The objective of the project is to develop a methodology for improving sweep efficiency and reducing CO₂utilization rates by performing a detailed post-mortem on a mature carbon dioxide project that relates actual reservoir performance to predicted performance.

Louisiana State University, Baton Rouge, LA
Denbury Resources Inc., Plano, TX

CO₂ displacement is a common improved recovery method applied to light oil reservoirs. The economic and technical success of CO₂ floods is often limited by poor sweep efficiency or large CO₂ utilization rates. Little Creek field in Mississippi is being studied to relate laboratory displacement results and simulated performance predictions to the historical reservoir performance to determine sweep efficiency, improve the understanding of the reservoir response to CO₂ injection, and develop scaling methodologies to relate laboratory data and simulation results to predicted reservoir behavior.

This project is studying the effectiveness of CO₂ flooding in a mature reservoir to identify and develop methods and strategies to improve oil recovery in CO₂ floods. The study of Little Creek field will identify strategies to improve recovery from the reservoir through understanding conformance control and sweep in the current operations. Based on the knowledge gained during this study, methods will be developed for predicting conformance control and sweep efficiency that can be extended to other CO₂ injection projects, in progress or planned, and ultimately yield improved oil recoveries due to CO₂ flooding and reduce CO₂ utilization rates.

The project was designed to be conducted in two primary tasks.

Task 1 is devoted to analyzing the historical performance of Little Creek field to evaluate the effectiveness of injection and production operations during the active CO₂ flooding process. The research team is continuing the process of comparing performance predictions to actual historical performance data, including displacement studies, conformance issues, and sweep efficiency. Laboratory displacement studies have been designed and will provide a means to correlate actual reservoir performance to simulation studies and to other laboratory studies for CO₂ flooding. Historical reservoir, production, and well data have been used to evaluate the effectiveness of CO₂ injection and sweep in Little Creek field. Relatively simple univariate regression techniques appear to be as accurate as more complex techniques and are easier to apply and evaluate and also require fewer ad hoc assumptions than the more complex techniques. Several production logs have been run in active areas of the field to evaluate injection profiles in individual wells to identify vertical injection conformance or isolate injection issues. Simulation work utilizing the information from these logs has been completed for an active area of the field. Cased-hole logs to try to evaluate behind pipe saturations were designed, and test case areas were evaluated to allow the research team to quantify the measurements as best they could. A single well test on a well in the Soso Field was proposed and Denbury offered to test a second well with limited data which is more similar to the majority of wells in the fields they operate. These tests were performed and the results are currently being evaluated. If successful, these cased-hole logs may also be run in selected wells to estimate oil saturation throughout the Little Creek reservoir to verify sweep efficiency or identify potential unswept segments of the reservoir especially in the northeast quadrant of the field, which has been shut-in for nearly 10 years and may be economically productive at current oil prices. Successful results may also be cause for expansion of the logging program to other fields operated by Denbury Resources to attempt to better understand sweep in those fields. Historical reservoir performance both prior to and after initiation of the CO₂ flood have been matched in simulation studies. These studies were undertaken in order to develop a system of dynamic reservoir models for use in predicting performance based on actual reservoir conditions. The emphasis has been on integrating the understanding developed in the various activities to improve CO₂ sweep. The remainder of this portion of the project will be to extend that understanding to other reservoirs by developing application guidelines and recommendations for efficient CO₂ sweep. Note that the project is also leveraging a large quantity of geological and engineering information provided to Denbury from Shell in order to evaluate potential differences between the original flood design and current operations.

Task 2 focuses on extending the findings of Task 1 to reservoirs that may be CO₂ flooding candidates. This includes reservoirs with light oils comparable to Little Creek field and also reservoirs with heavy oils (10-20^o API) where the potential for CO₂ flooding needs additional study, especially relative to sweep effects due to the viscosity variation between the CO₂ and viscous reservoir oils and due to the lack of complete miscibility between the oil and the CO₂. Denbury has several target reservoirs in eastern Mississippi that will serve as candidate reservoirs for this task and has agreed to consider implementing ideas developed to improve oil recovery. Work has begun on evaluating CO₂ displacement in heavy oil systems using the Martinville Field as a test case. Denbury has provided data related to this heavy (14^o API) oil flood which has experienced poor sweep due to early CO₂ breakthrough. A full fluid analysis has been provided and PVT modeling studies have begun as has a reservoir simulation study to understand the root causes for the poor performance. The task will then be to relate the results from this work to those in Task 1 in order to improve the confidence of recovery estimates made for this application, improve understanding of the feasibility of CO₂ flooding heavy oil reservoirs and understand obstacles that need to be overcome, and potential solutions to those obstacles.

Researchers have:

• Completed a parametric study of factors affecting the sweep efficiency of the miscible carbon dioxide flooding process.
• Based on the parametric study and simulation results, an operating strategy was proposed to maximize oil recovery in carbon dioxide floods. An MS Thesis based on this work was published in May, 2005.
• Initiated a production logging program and incorporated results into simulation studies.
• Reviewed porosity and permeability data from core analyses throughout the field and correlated these parameters with recovery and utilization.
• Used the correlation to guide history matching of the pilot area.
• Published SPE 113977 at the 2008 Improved Recovery Conference in Tulsa, OK based on the previous three items.
• Completed history matching of another region in the active portion of the field incorporating both the porosity and permeability correlations as well as the production logging results.
• Evaluated alternative operating methods to identify ways in which recovery might have been improved in the pilot area and might be able to be increased in the active region.
• An MS Thesis based on the above 6 items was completed and published in October, 2008.
• Evaluated cased-hole logging ideas for obtaining an estimate of current oil saturation. Published SPE 113887 at the 2008 Improved Recovery Conference in Tulda, OK based on this item.
• Obtained pulsed neutron and carbon-oxygen logs from two wells in the Soso Field (a similar reservoir to Little Creek with more activity). Analysis of these logs is nearly complete. Intend to publish a companion to the above paper based on this work.
• Investigated a variety of production data analysis techniques for evaluating flood performance with limited data. A univariate regression model was developed which was easier to implement and appears to be as reliable as more complex multivariate techniques. An MS Thesis based on this work should be complete by the end of August, 2009.
• Designed an experimental program to compare new sweep improvement techniques (developed elsewhere under several other DOE-sponsored projects). Preliminary tests have been completed and the comparison experiments are being conducted.
• Began a simulation study of sweep efficiency in heavy oil reservoirs.

(April 2010)
This project has been completed and the final report is available below under "Additional Information".

$678,070

$315,316 (32 percent of total)

NETL - Chandra Nautiyal (Chandra.nautiyal@netl.doe.gov or 281-494-2488) 
Louisiana State U. – Richard Hughes (rghughes@lsu.edu or 225-578-6038)

Final Project Report [PDF-2.00MB]

Publications
Tran, N. “Evaluation and Enhancement of Miscible Carbon Dioxide Flooding,” 2005 MS thesis, University of Oklahoma, Norman, OK.

Wiggins, M.L. and Hughes, R.G. “Evaluation and Enhancement of Carbon Dioxide Flooding Through Sweep Improvement—Annual Technical Report”, December 2005.