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Geologists Find Good Injectivity for CO2 into Alabama’s Citronelle Dome

Issue 11 and Volume 116.

The Citronelle CO<sub>2</sub> storage project drilled three new deep wells and three shallow groundwater monitoring wells and is using existing oil field wells to enable both near-surface and deep reservoir fluid sampling and pressure and temperature monitoring in, and above, the CO<sub>2</sub> injection zone. Courtesy of Electric Power Research Institute.
The Citronelle CO2 storage project drilled three new deep wells and three shallow groundwater monitoring wells and is using existing oil field wells to enable both near-surface and deep reservoir fluid sampling and pressure and temperature monitoring in, and above, the CO2 injection zone. Courtesy of Electric Power Research Institute.

By Andrew Maxson & Richard Rhudy, EPRI; Marian Stone and Richard Myhre, BKi

Quiet excitement marked the morning of Aug. 20, 2012, for the geologists and engineers of the Southeast Regional Carbon Sequestration Partnership (SECARB) as carbon dioxide (CO2) from a coal-fired power plant began flowing through a newly constructed pipeline, down an injection well and into the Paluxy Formation — 9,400 feet below ground surface. Easing in at 230 metric tonnes of CO2 per day, the amount was later ramped up to 400 tonnes per day, and then to the full design rate of 500 tonnes per day.

“Injection start represents a major project milestone and the culmination of four years of team building and outreach to Alabama communities and regulators,” said Jerry Hill of the Southern States Energy Board (SSEB), who serves as SECARB’s technical coordinator.”As part of this effort, we conducted site characterization and modeling, pipeline and well field design and construction, monitoring program development and equipment installation, and baseline data collection. We also acquired a first-of-a-kind permit in Alabama for injection of CO2 for geologic sequestration.”

Part of the world’s largest carbon capture and storage (CCS) project on coal flue gas, the CO2 injection and monitoring program is taking place in the Citronelle Oil Field, on the flank of the Citronelle Dome. Located about an hour’s drive north of Mobile, Ala., the Dome is a giant salt-cored anticline — a fold in the earth’s crust where strata slope downward from either side of a common crest. With its proven capability to trap oil and a lack of faults or seismic activity, the Citronelle Dome is an excellent prospect for storing large volumes of CO2.

The Paluxy Formation, nearly two miles deep at this part of the Dome, consists of 1,100 feet of sandstone interbedded with siltstone and shale. Overlying impermeable shales and clays prevent any upward flow of CO2 out of the Paluxy Formation. It is one of several well-sealed sandstone formations at the site, including the Upper Tuscaloosa Formation, which SECARB evaluated in a 2008 small-scale CO2 injection project in Mississippi, 60 miles west of the Citronelle Dome. Both during and after CO2 injection, a comprehensive monitoring program will enable SECARB researchers to characterize the suitability of this regionally extensive sandstone formation, and others like it, to store CO2 from Gulf Coast power plants and oil refineries.

“We’re in the middle of a well-characterized oil field, but we had little data on the brine-filled Paluxy Formation before this project,” said Rob Trautz, a senior project manager at the Electric Power Research Institute (EPRI). To fill this knowledge gap, EPRI and the Southern States Energy Board, which manages SECARB, assembled a world-class project team combining CO2 storage assessment and monitoring expertise with local drilling, construction and oil field operations experience.

Fielding the Right Team

Bringing together multiple organizations from the applied research, commercial industry and public resource management communities was exactly what the U.S. Department of Energy (DOE) envisioned when it created the Regional Carbon Sequestration Partnerships program. SECARB’s Citronelle CO2 storage project team includes EPRI; Advanced Resources International, which designed the injection and monitoring plan and built the reservoir model; the CO2 Capture Project and Lawrence Berkeley National Laboratory (LBNL), which are providing monitoring equipment and research scientists; and Denbury Resources LLC, the Citronelle landowner and mineral rights operator, which built and operates the CO2 pipeline and the injection and monitoring wells.

Geophone (situated on protective mat) about to be lowered into a wellbore at the Citronelle CO<sub>2</sub> storage project site to enable downhole seismic imaging of injected CO<sub>2</sub>. Courtesy of the Electric Power Research Institute.
Geophone (situated on protective mat) about to be lowered into a wellbore at the Citronelle CO2 storage project site to enable downhole seismic imaging of injected CO2. Courtesy of the Electric Power Research Institute.

The team began by assembling and reviewing available geologic data, by examining the casing and cement integrity of older wells surrounding the CO2 storage site (to be sure they wouldn’t allow entry and passage of the CO2 to shallower strata), and by collecting fluid and rock core samples from a newly drilled well. Findings were incorporated into a geologic model, and a reservoir simulator was used to predict the position and overall footprint of CO2 from injecting into multiple stacked sand layers within the Paluxy. Like a tall office building that uses multiple floors to minimize its footprint, injection into multi-layers limits the horizontal extent of the CO2 footprint, which in turn minimizes the area that needs to be monitored. Results from the simulations indicate the maximum travel of the CO2 will be less than 1,000 radial feet from the point of injection, and that the size of the plume at stabilization 10 years after injection will be less than 80 acres.

The pre-injection measurements also serve as a baseline that can be compared with data collected during and after CO2 injection operations.

