The Evolving Challenges of CCS Reservoir Engineering

Last week I had the pleasure of participating in an enlightening webinar sponsored by the Society of Petroleum Engineers entitled "Seven Unique Appraisal Challenges Derisking CCS Projects" presented by Robert Laronga. Following the enhancement and clarification of the Section 45Q tax credits for carbon sequestration a few years ago, many projects have entered the design and permitting phase. We're now getting greater insight into the technology and analysis needed to address the unique engineering problems and risks associated with these projects.

The seven challenges Laronga discussed in the webinar were: storage efficiency, storage and trapping mechanisms, unproven seal capacity, rock/brine/CO2 interactions, increasing pressure over time, injectivity over time and supercritical fluid properties. An eighth challenge, wellbore integrity of legacy wellbores, was omitted due to time constraints.

Many of these topics are familiar to geologists and petroleum engineers who have worked on hydrocarbon development projects, however, I the regulatory overprint on CCS projects to a higher level of technical diligence than most have encountered. Laronga stated that CCS projects have the most extensive coring, logging, and laboratory analysis programs he has seen, and they are comparable to what would be needed for a deepwater appraisal well. What is ultimately required at a given project is not only a function of the project details but also the regulator's discretion.

I can't touch on all the topics covered, but here are a few key takeaways from a reservoir engineer's perspective.

Multizone Evaluation is Necessary - This includes not just the saline aquifers, but also the overlying seal and potentially a back-up sealing layer and formations below the storage zones.

Complex Process Are at Work - In many oil and gas developments, the primary physical processes of interest are hydraulic and mechanical. In CCUS, however, we also must contend with chemical and thermal processes.

Capillary Pressure is Important - CO2 is not very effective at displacing brine. This limits how low the residual water saturation can go without excessively high pressures that could fracture the rock. It's also may be necessary to measure the capillary pressure attributes of the overlying seal - a step that's not typically done in oil and gas development.

?System Properties are Dynamic - It's common in oil and gas to consider changes in pressure and fluid composition over time, but the unique interactions between the fluids and rocks in CCUS projects complicates this. These interactions include mutual solubility between the CO2 and brine, the mineralization of CO2 (as a carbonate), the acidification of the brine that can react with the rock, and the precipitation of salts from the brine. These processes influence each other and all of them can affect porosity and permeability in complex and offsetting ways.

?Well Testing Becoming Revitalized - The nature of the wells drilled during the shale revolution resulted in a generation of reservoir engineers who didn't need to apply pressure transient analysis (and other) techniques very often, but these skills will be needed to monitor changes in CO2 injector's skin and permeability changes over time.

There was much more presented, but to learn more check out Laronga's SPE journal paper "Integrated Formation Evaluation for Site-Specific Evaluation, Optimization, and Permitting of Carbon Storage Projects".