CO2 storage and subsurface monitoring: closing Part 8

The 4D signature of CO2 injection

CO2 replaces brine in the saline-aquifer pore space. Under reservoir conditions (1-3 km depth, 35-80 MPa, 50-120°C), CO2 is SUPERCRITICAL: denser than gas (400-800 kg/m³) but less dense than brine (1000-1100 kg/m³). Elastically, CO2 behaves like a gas: low bulk modulus compared to brine. Gassmann fluid substitution predicts:

• Vp drop: ∼10-20% at high saturation (similar to natural gas exsolution from oil).
• Vs relatively unchanged: shear doesn’t care about fluid. Slight decrease from density change.
• Density drop: ∼2-4% depending on saturation and P,T.
• Ip drop: ∼15-25% at high saturation. STRONG 4D signal.
• Vp/Vs drop: ∼5-10%. Fluid-sensitive.

These changes make CO2 plumes highly VISIBLE in 4D amplitude difference: strongly NEGATIVE (Ip dropped) in the plume area, near-zero elsewhere. Sleipner (North Sea, since 1996) showed an initial pancake plume at year 2-3 that grew into a multi-layered cloud by year 20, all clearly imaged in repeat seismic.

Exercise — from baseline to alarm

• Open the widget in Baseline mode, Ip section view. You see the layered subsurface: overburden (yellow-orange shale), the tight caprock (red), the storage aquifer (blue-teal, low Ip), and a lower shale (orange). The injector’s location is marked with a yellow dashed line. No CO2 yet.
• Switch to Early injection. The Ip section shows a small BLUE anomaly forming around the injector in the reservoir — the CO2 plume. Now switch to 4D amp difference: the entire section is gray (no change) EXCEPT the plume area, which shows as dark BLUE (strong negative ΔIp). This is the classic 4D CO2 fingerprint.
• Switch to Inferred CO2 saturation: shows a bright yellow blob at the injector corresponding to where CO2 is. This is what you’d REPORT to the regulator — the CO2 volume and location.
• Switch to Mature storage. The plume has grown into a broad PANCAKE under the caprock, buoyantly migrated to the top of the reservoir. Lateral spread is driven by plume pressure gradient + reservoir permeability. In 4D: a wider blue amp difference signal. In saturation: a wider yellow plume.
• Now the critical scenario: switch to LEAK. The mature plume is still there in the reservoir, BUT there’s also a NEW BLUE ANOMALY in the overburden above the caprock — CO2 that has breached the caprock through a pre-existing fault or compromised wellbore. The alarm banner appears in red at top-right: ⚠ ALARM.
• On the leak scenario, switch to CO2 saturation view: a small column of yellow visible in the overburden, offset slightly from the injector (along a hypothetical fault plane). This is the critical product to hand to regulators.
• Back-and-forth between MATURE and LEAK: same reservoir plume (you’ve been injecting the same amount), but one scenario has containment, the other has a leak. Without 4D seismic monitoring, you would not know which situation you’re in. Project safety depends on it.

Integrated monitoring workflow

A full CO2 storage monitoring program integrates ALL the Part 7 + Part 8 workflows:

• Pre-injection (Parts 7 + 8.1 + 8.3): (a) 3D baseline seismic with full QI workflow (inversion + RPT + facies); identify storage reservoir and caprock. (b) Geomechanical model: σv, σH, σh maps; fault locations; caprock integrity threshold. (c) Anisotropic velocity model (§8.3) for accurate depth imaging. (d) Baseline petrophysics and saturation from any analog wells.
• Injection commissioning: inject slowly under regulatory oversight. Downhole pressure gauges track Pp in real time. Microseismic network listens for induced events.
• Year-1 monitor survey (§8.2): first 4D acquisition. Verify that the plume has formed as expected (small, contained). Calibrate the rock-physics model against observed 4D magnitude.
• Year 3-5 monitors: repeat 4D every 2-3 years. Track plume growth. Each monitor updates: plume volume, saturation map, pressure buildup. Integrate with reservoir simulation to forecast future plume evolution.
• Alarm protocols: any 4D signal in the OVERBURDEN (above caprock) triggers investigation. Stop injection, increase seismic coverage, characterize leak pathway, plan remediation.
• Post-injection monitoring (regulatory requirement, 20-50 years): continue 4D surveys at declining frequency (every 5-10 years). Verify CO2 stability after injection stops. Government transitions liability to state after verification period if no leaks.

