Advanced Monitoring Technologies and Their Application At the Secarb Phase III CO2 Storage Site Near Citronelle Alabama

Capturing CO₂ emitted by power stations and injecting it deep underground into geologic formations containing brine is one method proposed to prevent greenhouse gas concentrations in the atmosphere from increasing. Although the risk of CO₂ leakage from a properly characterized geologic storage site is expected to be low, monitoring of CO₂ injection is required by State and Federal agencies to detect CO₂ and brine leakage from the storage reservoir before it has an opportunity to impact the environment, to ensure that injection operations are safe and to maintain the structural integrity of the caprock or seal above the injection zone, which keeps the fluids permanently contained. For the most part, commercial technologies developed by the petroleum industry for oil and natural gas exploration and production are being adopted for use in monitoring CO₂ storage projects. However, advanced monitoring technologies and practices are needed that are cost effective, reliable, sensitive to small changes in fluid saturation and can withstand exposure to harsh downhole conditions, spanning the 30-60 year life expectancy of a typical power station and beyond.

The Southeast Regional Carbon Sequestration Partnership (SECARB) Anthropogenic Test is the world’s largest CO₂ injection project that receives pipeline CO₂ captured directly from a coal-fired electric power unit and stores it in the Southeast Unit of the Citronelle Oil Field near Citronelle, Alabama. As part of the SECARB Anthropogenic Test, Advanced Resources International (ARI) and the Electric Power Research Institute (EPRI) partnered with the CO₂ Capture Project (CCP) to field test an advanced package of monitoring tools to meet site-specific objectives in a dedicated monitoring well. During the planning stages of the SECARB Anthropogenic Test, the CCP was in the middle of funding a three-year research program at Lawrence Berkeley National Laboratory to design a Modular Borehole Monitoring (MBM) system, a next-generation package of fully integrated monitoring systems that would both control costs and deliver a robust suite of instruments with a reduced risk profile. The implementation of the MBM package in the dedicated D9-8 monitoring well required a team with experience in multiple disciplines, involving well completion experts from Denbury Resource Inc., a site-operations manager from ARI  and discipline experts at EPRI and CCP, as well as numerous subcontractors. The technical team deployed the innovative MBM system, capable of detecting CO₂ leakage behind casing and above zone, measuring in-zone pressure and temperature, evaluating vertical distribution of CO₂ in the reservoir, collecting fluid samples at reservoir pressures and seismic monitoring using a high-density fiber optic sensor array.

The backbone of the MBM system consists of integrated fiber-optic (FO) sensor array intended for long-term monitoring of CO₂ storage. The 9 mm diameter FO cable includes a single-mode fiber for acoustic monitoring of flow and active seismic imaging of the CO₂ plume using sources deployed at land surface or in nearby wells. The cable also includes a multi-mode fiber for performing distributed temperature sensing (DTS) embedded within copper conductors for performing heat-pulse interrogation of the near-wellbore environment. The temperature decay after the heat-pulse is turned off can be interpreted to determine the thermal conductivity of the near-wellbore (fluid, casing and rock) system. Changes in thermal conductivity caused by fluid substitution can be used to detect CO₂ migration and changes in saturation in the open well and behind casing using this technique. Installation of temperature and acoustic sensing fibers within the same cable assembly opens up truly integrated approaches that combine geophysical and hydrological monitoring that take advantage of the benefits of commercial grade telecom fibers including longevity (50 year life span), reliability, low cost and widespread availability. Spatial resolution on the order of one measurement every 1 to 3.3 ft (0.3 to 1 m) over a 6.2 mile (10 km) long FO cable is achievable using our system. Furthermore, as surface-based FO data acquisition systems improve, the MBM fiber-infrastructure will be able to perform with increasing sensitivity, with greater temporal and spatial resolution.

