An Improved Understanding of LNAPL Behavior in the Subsurface LNAPL - Part 1

Transcription

1 An Improved Understanding of LNAPL Behavior in the Subsurface LNAPL - Part 1 Dave Thomas Copyright 2011 Chevron Corporation All Rights Reserved Copyright claimed throughout, except where owned by others No part of these notes may be reproduced without written permission from Chevron. Contact D.G. Thomas at Chevron Energy Technology Company, dtgz@chevron.com,

2 ITRC Disclaimer and Copyright Although the information in this ITRC training is believed to be reliable and accurate, the training and all material set forth within are provided without warranties of any kind, either express or implied, including but not limited to warranties of the accuracy, currency, or completeness of information contained in the training or the suitability of the information contained in the training for any particular purpose. ITRC recommends consulting applicable standards, laws, regulations, suppliers of materials, and material safety data sheets for information concerning safety and health risks and precautions and compliance with then-applicable laws and regulations. ECOS, ERIS, and ITRC shall not be liable for any direct, indirect, incidental, special, consequential, or punitive damages arising out of the use of any information, apparatus, method, or process discussed in ITRC training, including claims for damages arising out of any conflict between this the training and any laws, regulations, and/or ordinances. ECOS, ERIS, and ITRC do not endorse or recommend the use of, nor do they attempt to determine the merits of, any specific technology or technology provider through ITRC training or publication of guidance documents or any other ITRC document. Copyright 2007 Interstate Technology & Regulatory Council, 444 North Capitol Street, NW, Suite 445, Washington, DC

3 Two Part Program Part 1 Key Concepts for LNAPL Characterisation Part 2 Principles & Methods for Evaluating LNAPL Extent & Stability 3

4 Outline of Part 1 What is LNAPL? How Does LNAPL enter soil & aquifers LNAPL Distribution in the Subsurface What is Residual Saturation? How Does LNAPL move? Why is LNAPL Inherently Stable? Oil Sheens 4

5 LNAPL? What is an LNAPL? Lighter than water Essentially immiscible Petrol is the favoured example Mix of 300 odd compounds not that simple! Photo with permission: ITRC 5

6 Simplified LNAPL Conceptual Model Source vs. Source Zone Utility Trench / Drain Explosion Hazard Exposure Risk Groundwater Supply Well Surface Water Vapours NAPL in Wells 6

7 How Does LNAPL Enter Soil & Aquifers? 7

8 Wetting Fluid Ability of liquid to maintain contact with a solid surface Water is usually the wetting fluid for water/oil/air mixtures in soil Air is usually non-wetting Wetting Non-Wetting 8

9 Entry Pressure LNAPL Water LNAPL Water Entry Pressure must be exceeded before LNAPL Can enter a water filled pore. 9

10 Capillary Pressure H c As pore size gets smaller, capillary rise gets bigger More difficult to push LNAPL into small pores H wt 11

11 Capillary Pressure (cm) Capillary Pressure (P c ) & Soil Type More difficult for LNAPL to displace water in clay 10 6 Clayey Silty Sand Fine Sand Entry Pressure, H c, is the maximum value of P c for 100% water saturation F-M Sand 1 Coarse Sand Water Saturation (%) Modified from ITRC Material 12

12 Entry Pressure Implications All non-wetting fluids are subject to entry pressure NAPL in NAPL-water system Air in any mixture NAPL needs to displace water to enter soil pores below the water table Easier for NAPL to air (unsaturated zone) than water (saturated zone below water table) 13

13 LNAPL Distribution in the Subsurface 14

14 Simplistic & Outdated Pancake Model LNAPL Water Grains No!! Mythbusters - Busted! 15

15 Vertical Equilibrium Model LNAPL Water Grains LNAPL penetrates below water table LNAPL and water coexist in pores Modified from ITRC Material 16

16 Elevation Capillary Pressure, P c Monitoring Well Water Table Water < 1 atm 1 atm > 1 atm 17

17 Elevation Capillary Pressure, P c Monitoring Well Oil Table Water Table LNAPL Water < 1 atm 1 atm > 1 atm 18

18 Elevation Capillary Pressure, P c Monitoring Well P c Oil Table Water Table LNAPL Oil Pressure = Water Pressure Water < 1 atm 1 atm > 1 atm 19

19 Saturation Distribution is Determined A Pc Water-LNAPL Interface < 1 atm 1 atm > 1 atm Pressure After RTDF (2005) B C Key Point: Higher the capillary pressure, the higher the LNAPL saturation 20

20 LNAPL Saturation is Variable Elevation (Capillary Pressure) Soil Type One metre LNAPL thickness in well Modified from ITRC Material 21

21 Height Above the LNAPL/Water Interface (ft) LNAPL Saturation & LNAPL Thickness ft Thickness 5 ft Thickness 2.5 ft Thickness 1 ft Thickness For a given soil type Higher thickness in well Higher capillary pressure Higher LNAPL saturation LNAPL Saturation (%) 22

22 Height Above Oil/Water Interface (cm) Real World Soils are Heterogeneous Soil #1 Soil #2 Soil # M e a s u r e d S o P r e d i c t e d S o Soil #4 Soil # H 2 O LNAPL LNAPL Saturation Modified from ITRC Material 23

23 Typical LNAPL Saturations LNAPL saturations in sands are typically low Range up to 30% Typically less than 10% Lower for finer grained soils 24