What’s at stake: the business and regulatory picture

CO2 storage is a regulated activity with multiple stakeholders:

• Regulators (EPA in US, OSHA + NEA in Norway, etc.) require 4D monitoring as a permit condition. The frequency and methodology are specified.
• Operators carry technical risk and economic liability. Proper monitoring = defensive documentation; leaks = remediation costs + reputational damage + possible criminal liability.
• Insurance carriers require 4D monitoring records to maintain coverage. An injection project without a monitoring plan is uninsurable.
• Carbon markets: stored CO2 qualifies for carbon credits ($50-200 per tonne depending on market) only if it’s VERIFIED to stay stored. 4D seismic is the verification method. A leak = the stored carbon is uncredited and the credits must be returned.
• Public: local communities have concerns about groundwater contamination (if CO2 leaks into drinking-water aquifers) and induced seismicity. Transparent monitoring builds trust.

A 1-million-tonne-per-year CO2 storage site has carbon-credit revenue of $50M-$200M per year. The 4D monitoring program costs ∼$2-5M per year. The ratio alone justifies the investment. And the cost of a single documented leak (remediation, lost credits, penalties) typically exceeds 10 years of monitoring costs.

Real-world case studies

• Sleipner (Norway, since 1996): world’s first commercial-scale CO2 storage. >20 MT stored. Annual 4D acquisition since start; over 20 monitoring surveys. Plume clearly imaged, evolving into a multi-layered structure. No leakage detected. Reference project for the industry.
• Snohvit (Norway): storage in deeper Tubaen Formation. Unexpected pressure buildup after 2 years; monitoring detected this early; injection was redirected to preserve storage integrity. 4D worked.
• In Salah (Algeria, 2004-2011): storage with surface heave monitoring via InSAR + 4D seismic. Detected fault-related pressure communication. Injection reduced to avoid caprock compromise. Closed in 2011 after storage target reached.
• Weyburn-Midale (Canada): CO2-EOR (enhanced oil recovery) combined with storage. Largest CO2 volume stored to date (>30 MT). 4D monitoring has tracked plume progression over 20 years of injection.
• North Sea Net Zero: the Endurance, Acorn, and several other projects under active development. Expected to inject 10-50 MT/year at full scale. 4D seismic monitoring plans in active permitting.

Common risks and their detection via 4D

• Caprock integrity failure: CO2 breaches the caprock seal and enters overburden. DETECTION: 4D amp difference anomaly ABOVE the caprock; any negative ΔIp in the overburden is the alarm.
• Wellbore integrity failure: CO2 migrates UP an abandoned well (common pathway). DETECTION: localized 4D anomaly offset from injector, aligned with known or suspected well locations. Requires extensive pre-injection wellbore-integrity survey.
• Fault reactivation: pressure buildup from injection triggers slip on a pre-existing fault, opening a leak pathway. DETECTION: microseismic detection (separate monitoring technology) correlated with 4D amplitude changes along fault planes.
• Overpressuring: injection rate exceeds reservoir capacity; pressure buildup threatens caprock integrity. DETECTION: time-shift in 4D (compaction from elevated effective stress); pressure gauges in monitoring wells.
• CO2 dissolving / fixing: over long timescales, CO2 dissolves in brine and eventually mineralizes. This REDUCES the 4D signal (dissolved CO2 doesn’t produce the same Ip drop). DETECTION: 4D amplitude decrease over long time — actually a positive sign (CO2 is becoming more permanently stored).

The way forward: QI for the energy transition

This textbook has covered 3D seismic interpretation from fundamentals to the frontier. QI as a discipline is not just about finding oil and gas. Increasingly, it’s about:

• CO2 storage: the largest growth area. CCS projects are being approved worldwide.
• Geothermal energy: reservoir characterization for engineered geothermal systems, fracture-controlled reservoirs.
• Hydrogen storage: salt caverns (simpler) and depleted gas reservoirs (requires QI verification of seal integrity).
• Groundwater management: aquifer characterization, saltwater intrusion monitoring, managed aquifer recharge.
• Critical minerals exploration: copper, lithium, nickel — many are found in deposits that seismic can help characterize.
• Construction-ready subsurface imaging: tunnels, underground storage, large foundations, nuclear waste repositories.

The QI workflow you’ve learned — data → inversion → rock-property transforms → probabilistic classification → volumetrics → monitoring — is REUSABLE across all these applications. The specific rock physics and mathematics change; the discipline and the respect for uncertainty carry over. Welcome to the post-textbook phase of your QI education.