1. Forms of Hydraulic Fractures at Shallow Depths in Piedmont Soils

2. Fracture Form What is it? Lateral extent, orientation, thickness Why important? Affects fracture function Field data for model calibration

3. Overview Objectives –Detailed description of fracture form –Infer processes of formation Field methods –Create fractures –Mapping Observations –Fracture geometry –Sand thickness –Sand movement (color distribution) Conceptual Model

4. Field Site Cohesive clayey, sandy loam Massive Local quartz Low K (<10 -6 cm/s) E = 5000 psi Friable sandy silt Relic foliation, highly variable Local quartz, kaolin, mica Moderate K (>10 -4 cm/s) E < 5000 psi 5-8 ft

5. 1 2 3 5 4 P 6

6. Slurry gel + sand

7. Slurry samples 4 samples – taken during injection Color Fraction

8. trenches Fracturing equipment F H I G

9. Field Mapping -Establish grid -Set up and trace -Depth -Thickness -Color distribution “Extra” features

10. Lateral extent (plan view) 8 cross sections ~100 linear feet 1161 measurement points Elliptical – aspect ratio = 1.4 : 1

11. Cross sections of fracture surface 1 2 3 4 5 6 7 8

12. Surface map View: N30°E, 30° above horizontal Injection casing

13. Cross sections of bottom surface Fracture G 7 8 11 12

14. H fracture surface map Bowl or spoon- shaped No downward propagation

15. Uplift ~ Elliptical dome Displacement eccentricity = 0.27 (0.12-0.27) Borehole eccentricity = 0.21 (0.12-0.21) Extent-- Uplift vs. sand

16. Sand thickness Sand thickness average: ~0.2 in. Sand thickness : uplift ~0.3-0.4 Varies over small distances (0.2-0.5 of mean) Trends over larger distances

17. Sand transport in fracture -Radial plug flow -Something else?

18. Leading edge of fracture

19. Channel Feature

20. 1 m 1 ft Covered Red Sand White Sand Blue Sand Injection casing Step on fracture surface. Dots on downthrown side Contact inferred Limit of red sand Approx. extend of fracture Strip of blue sand on frx surface Trench face

21. red white blue red + white red + blue red + white + blue Presence of sand colors within fracture

22. Individual color distributions Only percentages > 0.10 plotted Increasing color intensity = increasing percentage (black = white)

23. 1 m 1 ft Covered Red Sand White Sand Blue Sand Casing Step on fracture surface. Dots on downthrown side Contact inferred Limit of red sand Approx. extend of fracture Strip of blue sand on frx surface Trench face

24. Channel development Older sand pushed to the sides 1 2 3

25. Conceptual model of fracture growth and sand transport

26. Mechanical interactions on fracture propagation E1E1 E2E2 Fracture “feels” out to ½ its length Response to contrasts In Elastic Modulus

27. Modeling fracture form 2-D axisymmetric model of fracture propagation Predict form of fracture trace under various conditions Ratio of elastic modulus (E) between B horizon and saprolite theoretical observed (analysis by Qingfeng)

28. Conclusions Fractures of useful form can be created in shallow Piedmont soils -gently dipping, saucer- or spoon-shaped Uplift is a reliable means of interpreting fracture- -shape -sand thickness Downward propagation can (and does) occur -mechanical explanation New conceptual model of sand transport -Progressive, through-cutting Channel & Delta -fracture design and application