effects on the two target sands. The surfactant injection field test was completed by November 30, 2012. Data analysis is being performed to determine the effectiveness of this test.
• Acid treatment to determine whether carbonate precipitation or plugging by mobilized particles is responsible for the low injectivity to water in Sand 14-1. As in the surfactant test, above, acidification will continue for two or more months, if necessary, and be followed by an injection profile test.
• A step-rate test to determine the opening stress of the fracture at the injector and an evaluation of the feasibility of implementing ?smart wells? in which pressure is actively controlled to minimize opening of fractures and bypass of CO₂.

Project Start: January, 1 2007
Project End: February 28, 2013

Anticipated DOE Contribution: $4,692,957
Performer Contribution: $4,053,321 (46 percent of total)

Contact Information
NETL ? Chandra Nautiyal (chandra.nautiyal@netl.doe.gov or 281-494-2488)
University of Alabama at Birmingham ? Peter Walsh (pwalsh@uab.edu or 205-934-1826)
If you are unable to reach the above personnel, please contact the content manager.

Publications
J.C. Pashin and R.A. Esposito, "Citronelle Dome: A Giant Opportunity for Multi-Zone Carbon Storage and Enhanced Oil Recovery in the Mississippi Interior Salt Basin of Alabama," Annual Convention and Exhibition of the American Association of Petroleum Geologists, Long Beach, CA, April 1-4, 2007.

R.A. Esposito, J.C. Pashin, and P.M. Walsh, "Citronelle Dome: A Giant Opportunity for Multi-Zone Carbon Storage and Enhanced Oil Recovery in the Mississippi Interior Salt Basin of Alabama," 2007 Annual Convention of the Gulf Coast Association of Geological Societies and the Gulf Coast Section of the Society for Sedimentary Geology, Corpus Christi, TX, October 21-23, 2007.

R. A. Esposito, J. C. Pashin, and P. M. Walsh, "Citronelle Dome: A Giant Opportunity for Multi-Zone Carbon Storage and Enhanced Oil Recovery in the Mississippi Interior Salt Basin of Alabama," accepted for publication in Environmental Geosciences, June 2008.