Project Permitting

The project received a “Class V” permit, which covers research wells, for underground CO2 injection in November 2011 from the Alabama Department of Environmental Management. The permit application was submitted prior to the issuance of rules by the U.S. Environmental Protection Agency (EPA) for a new Class VI permit category specifically for CO2 injection wells designed for long-term geologic sequestration. However, Alabama regulators were aware of EPA’s work in this area, and included several Class VI well standards in the project’s permit. These provisions include periodic Area of Review updates and a monitoring program that covers surface CO2 monitoring, monitoring of the injection and wellbore annulus pressure, and frequent injection stream compositional monitoring. Monitoring results will be used to update the reservoir model and for the Area of Review updates.

Advancing CO2 Monitoring Technologies

The monitoring program exceeds compliance requirements in order to evaluate the adaptation of oil field tools and techniques for monitoring CO2, as well as test new and emerging technologies. Among the latter is a modular borehole monitoring system developed by Lawrence Berkeley National Laboratory (LBNL) with funding from the CO2 Capture Project, a partnership of energy companies.

custodial transfer station

LBNL’s multi-sensor monitoring tool has been installed in the project’s primary observation well to assess the rate of change in CO2 saturation in the formation as injection proceeds, sweep efficiency (a measure of pore space utilization for CO2 storage), and CO2-induced changes in geochemistry of the formation fluids. The modular borehole monitoring system includes a fiber optic cable for high-resolution down-hole temperature measurement; a heater cable allowing for “heat pulsing” to detect near-well CO2 leakage and provide other diagnostics; a down-hole pressure gauge for monitoring the pressure front; and a U-tube reservoir fluid sampling tool, which enables the continuous collection of reservoir fluids at in situ pressures.

Two strings of receivers or geophones have been deployed in the wells—a temporary long string and a semi-permanent short string of 18 geophones installed on tubing—enabling the collection of high-resolution data for imaging the CO2 dispersal. The project team hopes to obtain a cross-well seismic survey spanning 800 feet or more, which would be one of the longest cross-well surveys to date for CO2 measurement, according to Trautz.

The project will also test the effectiveness of different tracers, easily detectable substances added to injected CO2 to mark its position. By tracking the peak arrival times of the tagged CO2 at the observation well, researchers are able to infer the CO2 saturation between the wells—important information for reservoir modeling.

With funding from DOE and EPRI, four different groundwater sampling methods are being evaluated by the U.S. Geological Survey for cost-effectiveness and to assess the impact on sample quality due to depressurization as samples from deep wells are brought to the surface for analysis.

Alabama’s First Operational CO2 Pipeline Takes Tortoises and Wetlands into Account

The CO2 arrives at the injection site via Denbury Resources’ newly constructed 12-mile, 4-inch diameter carbon steel pipeline, commissioned in March 2012. Much of the pipeline’s route made use of a high-voltage transmission line corridor to simplify rights-of-way acquisition and construction access. Horizontal drilling (as opposed to trenching) was used for some sections to protect wetlands and avoid interference with surface features such as roads and railroad tracks. The project also identified 110 gopher tortoise burrows along the pipeline route and opted to drill horizontally beneath them, rather than relocating these protected reptiles.

CO2-specific design requirements for the pipeline included valve seals of Nylon, peroxide cured Buna N (HBNR-90/95), or ethylene propylene rubber; Teflon valve packing; low-temperature materials for valves used in blowdown service only; and mainline valve station similar to natural gas with blowdowns for maintenance.

Denbury pipeline purity requirements call for: >97% dry CO2 at 115ºF, 1500 psig (100 bar); <0.5 percent inerts (including N2 and argon); <30 lb water per MMSCF (<0.5 g/Nm3); and <20 ppm H2S. Samples of the CO2 stream are taken periodically at the custody meter station for permit compliance and regulatory reporting.

Hurricane and Lightning Test Project’s Resiliency

As Hurricane Isaac made landfall on Aug. 28, 2012, the project experienced an “unplanned test” of its shutdown and restart capability. Heavy rains led Denbury Resources to shut in the Citronelle oil field as a precautionary measure, which included shutdown of CO2 injection operations. At Alabama Power’s Plant Barry, vibration caused by lightning tripped sensors at the CO2 compressor. After restarting the compressor and reopening the injection well, downhole CO2 pressure and flow rates stabilized quickly. Further shutdowns, which are anticipated in response to the seasonal fluctuation of the CO2 supply, will allow additional assessment of the system’s ability to respond to real-world operating situations.

Looking Ahead

By late October, the project has stored over 18,000 tonnes of CO2, on its way to a planned injection of approximately 220,000 tons (200,000 tonnes). By the time injection operations and post-injection monitoring conclude in 2017–18, a wealth of data and experience applicable to future projects will have been gathered.

denbury co2 pipelin

The wells will be closed in accordance with state regulations, or possibly re-permitted and used by Denbury Resources for enhanced oil recovery.

“The Citronelle storage project demonstrates safe, secure CO2 injection and storage in regionally significant saline reservoirs in the U.S.,” said Hill.”It evaluates local storage capacity, injectivity, and trapping mechanisms, and tests the adaptation of commercially available oil field tools and techniques for monitoring CO2 storage.

“Another important aspect is the experience gained from the permitting process for CO2 pipelines and saline injection, and the opportunity to build stakeholder acceptance through education and outreach.”