Numerical Simulation of Basal Aquifer Depressurization in the Presence of Dissolved Gas

Transcription

1 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

2 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,

3 Motivation Walday Abeda (Canadian Natural) April 17,

4 Background Basal Aquifer Depressurization and Dissolved Gas April 17,

5 Background - Hydrogeology Oil Sand Water Sand April 17,

6 Background Aquifer Isopach Basal Aquifer thickness and mine footprint April 17,

7 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,

8 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,

9 Background Challenge of Gas NO GAS WITH GAS THEORETICAL DRAWDOWN CONE PW-A OW-A PW-A OW-A BUBBLE PRESSURE POINT April 17,

10 Drawdown (m) Drawdown (m) Background - Gas Locking Indicators 0 No gas locking Q1 25 Q Q 1 = 157 m 3 /d (24 igpm) Q 2 = 255 m 3 /d (39 gpm) Q 3 = 360 m 3 /d (55 igpm) Q Available drawdown to pump intake = 65.6m Separation between pumping well and observation well = 31.0 m Elapsed Time (min) DP-06 Datalogger DP-06 Datalogger DP-06 Manual DP-06 Manual DP well appeared to become gas locked Prepared By: 0 5 Q1 Prepared For: Canadian Natural Horizon Oil Sands Project 2007 Basal McMurray Watersands Drilling and Testing Program DP-06 Arithmetic Plot Figure No Q2 1) Excessive gas noted in discharge water Q3 Q4 2) Inflection point in drawdown curve indicative of change in effective permeability Q 55 1 = 157 m 3 /d (24 igpm) Q 2 = 333 m3/d (51 gpm) Q 60 3 = 655 m3/d (100 igpm) Q 4 = 982 m3/d (150 igpm) 65 Available drawdown to pump intake = 65.1m Separation between pumping well and observation well = 31.0 m Elapsed Time (min) Prepared By: April 17, Prepared For: DP-03 Datalogger DP-03 Datalogger DP-03 Manual DP-03 Manual Canadian Natural Horizon Oil Sands Project 2007 Basal McMurray Watersands Drilling and Testing Program DP-03 Arithmetic Plot Figure No. 13

11 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,

12 Problem Statement April 17,

13 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,

14 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,

15 Approach Characterization of Gas April 17,

16 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,

17 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,

18 Approach Modified Groundwater Flow Model April 17,

19 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,

20 Corrected Head (masl) Approach Modified Model SIM-NO GAS LOCKING DP-03-MEASURED DP-03-SIM DISCRETE STEPS OBW4-MEASURED OBW4-SIM Time (min) April 17,

21 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,

22 Approach Modified Model Effect on K Time = 0 days DP Well Rate (m 3 /day) DP DP DP DP DP DP Vertical well

23 Approach Modified Model Effect on K Time = 5.3 days

24 Approach Modified Model Effect on K Time = 9.4 days

25 Approach Modified Model Effect on K Time = 13.5 days

26 Approach Modified Model Effect on K Time = 17.6 days

27 Approach Modified Model Effect on K Time = 23.9 days

28 Approach Modified Model Effect on K Time = 30.3 days Head minimum at wells

29 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

30 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

31 Approach - Sample WL Contours Horizontal Wells NO GAS LOCKING WITH GAS LOCKING Head reduced to ~ 200 masl Improved performance over vertical wells;

32 Results Modified Groundwater Flow Model April 17,

33 Results Model Sequencing Area 2 Area 1: History Match Area 3 Area 6 Area 4 Area 5 April 17,

34 Elevation (m) Rate (m3/day) Pumping Rate (m3/day) 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 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 G G G G G G South Pit Basal Aquifer Piezometric Heads G G SIM G G SIM G G SIM G G SIM G G SIM RATE - AREA 1 RATE - AREA April 17,

35 Results Areas 3 and 4 were grouped due to aquifer connectivity + 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 April 17,

36 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,

37 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,

38 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,

39 Acknowledgements Canadian Natural Horizon Co-authors: Ken Baxter, Golder Associates Walday Abeda, Canadian Natural Walter Alexandru, Canadian Natural