Numerical Simulation of Basal Aquifer

Depressurization in the Presence of

Dissolved Gas

Karl P. Lawrence, Ph.D.

Rock Engineering Division

Golder Associates

Mississauga, Ontario, Canada

Outline

Motivation:

Walday Abeda, Canadian Natural

Background:

The importance of basal aquifer depressurization in oil sand mining and

the issue of dissolved gas

Problem Statement

Approach:

Sampling for Dissolved Gas;

Groundwater (GW) Modelling

Results & Conclusions

April 17, 2012 2

Motivation

Walday Abeda (Canadian Natural)

April 17, 2012 3

Background Basal Aquifer Depressurization and Dissolved Gas

April 17, 2012 4

Background - Hydrogeology

April 17, 2012 5

Oil Sand

Water Sand

Background – Aquifer Isopach

April 17, 2012 6

Basal Aquifer

thickness and mine

footprint

Background – Standard Modelling

General Modelling Process:

Construct 3D models to

simulate static groundwater

conditions and

transient flow

Calibrated to static/

pre-mining conditions

Predictive simulations to determine

necessary flow rates/volumes/schedules and number/location of DP wells

April 17, 2012 7

Background – Challenge of Gas

DP modelling is typically performed assuming single-phase (water) flow.

The concentration and composition of gas dissolved in the groundwater

may cause near well “gas-locking” during pressure reduction:

DP wells depressurize aquifer;

Pore pressure reductions lower the gas solubility;

When pressure reduces below critical value (“bubble pressure point”),

exsolution occurs; and

The effects include increase in effective specific storage or decrease in

effective permeability.

April 17, 2012 8

Background – Challenge of Gas

April 17, 2012 9

PW-A OW-A

THEORETICAL

DRAWDOWN CONE PW-A OW-A

BUBBLE PRESSURE

POINT

NO GAS WITH GAS

Background - Gas Locking Indicators

April 17, 2012 10

0

5

10

15

20

25

30

35

40

45

50

55

60

65

70

0 30 60 90 120 150 180 210 240 270 300 330 360 390 420 450 480 510 540 570 600 630 660 690 720 750 780 810 840 870 900

Elapsed Time (min)

Dra

wd

ow

n (

m)

DP-06 Datalogger OBW @ DP-06 Datalogger OBW @ DP-06 Manual DP-06 Manual

Canadian Natural Horizon Oil Sands Project

2007 Basal McMurray Watersands Drilling and Testing Program

DP-06 Arithmetic Plot

Prepared By: Prepared For:Figure No.

Separation between pumping well and observation well = 31.0 m

Q1 = 157 m3/d (24 igpm)

Q2 = 255 m3/d (39 gpm)

Q3 = 360 m3/d (55 igpm)

Q1

Q2

Q3

22

Available drawdown to pump intake = 65.6m

0

5

10

15

20

25

30

35

40

45

50

55

60

65

70

0 30 60 90 120 150 180 210 240 270 300 330 360 390 420 450 480 510 540 570 600 630 660 690 720 750 780 810 840

Elapsed Time (min)

Dra

wd

ow

n (

m)

DP-03 Datalogger OBW @ DP-03 Datalogger OBW @ DP-03 Manual DP-03 Manual

Canadian Natural Horizon Oil Sands Project

2007 Basal McMurray Watersands Drilling and Testing Program

DP-03 Arithmetic Plot

Prepared By: Prepared For: Figure No.

Separation between pumping well and observation well = 31.0 m

Q1 = 157 m3/d (24 igpm)

Q2 = 333 m3/d (51 gpm)

Q3 = 655 m3/d (100 igpm)

Q4 = 982 m3/d (150 igpm)

Available drawdown to pump intake = 65.1m

Q1

Q2

Q3

Q4

13

No gas locking

DP well appeared to become gas locked

1) Excessive gas noted in discharge

water.

2) Inflection point in drawdown curve

indicative of change in “effective”

permeability

Background - Effect of Gas Locking

DP wells have a reduced efficiency and area of influence; DP is locally

limited;

Additional DP wells may be required to achieve target water level

elevations;

Failure to identify dissolved gases can result in:

1. Significant scheduling delays

2. Operational issues

3. Stability issues

4. And ….

April 17, 2012 11

Problem Statement

April 17, 2012 12

Problem Statement

Existing GW Flow Model and DP Well Network

If we do not account for dissolved gas in flow model predictions:

Discharge water volumes are over-estimated;

Rate of depressurization is over-estimated; and potentially

Number of required wells is under-estimated.

Primary objectives of numerical simulations:

1. Simulate the effects of gas-locking of DP-wells;

2. Design a DP well network capable of achieving target DP; and

3. Maintain safe workplace; save money; avoid scheduling delays.

April 17, 2012 13

Problem Statement

This is a reservoir engineering challenge applied to a large scale

groundwater flow modelling and mine planning scenario. Sampling

techniques to quantify gas were available from the oil industry but they

needed modification. Groundwater flow modelling needed to account for

variable permeability. A multi-disciplinary approach was required and a

meeting of minds to develop a management tool for Canadian Natural.

April 17, 2012 14

Approach

Characterization of Gas

April 17, 2012 15

Approach – Characterization of Gas

The primary milestones in developing a management tool were:

Characterization of dissolved gas through collection of pressurized

field samples and laboratory analysis;

Characterization of dissolved gas/effective permeability relationships

through multi-phase flow modelling to generate relative permeability

curves (Canadian Natural reservoir engineering); and

Incorporation of gas effects in groundwater flow modelling:

Standard approach if aquifer pressure > bubble point pressure

Modified approach with transient effective permeability when aquifer

pressure < bubble point pressure (based upon relative permeability

curves)

April 17, 2012 16

Approach – Characterization of Gas

AGAT Laboratory modified an oil

field bottom hole sampler for low

pressure conditions.

