# Presentation: 875246

## G. Michael Hoversten
Erika Gasperikova

Lawrence Berkeley National Laboratory

- Erika Gasperikova
- Lawrence Berkeley National Laboratory

- Novel Geophysical Monitoring Techniques for CO2 Sequestration

## Outline

- Project plan
- Geophysical monitoring examples for test site
- Tilt measurements
- Gravity
- Crosswell EM, surface electric dipole EM
- Self Potential
- Future work on Schrader Bluff Model
- Seismic properties of CO2 saturated rocks
- CO2-hydrocarbon gas mixtures
- Effects of residual hydrocarbon gas
- Conclusions

## Plan for MonitoringTechnique Comparison

- Leverage CCP project with GeoSeq & NGOTP (National Gas & Oil Technology Partnership) projects
- Make numeric estimates of CO2 saturations to quantify resolution of techniques through inversion where possible
- Use existing flow simulation models until a realistic large scale model could be obtained
- Liberty Field, South Texas, GeoSeq demo
- Thin sand (10m) not representative of large scale sequestration
- This has highlighted Tilt & SP as having potential
- Acquire and validate SP modeling
- Debug & validate 3D controlled source EM modeling code
- Schrader Bluff Alaska model now being built
- Realistic depths and volumes
- Compare non-seismic techniques on this model

## 5 & 100 days calculations

- Background from interpolated logs
-  Flow simulation is inserted
-   conductivity ~ 25%
- EM & DC average over large volume
-  velocity ~ 3%
-   density ~ 1.5%
-   Acoustic Impedance ~    5%
- Seismic reflection sensitive to changes at interfaces
- Largest contrast at upper boundary

**Plan view**

**North-south**
**cross section**

**Injection well**

## Tilt & Gravity measurements

- Gravity shows CO2 moving to the north & west from injection well
- Shows current location of CO2
- Pressure change inversions from surface tilt show the high-permeability zones in injection unit
- Indicates where future flow will occur

## Gravity Response

- Triangular 3D disk
- surface at 1000 & 2000m
- 100% CO2 in 20% porosity
- 240m radius – 100m thickness would contain 41 days CO2 production from 1000 MW  coal fired power plant
- (US average 948kg/MWh)
- Gravity response from deep storage sites will only be measurable for large volumes
- Generate matrix of Peak and half-width gravity response as function of Density contrast, Depth, Volume (inverted cone model)
- Detection & simple measure of resolution
- Inversion will provide quantitative estimates of resolution

## Predicted RCO2 & Geophysical Images

- Individual geophysical images are fuzzy and inconsistent
- Sensitive to combinations of reservoir parameters
- RCO2 image has high spatial resolution and strong correlation with injected CO2
- Separating Pp & Sw effects allows “seeing” the CO2 effects

## 3D EM Inversion

- Previous water flood model
- CO2 & Oil are equivalent in Archie’s Law so results are relevant for CO2 in brine formation
- Maps Sw out-of-plane of crosswell section
- In results to date - 10% maximum error in region bounded by wells
- Surface grounded electric dipole source has been added for model study of Schrader Bluff

## Self-potential Sources

- The multiplicity of sources can be either an advantage or a disadvantage.  On the one hand, a number of phenomena can be studied with the technique, on the other hand, the possibility of a number of different sources can sometimes be confusing. – Sill (1983)
- two electrolytes of different concentration being in contact with one another
- electrolytes flowing through a capillary system or porous media
- an electrolyte in contact with a solid
- electromagnetically induced telluric currents

## Electric Potential from fluid flow

- Coupled flow equations simplified
- When the effect of the induced electric potentials() on the primary fluid flux () is small the coupled flow equations decouple to give (1) & (2)
- With no impressed current (J)
- convection & conduction currents balance resulting in (3)
- The coupling coefficient L between flow and potential is not characterized for CO2
- L from the literature
- Water:               - 15
- Benzene:           - 100
- The more resistive the fluid the larger L