The MBM system also includes a semi-permanent tubing-deployed, 18-level geophone array (an array of 3-component and vertical geophones) used for seismic imaging and tracking of the CO₂ plume as it moves through the formation away from the nearby injection well. The custom hydraulic clamping system provides acoustic coupling between geophones (in the annulus between tubing and casing) and the formation, increasing the strength of the detected acoustic signal and thus significantly improving the signal to noise ratio. An offset vertical seismic profile (VSP) survey was performed in the open well prior to completion using a wireline conveyed 80-level, 3-component geophone array prior to CO₂ injection, which will be repeated again after CO₂ injection ends in 2-3 years. The 18-level MBM array provides intervening snap shots of the CO₂ plume move-out in time and also serves as a reference point for comparing the quality of VSP results obtained using the single mode fiber for distributed acoustic sensing (DAS), a new and emerging technology.

A U-tube fluid sampler (Freifeld et al., 2005) was also installed as part of the MBM, which improves upon the original design by using a single, tube-in-tube control line instead of two. The intake for the U-tube sampler is located below the hydraulically-set isolation packer, allowing for the collection of brine and/or CO₂ from the injection interval. The U-tube is capable of obtaining brine samples at reservoir pressure at high sampling frequency (approximately one sample per hour) and becomes self-lifting once CO₂ breakthrough occurs. Brine samples are currently being collected using the U-tube and analyzed in the laboratory for compliance purposes and to measure changes in pH associated with arrival of the dissolved CO₂ plume prior to free-CO₂ breakthrough. Assuming CO₂ breakthrough occurs at the observation well, gas-phase tracers will be introduced into the CO₂ at the injector and tracer arrival times will be measured using the U-tube to determine interwell saturations, helping to characterize reservoir dynamics. The project team is also comparing different fluid sampling methodologies as part of this research project to determine the best available practices for sampling deep wells. Fluid samples collected from great depth, depressurize and cool, potentially resulting in changes in brine hydrochemistry, which is not desirable from a characterization or compliance standpoint. Therefore, the study compares water quality results by collecting samples using four sampling methodologies including: 1) the U-tube; 2) Kuster sampler lowered into the well using a slickline; 3) pumping using an electrical submersible; and 4) N₂ gas lift. In general, samples collected using the Kuster and u-tube show the least amount of impact on groundwater pH and reactive species (e.g., carbonates), whereas pumping and gas lift sampling should be limited to major and unreactive solutes.

Deployment of the MBM system in the well was facilitated by bundling the seven control lines together in a polypropylene-jacketed flatpack typically used in off-shore applications. Bundling the control lines into one flatpack reduced the number of spools required from 8 to 2 (one flatpack and one control line for the geophones) greatly simplifying the installation and reducing the risk of damaged lines from pinching in the work-over rig slips. In addition, a non-rotating off-center overshot was used to couple the uphole, dual-mandrel hydroset packer assembly to the 450 ft (137 m) long bottom hole assembly consisting of a perforated tail pipe that provided structural support for the instruments and facilitated wire-line logging below the packer. The instrument control lines passed through and were sealed at the packer using bulkhead compression fittings thus isolating the annulus (where pressure monitoring is typically required) from the downhole section potentially exposed to CO₂. Due to the fact that the packer could not rotate after the compression fittings were tightened and seals were made, the overshot assembly located beneath the packer facilitated the direct slip connection of the upper assembly onto the bottom hole assembly without rotating the 2-7/8 inch (7.3 cm) production tubing. Finally, the bottom of the packer was landed at a depth of about 9,400 ft (2,865 m) and the entire completion depth was 9,850 ft (3,002 m), taking approximately four, 24-hr days to install. 

To date, all of the MBM instruments continue to operate reliably. A mobile monitoring office installed on-site has internet connectivity that allows remote access to pressure/temperature and DTS data. The power supply that energizes the heat-pulse FO cable is also remotely operable, allowing periodic heat-pulse tests without requiring on-site personnel. It is anticipated that the MBM will continue to operate throughout the lifetime of the SECARB Anthropogenic Test to provide key observational data to compare with, and to calibrate predictive models of CO₂ plume behavior.

Ref:

Freifeld, B.M., Trautz, R.C., Yousif K.K., Phelps, T.J., Myer, L.R., Hovorka, S.D., and Collins, D., 2005, The U-Tube: A novel system for acquiring borehole fluid samples from a deep geologic CO2 sequestration experiment, J. Geophys. Res., 110, B10203, doi:10.1029/2005JB003735.