Sample of water collected under low

flow conditions at ambient aquifer

pressure adjacent to well screen.

Estimate saturation pressure (about

300 kPa) and gas composition.

Open top well makes

characterization approximate

however, consistency obtained

across site

April 17, 2012 17

Approach

Modified Groundwater Flow Model

April 17, 2012 18

Approach – Modified Model

Needed to modify existing groundwater flow model (MODFLOW) to

account for dissolved gas issues and transient effective permeability.

Modification involved:

Interruption of MODFLOW simulation at discrete time steps;

Automated update of hydraulic conductivity during discrete steps based

upon:

Current aquifer pressures; and

Permeability reduction relationship.

April 17, 2012 19

Approach – Modified Model

April 17, 2012 20

210

220

230

240

250

260

270

280

290

300

0 50 100 150 200 250 300 350

Co

rre

cte

d H

ead

(mas

l)

Time (min)

SIM-NO GAS LOCKING

DP-03-MEASURED

DP-03-SIM

DISCRETE STEPS

OBW4-MEASURED

OBW4-SIM

Approach – Modified Model

Relative permeability modification incorporated;

Steady-state calibration performed;

Transient calibration based upon:

Two DP well testing programs; and

History match performed based upon 1.5 years of historical pumping

data.

April 17, 2012 21

Time = 0 days

Vertical well

DP Well Rate

(m3/day)

DP-01 500

DP-02 50

DP-03 200

DP-04 250

DP-05 300

DP-06 300

02

01

03 04

05

06

Approach – Modified Model

Effect on K

Time = 5.3 days

Approach – Modified Model

Effect on K

Time = 9.4 days

Approach – Modified Model

Effect on K

Time = 13.5 days

Approach – Modified Model

Effect on K

Time = 17.6 days

Approach – Modified Model

Effect on K

Time = 23.9 days

Approach – Modified Model

Effect on K

Time = 30.3 days

Head minimum at wells

Approach – Modified Model

Effect on K

Approach – Modified Model

Effect on Depressurization

Permeability Relationship:

30% Reduction in Permeability / 10 m head reduction

Pump for 30 days:

Permeability reduction results in

steeper drawdown curve;

Radial extent of depressurization

reduced.

Example

Head at pump = 210 masl

Head at 8 m from well: 222 masl

Head at 25 m from well: 225 masl

225 230

220

225

Approach - Sample WL Contours

Vertical Wells

NO GAS LOCKING WITH GAS LOCKING

Head reduced to ~ 200 masl Head reduced to ~ 200 masl

at DP wells but only to ~ 230 masl

elsewhere

Approach - Sample WL Contours

Horizontal Wells

NO GAS LOCKING WITH GAS LOCKING

Head reduced to ~ 200 masl Improved performance over

vertical wells;

Results

Modified Groundwater Flow Model

April 17, 2012 32

Results – Model Sequencing

April 17, 2012 33

Area 1: History

Match

Area 6

Area 2

Area 3

Area 4 Area 5

Results

Modified Modflow approach

calibration:

Gas-locked DP well field and

simulated test showed an

excellent match.

Satisfactory match to complicated

DP well pumping history involving

33 scheduled stress (pumping)

periods; for

19 pumping wells.

April 17, 2012 34

-700

-600

-500

-400

-300

-200

-100

0

0 100 200 300 400 500 600 700 800 900 1000

Pu

mp

ing

Rat

e (

m3/

day

)

Time (days since startup)

DP-01 DP-02

DP-03 DP-04

DP-05 DP-06

DP-07 DP-08

DP-10 DP-12

DP-13 DP-14

DP-15 DP-16

DP-17 DP-18

DP-19 DP-20

DP-32 G10-044

G10-047 G10-049

G10-050 G10-058

G10-059

0

200

400

600

800

1000

1200

1400160

180

200

220

240

260

280

300

Rate

(m

3/d

ay)

Ele

vati

on

(m

)

South Pit Basal Aquifer Piezometric Heads

G10-041 G10-041-SIM

G10-042 G10-042-SIM

G10-043 G10-043-SIM

G10-045 G10-045-SIM

G10-057 G10-057-SIM

RATE - AREA 1 RATE - AREA 2

Results

Areas 3 and 4 were grouped due to aquifer connectivity

April 17, 2012 35

Existing/Planned Wells

Additional wells required

due to gas-locking

+

+

An additional 7 to 8

pumping wells were

required in each modelled

area to reach the DP target

Results

Sequential mining stages modelled;

Channelized aquifer was advantageous in that aquifer lobes could be

isolated (as a result of gas-locking).

We could use gas locking to our advantage to cut off the pressure

support to portions of aquifer requiring ongoing pit perimeter

depressurization;

Approximately twice as many wells were needed than originally planned;

and

Horizontal wells could potentially improve depressurization.

April 17, 2012 36

Conclusions

A pro-active approach was taken in addressing dissolved gas.

Other sites may experience the same issues in the future.

Substantial scheduling delays and

costs can be incurred if dissolved

gas is not identified and considered

in the pre-construction engineering

analysis.

Characterization of dissolved gas

is key!

April 17, 2012 37

Future (On-going) work

Discrete nature of MODFLOW modification presents subjective

interpretations;

Options:

1. Modify MODFLOW source code – achievable but pre/post-processing

issues arise;

2. Port modelling to FEFLOW – additional modules are permitted through

IFM to induce continuous effective permeability reduction in time

(without interruption to the simulation)

Option 2 is the focus of current work.

April 17, 2012 38

Acknowledgements

• Canadian Natural Horizon

• Co-authors:

Ken Baxter, Golder Associates

Walday Abeda, Canadian Natural

Walter Alexandru, Canadian Natural