numerical simulation of basal aquifer depressurization in the … · 2020. 12. 16. · feflow model...
TRANSCRIPT
-
Numerical Simulation of Basal Aquifer Depressurization in the Presence of Dissolved Gas – An Update
Rudy Maji, Ph.D. Karl Lawrence, Ph.D. Ken Baxter, M.Sc., P.Geol. Scott Morton, P.Geol.
-
What to Take Away From this Presentation
Reservoir modelling typically considers multiphase flow;
Groundwater modelling has only dealt with single phase flow;
A regional scale reservoir model is required to address the dissolved gas issues during basal aquifer depressurization in advance of mining;
Originally, this challenge was addressed through a modified application of MODFLOW (finite difference approach);
This presentation provides the results of a better approach developed using FEFLOW (finite element approach).
April-16-14 2
-
Outline
Background
Key Questions to be Answered
Ground Water Modelling Approach Model Construction Model Calibration Prediction Results
Summary and Conclusions
April-16-14 3
-
Background
April-16-14 4
-
Background – Project Location
Calgary, Alberta, Canada
Fort McMurray, Alberta, Canada
April-16-14 5
-
Background - CNRL Horizon Oil Sands Project
Located about 75 km north of Fort McMurray in northeastern Alberta
Open pit mining operation and oil sands plant for extracting and upgrading bitumen from the McMurray Oil sands deposit.
April-16-14 6
-
Background - CNRL Horizon Oil Sands Project
Prior to mining the piezometric elevations within primary aquifer varied between 276 m to 280 m asl
An aquifer depressurization plan consisting of 6 pumping wells were planned around the South Pit to depressurize the basal aquifer
April-16-14 7
-
Background - CNRL Horizon Oil Sands Project
DP network operational since 2009
In early 2012, the DP well network consisted of 35 vertical in-pit pumping wells and 2 horizontal pumping wells
Depressurization has been achieved in the area of active mining.
2006 DP Well Network Planning
2009 DP Well Network Operational
2010-2011 Mine Sequence Excavation
April-16-14 8
-
Background – Why? Gas Locking
If there are substantial amounts of dissolved gas in the groundwater, “gas-locking” may occur during the depressurization of aquifer
Pore pressure reductions lower the gas solubility;
When pressure reduces below critical value (“bubble pressure point”), exsolution occurs;
Presence of gas ↑ effective specific storage or ↓ effective permeability;
Permeability reduction results in steeper drawdown curves and reduced depressurization extent.
April-16-14 9
-
Background – Effects of Gas Locking
Reduction in DP well efficiency – depressurization is limited locally;
Additional DP wells may be required to achieve the target water level elevations.
Cost and schedule implications.
April-16-14 10
-
Background – Basal Aquifer Isopach
Mining starts from the west edge of the pit
South Pit Contours
April-16-14 11
-
Background – Key Indicators
0
5
10
15
20
25
30
35
40
45
50
55
60
65
700 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
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
April-16-14 12
-
Key Questions
April-16-14 13
-
Key Questions to be Answered
How many wells are required to be installed in the DP network?
How much water to be handled?
April-16-14 14
-
Modelling Approaches
April-16-14 15
-
Groundwater Modelling Approaches
Develop Regional Model in Multiphase Flow Modelling Framework. Numerically complex; Computationally memory extensive.
Develop Alternative Model to Build the Solution for Gas Locking Problem
in the Single-Phase Modelling Framework; Easy to solve using an existing regional model; Computationally less intense.
Regional Model in Visual MODFLOW; Regional Model in FEFLOW.
April-16-14 16
-
Single Phase Flow Modelling - MODFLOW
MODFLOW model was revised to account for dissolved gas issues The hydraulic conductivity values were revised based on the
simulated pressure at discrete time steps.
April-16-14 17
-
MODFLOW Approach – Discrete Reduction
210
220
230
240
250
260
270
280
290
300
0 50 100 150 200 250 300 350
Corr
ecte
d H
ead
(mas
l)
Time (min)
SIM-NO GAS LOCKING
DP-03-MEASURED
DP-03-SIM
DISCRETE STEPS
OBW4-MEASURED
OBW4-SIM
18 April-16-14
-
Single Phase Flow Modelling - FEFLOW
An external module was developed in FEFLOW Automated update of hydraulic conductivity during each timestep
based on: Aquifer pressures; and Relative permeability relationship.
