Case Study: Measuring Residual CO2 Saturation Near an Injection Well

CO2 saturation logging with RST

Challenge: For safer operations, research partners required accurate prediction of the injected CO2 plume.

Solution: RST reservoir saturation tool measurements yielded high-resolution results.

Result: The data collected confirmed the containment of residual CO2 saturation.

Injecting carbon dioxide (CO2) brings a weighty list of variables, parameters, and potential outcomes. One-size-fits-all does not apply when it comes to modeling, so accurately predicting the evolution of injected CO2 plumes is a challenge—but critical to safer operation and a project’s success.

At the Frio research project near Houston, Texas,Schlumberger was called upon for logging expertise to help with measurements of CO2 saturation in two wells, both in the injection interval, and also above.

From equations to real-world situations

One parameter that all CO2 injection models depend upon is the residual CO2 saturation as it moves through water-saturated rock. Extensive laboratory work on cores has helped to estimate this parameter. Scientists used rock, water, and CO2 properties to build predictive equations. In a laboratory setting, the equations work. But what about in real subsurface conditions? The equations needed to be verified. The Frio project was the first attempt to measure this parameter after injection.

Finding an effective way to measure residual CO2 saturation

Many approaches were used at the Frio project to measure the residual CO2 saturation. The methods yielded varying amounts of resolution. Schlumberger used sigma logging with RST. The measurements made with the RST reservoir tool in both the injection and observation wells appeared to give high-resolution results, with answers that confirmed results from other methods used.

Overcoming complexities to achieve measurable results

The project presented two significant challenges. The first was to measure the CO2 saturation in the water at its maximum level during injection. The second was to measure the residual CO2 saturation in the reservoir long after the plume had continued up the structure.

There were two wells available for logging. The injection well was down dip of the observation well (11˚ structural dip), and they were 30 meters apart. Researchers anticipated that the CO2 plume would quickly migrate to the top of the reservoir and travel up the structure past the observation well.

A systematic dual-well approach to logging

Prior to any CO2 injection, Schlumberger ran a baseline log. Repeat logs were then run on both wells:

  • during injection, immediately following CO2 breakthrough at the observation well
  • 2 days after injection activities ceased
  • 1 month after injection
  • 9 months after injection.

Successive logs were processed to calculate the CO2 saturation.

Results at the injection and observation wells

Using RST and our processing methods, Schlumberger was able to ascertain the saturation profile.

Injection well

Although the CO2 was injected in the middle of the sand (with a minor shale layer above the perforation), the CO2 quickly rose to the top of the zone and 40% CO2 saturation. Soon after injection stopped, the highest saturation was down to less than 20%. In a month, it appeared that the saturation of all zones contacted by CO2 had stabilized at 8–15% CO2 saturation.

Observation well

CO2 appeared in the top 11 feet of the observation well and achieved a maximum of 45% CO2 saturation. After injection stopped, it quickly dropped to less than 20% saturation. One month later, the CO2 had stabilized at 8–12% saturation.

The measure of success

The data collected will help constrain residual CO2 saturation in similar reservoirs. Since residual saturation is the primary trapping mechanism in the early injection time, this information will help make predictive plume simulations much more accurate. Many studies quote residual saturations measured in the laboratory at more than 20%, which is much higher than measurements at this site. Lower saturations mean that a given volume of CO2 will develop a larger areal extent, for which careful consideration must be given in permitting and monitoring CO2 evolution.

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RST Measurements Yield
High-Resolution Results

The injection well saturation profile over time.
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