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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
Fracture Form What is it? Lateral extent, orientation, thickness Why important? Affects fracture function Field data for model calibration
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
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
1 2 3 4 5 6 P
Slurry samples Color Fraction 4 samples – taken during injection
F G H I Fracturing equipment trenches
FieldMapping -Establish grid -Set up and trace -Depth -Thickness -Color distribution “Extra” features
Lateral extent (plan view) 8 cross sections ~100 linear feet 1161 measurement points Elliptical – aspect ratio = 1.4 : 1
Cross sections of fracture surface 8 7 6 5 4 3 2 1
Surface map Injection casing View: N30°E, 30° above horizontal
Cross sections of bottom surface Fracture G 12 11 8 7
H fracture surface map Bowl or spoon- shaped No downward propagation
Uplift ~ Elliptical dome Displacement eccentricity = 0.27 (0.12-0.27) Borehole eccentricity = 0.21 (0.12-0.21) Extent-- Uplift vs. sand
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
Sand transport in fracture -Radial plug flow -Something else?
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
red white blue red + white red + blue red + white + blue Presence of sand colors within fracture
Individual color distributions Only percentages > 0.10 plotted Increasing color intensity = increasing percentage (black = white)
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
1 Channel development 2 3 Older sand pushed to the sides
Conceptual model of fracture growth and sand transport
E1 E2 Mechanical interactions on fracture propagation Fracture “feels” out to ½ its length Response to contrasts In Elastic Modulus
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)
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