GEOL 4334
Lab 08: Gelatin Fracks!
v. 2015
LAB 08: Gelatin Fracs! Experimental fracturing of gelatin & Fracture Characterization
Objectives: This lab will develop & exercise the skills of 3D visualization and description of geologic
structures using correct terminology and will acquaint you with:
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How to plan and evaluate an experiment that reproduces natural fractures
Evaluating the interaction between differential stress, strain, and failure
Extrapolating experimental fracture results to natural settings
Developing testable predictions about fracture orientation and shape in natural settings
Materials: pencils, protractor, ruler, camera***, gelatin, plaster-of-Paris, syringe, tracing paper, 2 L soda
bottle (cut off the top half), Rose diagram
***You may use cell-phone cameras to photograph the experiment; please do not use your cell phone to
text or otherwise communicate outside of the classroom. These images may be included, in addition
to the sketches. However, sketches are REQUIRED; photographs are optional.
Plaster-of-Paris recipe: 2 parts water: 5 parts plaster, mix thoroughly – should be viscosity of thick
pancake batter or Jif peanut butter (the smooth kind, not crunch!)
Gelatin recipe: 1cup water; 1 packet Jell-O (1/4 cold water and ¾ hot water).
The geometry of fractures
propogation
direction
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1. main joint face
2. fringe
3. plumose structure
4. trace of twist hackles
5. hackle face
6. contact between lithologies “shoulder”
7. trace of main joint face
8. “ribs” or arrest lines; “paleo-tip lines” of the fracture
convex toward propagation direction
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GEOL 4334
Lab 08: Gelatin Fracks!
v. 2015
Fracture Modes
Structures associated
with fractures/joints
Note how the trace
of the twist hackle
forms “en echelon”
cracks
images from Twiss and
Moores, 2002 and Van
der pluijm Structure
textbooks
trace of
hackle on joint face
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GEOL 4334
Lab 08: Gelatin Fracks!
v. 2015
images from Twiss and
Moores, 2002 and Van
der pluijm Structure
textbooks
Field pictures of joint
surfaces. Note plumose
structures, twist hackles,
and other features.
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GEOL 4334
Lab 08: Gelatin Fracks!
v. 2015
Part 1. Making your own fracture!
During this experiment you will apply a differential stress to the
gelatin-filled bottle, inject the gelatin with plaster-of-Paris, and
observe how cracks propagate. The resulting fractures provide
clues as to how natural fractures in rocks form and evolve.
The figure (right) displays the experimental set-up and additional
plaster added to the top of the jello. This will form a base to the
fracture mold when it is solidified and extracted.
Procedure & Worksheet
A few pointers:
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Work in groups of 2-4.
Don’t let gelatin sit out for more than an hour before
injecting.
Molds should solidify in ~45-60 minutes using the above
recipe.
To extract, cut around gelatin and mold and remove by hand. Then remove excess gelatin
carefully.
Clean syringes immediately after injecting plaster.
DO NOT PUT PLASTER DOWN THE SINK
1. Before beginning the experiment, measure the diameter of the bottle prior to applying the
differential stress (i.e., the clamp). This will be your initial value to assess the amount of strain the
gelatin has undergone. Briefly predict and describe the geometry and orientation of the injected
plaster-of-Paris (5 pts).
Draw an annotated picture of the experiment including your prediction of the shape of the plaster
mold (i.e., fracture orientation). (5 pts)
Sketches:
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GEOL 4334
Lab 08: Gelatin Fracks!
v. 2015
Make approximately 200 ml of plaster-of-Paris using the above recipe. Apply the clamp to the bottle to
create a differential stress. Slowly and steadily inject the plaster-of-Paris into the gelatin and observe.
(Allow the plaster to set for at least 45 minutes – preferably more.)
2. After injecting the plaster-of-Paris, draw the experimental set-up in map and cross-section views.
Note the orientation of the differential stress in your illustrations.
Sketches (5 pts):
3. While the plaster is hardening, carefully draw and measure the axial ratio of the deformed
canister without disturbing the plaster mold. Calculate the strain that the bottle has undergone by
comparing the axial ratio of the undeformed bottle and the deformed bottle. (clue: calculate
elongation).
Sketches & Calculations (10 pts):
4. In a sentence or two, relate the geometry of the axes of the deformed ellipse to the principal
stresses that you have applied to the bottle. (5 pts)
5. Describe the shape and orientation of the resultant crack and crack-filling material? How did it
differ from your initial hypothesis? Develop a hypothesis to explain the geometry of the crack with
respect to principle stresses. (10 pts)
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GEOL 4334
Lab 08: Gelatin Fracks!
v. 2015
Once the plaster-of-Paris has set, gently remove it from the gelatin. For best results, cut out gelatin
around mold and slowly remove by hand. Then, remove excess pieces of gelatin carefully.
6. Observe the delicate structure on the plaster mold.
a. Draw an annotated picture of the mold noting any features that have formed. Be sure to
include scale. (10 pts)
b. Describe and define all of the features that occur on your mold and which you can
recognize in natural fractures. Be sure to note the Mode of fracturing that has occurred.
(10 pts)
Sketches:
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GEOL 4334
Lab 08: Gelatin Fracks!
v. 2015
Part 2. In-situ fracture analysis & characterization.
An image of a fractured, horizontal bedding plane is provided. Observe the geometry and evaluate how many
fracture sets may be defined. There are at least 3 distinct sets of fractures (F1, F2, F3, from oldest to
youngest).
• Using a piece of tracing paper, neatly construct a map of the fracture sets. On your map be sure to include a
scale and north arrow.
• In the space below, compose a brief description of each of the fracture sets that helps to distinguish them.
• Measure the trend of as many fractures or each set as possible and record the data in the provided table.
• Construct a rose diagram to illustrate the orientation of the fracture sets (see Lab 2 for a refresher on plotting
rose diagrams).
Fracture set # ____
Fracture set # ____
Fracture set # ____
Fracture set # ____
Fracture set # ____
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GEOL 4334
Lab 08: Gelatin Fracks!
v. 2015
Fracture table.
Frac # _____
Frac # _____
Frac # _____
Frac # _____
Frac # _____
Summary Characteristics:
Summary Characteristics:
Summary Characteristics:
Summary Characteristics:
Summary Characteristics:
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GEOL 4334
Lab 08: Gelatin Fracks!
v. 2015
Questions:
Answer the following questions on a one-page, typed sheet with your full name, TA, and lab time in the upper
right corner of the page.
1. Briefly describe the average orientation of the different fracture sets. What is the average angular
relationship between the different fracture sets?
2. In evaluating your map is there any systematic offset of one fracture set by another? If so, explain.
3. Relate the results of your jello experiment to your map of natural fractures in terms of the orientation of the
compressive stress axes. How do you rectify your observations of natural fractures with your experimental
results?
4. What orientation would you steer your well bore to maximize any in situ fracture permeability? Explain your
answer.
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GEOL 4334
Lab 08: Gelatin Fracks!
r
No
v. 2015
th
Fractures in horizontal bed of siliceous shale. Note north arrow and hand lens (14 mm wide) for scale.
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GEOL 4334
Lab 08: Gelatin Fracks!
Rose diagram
°
30
N
360°
03
0°
0°
06
30
0°
3
v. 2015
E, 090°
0°
12
24
0°
W, 270°
21
0°
°
S,
180°
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0
15