The project goal was to perform unique laboratory experiments with artificial fractured cores (AFCs) and X-ray computer tomography (CT) to examine the physical mechanisms of bypassing in hydraulically and naturally fractured reservoirs that eventually result in less efficient CO2 flooding in heterogeneous or fracture-dominated reservoirs.

Program
This project was in response to DOE's solicitation DE- PS26-01NT41048, Reservoir Efficiency Processes. The goal of this solicitation was to reduce the amount of oil bypassed due to the poor sweep of carbon dioxide and to increase oil predictability and improve oil extraction using new technologies.

Texas Engineering Experiment Station (TEES)
Texas A&M University
College Station, TX

The primary goal of this research is to maximize the potential of CO2 flooding in the United States. As more technical knowledge accumulates, it becomes clear that natural and hydraulically induced fractures often dominate pattern or reservoir sweep efficiency. As the level of sophistication grows, low permeability reservoirs become more amenable to EOR via CO2. Low-permeability reservoirs are usually characterized by brittle matrix rock, which cracks under natural or induced conditions.

Many of the issues involved in saturation distribution during CO2 injection have been tested in Berea cores above and below miscibility pressure. However, the level of heterogeneity rarely, if ever, includes the presence of natural fractures. This is not coincidental, since the level of experimentation required is high in order to develop useful interpretations. The fact remains that reservoir heterogeneity dominates the performance of gas injection. Hydraulic or natural fractures can exert a major influence on the economics of CO2 injection projects. But the fundamental mechanisms of transfer in fracture systems are virtually unexplored. The transfer of injected gas from hydraulically induced or natural fractures determines the ultimate displacement and sweep efficiency. It is the intent of this proposed work to advance the understanding of this dynamic process and determine the implications on the ultimate performance of bypassing reserves during CO2 injection.

Results
The project used an X-ray CT scanner to image saturation profiles of flow patterns for direct measurement of bypassing mechanisms and to measure bypassed oil in order to optimize CO2 flooding efficiency. With this equipment, researchers have established the relationship between fracture aperture distribution and overburden pressures. They found that CO2 gravity drainage still plays an important role in oil recovery, even in a short-matrix block. CO2 sweep efficiency was improved significantly by controlling the CO2 mobility in the fracture with viscosified water and placing a cross-linked gel in the fracture.

Benefits
In the United States, oil that is potentially producible by advanced recovery methods amounts to 200 billion barrels. Of the available advanced oil recovery methods, gas injection has the greatest potential for additional oil recovery from domestic light oil reservoirs. CO2 flooding is the most promising gas injection technique for widespread use among enhanced oil recovery (EOR) technologies.

New CO2 projects are commencing in the U.S. and internationally each year. CO2 suppliers are drilling new CO2 production wells to increase available CO2 for delivery, and plans are under way to increase current pipeline capacities in areas where there are a lot of CO2 floods, such as the Permian Basin of New Mexico and Texas. Also, other areas in North America, such as the Wyoming-to-Canada corridor, California, and the Mississippi River region, continue development or consideration of extending the current CO2 pipeline networks to more-distant reservoirs. However, there are many reservoirs that are not being considered for CO2 flooding or any type of EOR methods because of extreme heterogeneity, or natural fractures. Because of CO2 flooding's huge potential, efforts to overcome this challenge offer potential to bolster the Nation's economic and energy security.

Project Summary
Among the project highlights:

• Advanced imaging technology has been employed to 1) characterize matrix and fracture systems, 2) image saturation profile, and 3) investigate transfer and bypassing mechanisms in order to optimize CO2 flooding efficiency.
• The new laboratory experiments have been developed to 1) demonstrate the effect of different overburden pressures and injection rates on fracture aperture and matrix and fracture productivities, and 2) mitigate bypassing mechanisms that will result in less bypassing and more efficient CO2 flooding in fracture-dominated reservoirs.
• Different CO2 injection rates and WAG (water-alternating-gas) injection ratios, along with increasing water viscosity in the WAG process and placing a gel-polymer in the fracture system were conducted as the laboratory scale to improve CO2 flooding efficiency.
• The laboratory techniques have been used to reduce CO2 bypassing and optimize CO2 flood design in the Wasson Field of west Texas.
• Analytical and numerical modeling have been performed to 1) investigate the effect of fracture aperture at variable overburden pressure, 2) study the effect of different rock heterogeneity on flow path contributors, 3) validate the use of cubic law equation, 4) examine the transfer mechanism during core flooding in fractured cores, and 5) assess the effect of grid orientation in different mobility ratios.
• A new discrete fracture simulator with flexible and unstructured gridding techniques was developed to accurately model the fluid flow through fracture networks with multiple orientations.

(August 2005)
All the proposed tasks have been completed on time. The final report is being prepared.

Publications (partial list)
Schechter, D.S., et al. Investigation of Efficiency Improvement during CO2 Injection in Hydraulically and Naturally Fractured Reservoirs, Semi-Annual Progress Report (DOE Contract No.: DE-FC26-01BC15361), Oct 2001-March 2002, April 2002-October 2002, October 2002-March 2002, April 2003-October 2003, November 2003-March 2004, April 2004-October 2004, and October 2004-April 2005.

Kaul, S.P., Putra, E., and Schechter, D.S., Spontaneous Imbibition Simulation with Rayleigh-Ritz Finite Element Method, paper SPE 90053, presented at the SPE International Petroleum Conference, Puebla, Mexico, November 8-9, 2004.

Muralidharan, V., Chakravarthy, D., Putra, E., and Schechter, D.S.,:Simulation of Fluid Flow through Rough Fractures, paper SPE 89941, presented at the SPE International Petroleum Conference, Puebla, Mexico, November 8-9, 2004.

Chong, E., Syihab, Z., Putra, E., and Schechter, D.S., A Unique Grid-Block System for Improved Grid Orientation, paper SPE 88617 presented at Asia Pacific Oil and Gas Conference and Exhibition (APOGCE), Perth, Australia, October 18-20, 2004.

Muralidharan, V., Chakravarthy, D., Putra, E., and Schechter, D.S., Simulation and Imaging Experiments for Flow through a Fracture Surface: A New Perspective, SPE paper presented at the International Student Paper Contest, Houston, TX, September 26-29, 2004.

Muralidharan, V., Putra, E., and Schechter, D.S., Experimental and Simulation Analysis of Fractured Reservoir Experiencing Different Stress Conditions, paper CIPC 2004-229 presented at the Annual Technical Meeting of the Petroleum Society, Calgary, Canada, June 8-10, 2004

$937,000

$235,000 (20% of total)

NETL - Daniel J. Ferguson (dan.ferguson@npto.doe.govor 918-699-2047)
TAMU- David Schechter (schech@spindletop.tamu.edu or 979-845-2275)