Surveillance and optimization: critical to CO2 flooding

Knowledge about how to optimize a CO₂ EOR project has grown dramatically in recent years as the number of active CO₂ EOR floods worldwide has more than doubled, from 65 in the mid-’90s to more than 145 today. During this same time period, U.S. projects have increased from 55 to 130.

CO₂ EOR’s growing popularity is due in large measure to its efficiency at “cleaning” oil from the rock. But since the properties of CO₂ in the reservoir are such that overall “sweep” through the reservoir will likely be lower than during a waterflood, it is important to know that the CO₂ is doing its job, contacting oil and moving it. And now that supplies of CO₂ are constrained, the cost of CO₂ for EOR projects is higher, and surveillance is even more important. Therefore, active reservoir management and ongoing pattern management is critical. This can only be achieved through an active, ongoing surveillance and optimization effort. Without such an effort, a CO₂ flood’s performance can easily and quickly spiral out of control, and its profitability can suffer.

Surveillance

Surveillance operations can be grouped into two categories: routine and nonroutine. Routine surveillance is these basic data that are required to monitor and analyze the flood performance on a daily or monthly basis. It should have a reasonable cost associated with it and is part of the reservoir management team’s data gathering plans. It includes production well tests, injection rates and pressures, reservoir pressures, injection profiles, and corrosion.

Several techniques can be applied to minimize the adverse sweep. (Source: Melzer Consulting)

Nonroutine surveillance is generally more focused on helping solve a specific problem. It also is likely more expensive. Examples of nonroutine surveillance include transient pressure analysis, production profile logs, borehole image logs, saturation logs, coring and special core analysis, and seismic methods.

Optimization

Several techniques can be applied to minimize the adverse sweep. Water-alternating-gas (WAG) management is the most widely used sweep control technique used for CO₂ EOR projects. WAG management refers to the selection of the optimum WAG ratio and cycle slug sizes for each injector in the project.

The process requires the analysis of the surveillance data obtained, the development of recommendations based on the analysis and the implementation of those recommendations. Important considerations include the incremental oil production and gas production as well as facility constraints such as gas recycling capacity.

Both increasing the WAG ratios (tapering) and reducing the cycle sizes can improve the sweep and, with it, the CO₂ utilization (the amount of CO₂ needed to produce an incremental barrel of oil). They also can both result in more optimal CO₂ retention and therefore help reduce the gas production rate. Reduced half-cycle sizes have the additional benefit of a higher CO₂ processing rate and a reduced fluctuation in produced rates that are inherent with the WAG process.

We never know enough about a reservoir to expect a pre-deployment reservoir management plan to be perfect. As a result, CO₂ miscible floods are often complex to manage. It is imperative that operators of such floods dedicate the needed talent and resources to do the proper surveillance and analysis to optimize flood performance and flood profitability.