- Convection
- current

- Conduction
- current

## Liberty Field SP model

- 2D solution of coupled flow & electric potential problem
- CO2 injection rate
- 1.6 l/s (80,000 kg/day)
- 4.0 l/s (250,000 kg/day)
- CO2 properties
- density   = 788 kg/m3viscosity = 73 uPa-s

## SP Response

- Liberty Field
- 250,000 kg/day (4 l/s)
- 10m thick layer
- 20% porosity
- 150 mDarcy
- Depth 1500m
- Scleipner
- 106 tons/year (40 l/s)
- 200m thick layer
- 20% porosity
- 1 Darcy
- Depth 1000 m

##  SP spatial response

- Layer truncated at 300m
- Increased response due to increased fluid flux at constant injection rate
- 10 mV anomaly caused by layer truncation
- Spatial information about flow structure causes measurable signal
- SP inversion will quantify what parameters can be extracted from surface SP measurements in complex structure

## SP future direction

- Lab measurements of Coupling Coefficient L
- CO2 in brine-saturated sandstone
- Small scale field test at Liberty Field site
- 2D  surface lines in time-lapse mode ~ $15K
- If L is large enough & field test sees the injection
- Couple SP calculations to Tough2
- Develop inversion code for SP

## Schrader Bluff CO2 Study

**West Sak/Schrader**
**Core Development Area**

- 26

- 0

- Miles

- ANWR

- Prudhoe Bay

- Endicott

- Point Thompson

- ANWAR

- Milne Point

- North Star

- Liberty

## Schrader Bluff Formation

- 3D Flow simulation of CO2 injection into the OA & OB sands
- Petrophysical lab measurements & log data are input to generate a rock-properties model to relate flow simulation to geophysical models
- Geophysical modeling
- Cost estimates

## Schrader Bluff study

- Surface Tilt
- Self Potential
- Will require some laboratory measurements of coupling coefficients
- Gravity and gradients
- Surface controlled source electromagnetic imaging

## Conclusions (1)

- Gravity, Tilt and SP (of the non-seismic methods) may be combined to provide sufficient information about aerial distribution of injected CO2
- Gravity will probably not be sensitive enough for deep (2000m) reservoirs, but may be used for shallow (1000m) reservoirs
- Large quantities may still be seen at 2km depth
- Surface EM & MT probably don’t offer the cost/benefit ratio necessary
- Controlled source electric dipoles will be investigated because they offer the best spatial resolution of all surface EM techniques
- Cross-well seismic & EM may play a limited role in high-resolution characterization studies
- Use in initial demonstration projects for assurance
- Used at start-up of projects for assessment of flow simulation predictions

## Conclusions (2)

- SP looks promising enough from simple models to pursue
- Lab measurements of coupling coefficients
- Field test at Liberty Field
- Seismic properties of CO2 are more complex than hydrocarbon gas
- Stronger pressure-temperature effects
- Multi-valued bulk modulus as function of pressure
- Saturation levels of CO2-hydrocarbon gas mixtures in shallow reservoirs can not be uniquely determined by velocity alone
- Residual gas saturation significantly reduces the Vp sensitivity to changes in CO2 saturation

## Effect of Residual Gas Saturation

- Reflection Coefficient at top of brine filled sand embedded in Gulf Coast Shale
- Effect of residual Sg is greatest above CO2 phase transition depth, where CO2 is in gas state
- Residual Sg reduces the effects of changing SCO2
- Residual Sg causes R vs. SCO2 to be more linear

**Quartz Sand 20% porosity, Sg = 0**

**Quartz Sand 20% porosity, Sg = 0.01**

## CH4-CO2 mixtures

- Kmix vs. CO2-CH4 mole fraction is multi-valued for pore pressures between 10-15 MPa (1450-2175 psi)
- Hydro-static pressure between
-       3000-4000 ft
- Higher molecular weight hydrocarbons – CO2 mixtures behave similarly

- Assuming 1 psi/ft overburden pressure, bulk Vp is multi-valued below 11 MPa effective pressure
- This effect will complicate interpretation of CO2 saturations from Vp changes in shallow reservoirs