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Forms of Hydraulic Fractures at Shallow Depths in Piedmont Soils

Forms of Hydraulic Fractures at Shallow Depths in Piedmont Soils. Fracture Form. What is it ? Lateral extent, orientation, thickness Why important ? Affects fracture function Field data for model calibration. Overview. Objectives

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Forms of Hydraulic Fractures at Shallow Depths in Piedmont Soils

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  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 4 5 6 P

  6. Slurrygel + sand

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

  8. F G H I Fracturing equipment trenches

  9. FieldMapping -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 8 7 6 5 4 3 2 1

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

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

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

  25. Conceptual model of fracture growth and sand transport

  26. E1 E2 Mechanical interactions on fracture propagation Fracture “feels” out to ½ its length Response to contrasts In Elastic Modulus

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

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