# Presentation: 231279

## G. Michael Hoversten1
Roland Gritto1
John Washbourne2
Thomas M. Daley1

1 Lawrence Berkeley National Laboratory USA
2 TomoSeis Inc

Seventy Second Annual meeting, Society of Exploration Geophysicist
October 6-9 , 2002 Salt Lake City

- Roland Gritto1
- John Washbourne2
- Thomas M. Daley1
- 1 Lawrence Berkeley National Laboratory USA
- 2 TomoSeis Inc
**Seventy Second Annual meeting, Society of Exploration Geophysicist**
**October 6-9 , 2002 Salt Lake City**

- Fluid saturation and pressure prediction in a multi-component reservoir by combined seismic and electromagnetic imaging

**Notes:**
This presentation will describe a time-lapse crosswell seismic & electromagnetic imaging experiment carried out during CO2 injection in a producing oil field.

Just as in CO2 sequestration into a depleted oil and gas reservoir, this represents a complex multi phase fluid environment where the geophysical signature is a function of many changing reservoir parameters.  

The objective of the talk is to demonstrate a methodology for isolating the effects of the CO2 so that we can accurately image the spatial location of the injected CO2 and quantify the quantities in place.

## Objectives

- Improve spatial resolution of CO2 placement prediction
- When compared to interpreting time-lapse geophysical images
- Provide quantitative predictions of the amount of CO2 in the formation

## Outline

- Define the geophysical problem
- Summary of the process
- Description of the Experiment
- Rock Properties Model
- Data processing for CO2 quantity and location
- Conclusions

**Notes:**
I can not cover all of the details in the time allotted so I will concentrate on the main features and results.   All of the relevant details are given in the 4 page expanded abstract which can be seen online on the conference web page.

## Definitions

-    		Change in
-  Vp 		Compressional velocity
-  Vs 		Shear velocity
-   Electrical conductivity
-  Pp 		Pore pressure
- Fluid properties are a function of Pp
-  Peff		Effective pressure (lithostatic –Pp)
- Dry frame properties are a function of Peff
-  SCO2  	CO2 fluid saturation
-  RCO2  	CO2/oil ratio (the amount of CO2 dissolved in oil)
-  Sw 		Water saturation
-  Sg		Hydrocarbon gas saturation

## Separating Fluid effects with geophysical data

- +Sw

- -Sw

- -Pp

- +Pp

**Open: S****g****=0**

- Additional data is required to separate Pp, Sw, Sg

- Changes are relative to reference values of Pp & Sw
- Conditions prior to CO2 injection
- With no gas, Vp – Vs crossplot  provides unique Pp & Sw values

## Separating fluid effects

- Estimate Sw from electromagnetic (EM) 
- independent from the seismic
- Estimate Pp form seismic Vs & Sw
- Estimate      from  Sw  and  Pp
- Estimate residual            =              -
- Residual is a function of RCO2  & SCO2
- Use             to estimate RCO2SCO2
- Assumptions
- Initial Sg equal to average log value
- Partitioning between RCO2SCO2

## Effects of in-situ gas on 

-  Vp = f(P, Sw, Sg)
- Initial hydrocarbon gas (CH4) reduces the Vp caused by P & Sw
- If we do not account for Sg that is present,                will be positive
- There is a strong correlation between logged Sg and +
- Recalculate                  assuming
- Sg = 0.02 where                  is positive

## CO2 Pilot Well Pattern

- 4 H2O injectors converted to CO2 injectors
- Observation wells OB-C1 and OB-C2 for crosswell geophysics
- Hydraulic fractures for 11-8W &12-7W connect
- Surface tilt measurements
- Injected tracer testing

- Crosswell
- Image Plane

- Hydraulic
- Fractures

## Linking the reservoir & geophysics

- Velocity
- Hertz-Mindlin & Hashin-Strikman (effective dry rock bulk modulus)
- Soft rock, dry frame K(Peff)
- Gassmann (fluid substitution)
- Density
- Mixing law
- Electrical Conductivity
- Archie’s Law
- Simplex inversion for model parameters
- Input (, Sg, Sw, P,T,oil API, brine salinity)
- Output (grain - ,,K, critical gas density, Archie parameters)

## Core measurements validate rock-properties model

- Based on core measurements
- Z. Wang
- Oil & Brine Saturated
- 5% velocity anisotropy
- matches tomographic velocity anisotropy for crosswell data
- Model matches vertical lab Vp
- sonic log measures vertical Vp
- The model predicts Pressure effects

## Time-lapse Geophysical signature

- Starting Vp from 
- Most a priori  information
- Starting Vs from Vp
- High correlation between  and permeability log
- 5% V anisotropy matches core anisotropy
- Correlation between mapped fault and Vs & Vp
- Pressure effect
- Large changes above injection interval

## Reference Values & Sensitivities

- Reference Pp from production history matched flow simulation up to the start of CO2 injection
- Reference Sw, Sg and Porosity are set to average log values in the reservoir at the start of CO2 injection
- Sensitivity of predicted RCO2 to reference values (in order of importance):
- Sg
- Sw
- Pp
- Porosity

## Linking  geophysics to  reservoir

- Linking  geophysics to  reservoir

**Obs.**** ******

****** ****- ********S****w**

**Calc. ********S****w**

**Obs. ********V****s**

**Calc. ********P****eff**

**Obs. ********V****p**

**Residual ********V****p**

******V****s**** = f(********P****eff****,********S****w****)**

**Petrophysical**
** ****Model + **
**Calc. ********P & ********S****w**

**=**

**Calc. ********V****p**

**Obs.-Calc. V****p**

**S****g****=0.02**

**S****g**** = 0.0**

******V****p**** (-)**

******V****p**** (+)**

******R****CO2**

## 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

## Conclusions (1)

-  Vp & Vs are not sufficient to determine
- P, Sw and the effects of gas (CO2 or CH4)
- without assumptions, or more data
- Electromagnetic imaging of  can provide an independent estimate of Sw
- A detailed rock-properties model is required
- links Vp, Vs & to P, Sw and RCO2
- Predictions are only as good as the rock-properties model
- Errors in reference values of fluid saturations have a larger impact on predicted changes in RCO2 (SCO2) than errors in reference pressure or porosity

## Conclusions (2)

- Removing the P and Sw effects from Vp allows interpreting the residual Vp for effects due to CO2
- Quantitative estimates of CO2 in place
- Better spatial resolution
- Quantitative monitoring of CO2 in oil & gas reservoirs requires multiple geophysical and wire line data sets to separate the complex effects of pressure and the multiple fluid saturations
- Repeat logs provide better starting models for inversion and constrain parameters such as *in-situ* Sg

## Acknowledgements

- Support was provided by the Assistant Secretary for Fossil Energy, Office of Coal and Power Systems through the National Energy Technology Laboratory of the US Department of Energy under Contract No. DE-AC03-76SF00098
- Chevron Petroleum Co. provided site access and data
- Mike Morea
- Electromagnetic Instruments provide crosswell EM data
- Mike Wilt