April-16-14 19
-
FEFLOW Approach - Continuous Reduction
20 April-16-14
0
200
400
600
800
1000
12000
10
20
30
40
50
60
70
0 100 200 300
Time [min]
Pum
ping
Rat
e [m
3/da
y]
Draw
dow
n [m
]
DP-03
DP-03 - SIM
OBW
OBW - SIM
Pumping Rate
-
Single Phase Flow Modelling - FEFLOW
Gas characterization Pressurized sampling
Multi-phase flow (reservoir) modelling CNRL
Relative permeability relationship 30% reduction in permeability for every 10 m reduction in head below the bubble point pressure (BPP)
April-16-14 21
-
Single Phase Flow Modelling - FEFLOW
𝐾𝐾 = 1 − 𝑝𝐻−ℎ𝐷 × 𝐾 … obtained from the relative permeability and
pressure saturation relationship
p% of permeability reduction was found for D m of pressure drop
H = 240 masl (BPP) h = head in each node K = Original Hydraulic Conductivity
April-16-14 22
-
Calibration to a Pumping Test
April-16-14 23
With Gas-Locking – 30% reduction in K per 10 m
0
200
400
600
800
1000
12000
10
20
30
40
50
60
70
0 100 200 300
Time [min]
Pum
ping
Rat
e [m
3/da
y]
Draw
dow
n [m
]
DP-03
DP-03 - SIM
OBW
OBW - SIM
Pumping Rate
Calibration (Well Head)
Observed
Simulated
-
Comparison Between Approaches
FEFLOW Model
Reduction in effective permeability is continuous (i.e. at every timestep) Implemented through external module (IFM) which is linked to FEFLOW at run-time separate module created to
reduce K
shorter runtimes
minor user interaction (only need specify K reduction function)
Visual MODFLOW Model
Reduction in effective permeability is discrete (i.e. not at every timestep) The K field was revised at discrete timesteps based on the simulated heads and bubble point pressure model run was interrupted to
reduce K
longer runtimes
major user interaction
April-16-14 24
-
History Match
April-16-14 25
-
History Match – Pumping Wells
Pumping schedule irregular
Horizontal wells are
modelled
40 scheduled stress (pumping) periods
43 pumping wells
April-16-14 26
-
History Match – DP Obs Wells (with gas-lock)
April-16-14 27
-
History Match – DP Obs Wells (no gas-lock)
April-16-14 28
-
History Match – DP Obs Wells (with gas-lock)
April-16-14 29
-
History Match – DP Obs Wells (no gas-lock)
April-16-14 30
-
History Match - Summary
Detailed history match of operational volumes completed with irregular pumping schedule; 2 horizontal wells; 41 vertical wells; 40 scheduled stress (pumping) periods
Suitable match obtained given approach and uncertainties
April-16-14 31
-
Prediction Results
April-16-14 32
-
Estimation of Flow Volumes and Number of Wells
North South Total [m3/day [m3/day [m3/day 2012 711.23 98.46 809.69 2013 655.55 292.44 947.99 2014 482.81 555.90 1038.71 2015 457.22 662.96 1120.18
No Gas Locking No Gas Locking – Estimate # Wells
With Gas Locking – Estimate # Wells
North South Total
8 1 9
7 3 11
5 6 12
5 7 12
Number of Wells Operating
North South Total
24 3 27
22 10 32
16 19 35
15 22 37
Number of Wells Operating
April-16-14 33
-
Summary and Conclusions
April-16-14 34
-
Summary and Conclusions
Characterization of dissolved gas during the early stage of the project is critical;
A predictive tool has been developed to design the DP well network for gassy aquifers;
Greater number of wells are required to depressurize a given aquifer in the presence of dissolved gas;
The presence of dissolved gas could significantly impact the overall project schedule and budget.
April-16-14 35
Numerical Simulation of Basal Aquifer Depressurization in the Presence of Dissolved Gas – An UpdateWhat to Take Away From this PresentationOutlineBackgroundBackground – Project LocationBackground - CNRL Horizon Oil Sands ProjectBackground - CNRL Horizon Oil Sands ProjectBackground - CNRL Horizon Oil Sands ProjectBackground – Why? Gas LockingBackground – Effects of Gas LockingBackground – Basal Aquifer IsopachBackground – Key IndicatorsKey QuestionsKey Questions to be AnsweredModelling ApproachesGroundwater Modelling ApproachesSingle Phase Flow Modelling - MODFLOWSlide Number 18Single Phase Flow Modelling - FEFLOWFEFLOW Approach - Continuous ReductionSingle Phase Flow Modelling - FEFLOWSingle Phase Flow Modelling - FEFLOWCalibration to a Pumping TestComparison Between ApproachesHistory MatchHistory Match – Pumping WellsHistory Match – DP Obs Wells (with gas-lock)History Match – DP Obs Wells (no gas-lock)History Match – DP Obs Wells (with gas-lock)History Match – DP Obs Wells (no gas-lock)History Match - SummaryPrediction ResultsEstimation of Flow Volumes and Number of WellsSummary and ConclusionsSummary and Conclusions