24 What Is Residual Saturation & How Does LNAPL Move? 25

25 Mobile and Residual LNAPL can move if total saturation exceeds residual saturation Traditionally, LNAPL remediation has focused on the mobile (recoverable) component In many cases Residual LNAPL is the key issue! 26

26 Creation of Residual Capillary trapping mechanisms for gasoline flow in water-saturated soil (after Chatzis et al. 1983) 27

27 Total vs Recoverable (Mobile) LNAPL Courtesy Mark Lyverse, Chevron ETC 30

28 LNAPL Flow Darcy s Law Different equation for each fluid LNAPL Relative Permeability LNAPL Hydraulic Conductivity Intrinsic Permeability of soil Fluid Properties 31

29 Conductivity & LNAPL Type Modified from ITRC Material 32

30 Relative Permeability LNAPL Permeability is Relative to Saturation 1 NAPL (k ro ) Water Soil Pore Volume 0 0 Water Saturation 100% 100% NAPL Saturation 0 33

31 LNAPL Conductivity & Depth Modified from ITRC Materials 34

32 NAPL Saturation - and Therefore Relative Permeability, Will Vary Throughout the Plume Relative Permeability at the edges of a NAPL plume are low because of low LNAPL saturations. Courtesy Mark Lyverse, Chevron ETC 36

33 Relative Permeability (continued) 27% 14.8% Higher LNAPL saturation in coarser-grained soil 47% 2.7% Percent finegrained Percent LNAPL LNAPL relative permeability is not uniformly distributed soil heterogeneity controls Higher LNAPL saturation in coarser-grained soil higher relative permeability higher potential LNAPL flow rate Mark Adamski Soil with fluorescing LNAPL 37

34 Why Is LNAPL Inherently Stable 38

35 Why are Most LNAPL Plumes Stable? LNAPL Gradients Not the same as hydraulic gradient Initially large but dissipate rapidly LNAPL Saturation Low saturation leads to low LNAPL permeability near margins of source zone Residual Saturation Soil has capacity to hold oil against drainage Photo: With Permission ITRC Entry Pressure Water acts as a capillary barrier against continued LNAPL spreading 39

36 Case Example: LNAPL Release and Spreading Approximate Spreading Rate (ft/day) Groundwater Flow Release Location Pipeline Location Change in Plume Area from 8/01 to 12/ Feet per day Sweet Texas Crude Unknown release volume Began LNAPL Gradient Evaluation Feet per year 0 1/1/00 7/19/00 2/4/01 8/23/01 3/11/02 9/27/02 4/15/03 40

37 NAPL Migration Large volume release on known date from a crude oil pipeline LNAPL Plume Genesis time = mths 6 mths 9 mths 1 year 2 year 3 year High LNAPL saturation and high LNAPL gradient result in fast initial velocity Low saturation & flat LNAPL gradient result in slowing down of the LNAPL plume and pore-entry pressure results in ultimate stoppage of the plume Data from API Interactive LNAPL Guide,

38 Oil Sheen Behaviour 42

39 Oil Sheen Oil sheen a thin oil layer that exists at the air-water interface Very low LNAPL volume (can be a few molecules in thickness) 43

40 Sheen Fundamentals Spreading Coefficient g w,a go,a oil g w,o d o air Non-spreading Spreading Surface water Groundwater With Permission: Dr. Julio Zimbaron, CSU 44

41 Controls on leading edge transport Wetting Spontaneous imbibition Non-wetting impeded by displacement pressure With Permission: Dr. Julio Zimbaron, CSU 45

42 Diesel Sheen Animation With Permission: Dr. Julio Zimbaron, CSU 46

43 Diesel Sheen Animation With Permission: Dr. Julio Zimbaron, CSU 47

44 Capillary Barrier Discontinuity in air phase can stop imbibition Capillary rise g.s. NAPL imbibition Capillary barrier Coarse Sand Fine Sand With Permission: Dr. Julio Zimbaron, CSU 48

45 Summary of Key Points

46 Basic Key Concepts LNAPL saturations are complex & dependent upon multiphase theory (not just oil floating on water) LNAPL saturation is variable Coarser grained soils will have a higher LNAPL saturation LNAPL saturations will be lower at margins of source area All LNAPL is not recoverable Residual saturations 50

47 Key Concepts NAPL Entry Pressure There is a minimum entry pressure that is needed for NAPL to displace water & move into soil This means: If NAPL is mobile at your site, significant NAPL gradients will typically be apparent (i.e. mounding of NAPL in monitoring wells relative to the water table elevation) 51

48 Key Concepts LNAPL Mobility LNAPL plumes are generally stable after short periods of time (most are stable within 1 to 2 years) Water forms a capillary barrier to LNAPL movement LNAPL gradients dissipate rapidly Low LNAPL saturations at margins of LNAPL source zone This means: LNAPL recovery is unlikely to be effective at reducing risk profile unless a release is recent (i.e. Recovery can be initiated within a few months to a year of a release) 52

49 Key Concepts LNAPL In Wells LNAPL in wells does NOT mean that LNAPL is mobile LNAPL in wells is typically limited to a smaller subset of the total area where residual LNAPL is present Area of residual LNAPL is key risk driver This means: If an LNAPL plume is stable then removal of LNAPL in wells will not reduce risk profile of site. 53

50 Thanks for Listening Questions?