Nano-indentation tests to characterize hydraulic fractures

1. A method comprising:

positioning a rock sample within a Scanning Electron Microscope (SEM);

within the SEM, nano-indenting the rock sample from a surface of the rock sample to a specified depth within the rock sample, the specified depth less than a thickness of the rock sample;

imaging, using the SEM, nano-indenting the rock sample from the surface of the rock sample to the specified depth within the rock sample;

while nano-indenting the rock sample, measuring a plurality of depths from the surface to the specified depth and a plurality of loads applied to the sample, each load associated with a respective depth;

determining, from the plurality of loads and the plurality of depths, a change in load over a specified depth;

determining, from the change in load over a specified depth, an energy associated with nano-indenting the rock sample;

determining, from a SEM image of the nano-indented rock sample, an indentation volume responsive to nano-indenting the rock sample; and

determining an energy density associated with the rock sample based, in part, on the determined energy and the indentation volume.

2. The method of claim 1 , further comprising imaging, using the SEM, nano-indenting the rock sample from the surface of the rock sample to the specified depth within the rock sample.

3. The method of claim 1 , wherein the thickness of the rock sample is substantially 1 millimeter, and wherein nano-indenting the rock sample comprises:

positioning a nano-indenter on the surface of the rock sample; and

penetrating the surface of the rock sample using the nano-indenter by a distance that is at least equal to 10 nanometers and less than a thickness of the rock sample.

4. The method of claim 1 , further comprising:

penetrating the surface of the rock sample using the nano-indenter by the distance to a depth; and

while penetrating the surface of the rock sample to the depth, measuring a load applied by the nano-indenter on the rock sample from the surface to the depth.

5. The method of claim 1 , further comprising:

performing, using the nano-indenter and within the SEM, a nano-indentation experiment; and

determining a physical property of the rock sample based on a result of the nano-indentation experiment.

6. The method of claim 1 , wherein the thickness of each rock sample is between seven and ten times a depth to which the rock sample is nano-indented.

7. The method of claim 1 , further comprising:

generating a plot of the plurality of loads versus the plurality of depths; and

determining a plurality of slopes of the generated plot corresponding to a plurality of changes of load over specified depth, the plurality of changes of load over specified depth including the determined change of load over specified depth.

8. The method of claim 7 , wherein each slope of the plurality of slopes represents a failure of the rock sample responsive to the nano-indenting, wherein the method further comprises associating a failure type to the determined change of load over specified depth.

9. The method of claim 7 , wherein determining, from the change in load over the specified depth, the energy associated with nano-indenting the rock sample comprises determining an area of a portion of the plot that corresponds to the change in load over the specified depth.

10. The method of claim 9 , wherein determining, from the change in load over the specified depth, the energy associated with nano-indenting the rock sample comprises multiplying the change in the load by the specified depth.

11. The method of claim 10 , wherein determining the energy density associated with the rock sample based, in part, on the determined energy and the indentation volume comprises dividing the energy by the indentation volume.

12. The method of claim 1 , wherein, while nano-indenting the rock sample, measuring a plurality of depths from the surface to the specified depth and a plurality of loads applied to the sample, each load associated with a respective depth comprises:

penetrating the surface of the rock sample using the nano-indenter by the distance to a first depth of the plurality of depths; and

while penetrating the surface of the rock sample to the first depth, measuring a load applied by the nano-indenter on the rock sample from the surface to the first depth.

13. The method of claim 1 , wherein a distal end of the nano-indenter has a pyramid shape terminating at a penetrating tip, wherein determining, from the SEM image of the nano-indented rock sample, an indentation volume responsive to nano-indenting the rock sample comprises:

determining, from the SEM image, a cross-sectional area of a region penetrated by the nano-indenter; and

determining the indentation volume based on the cross-sectional area and the specified depth.

14. A method comprising:

nano-indenting a rock sample to a depth less than a thickness of the sample while the rock sample is within a Scanning Electron Microscope (SEM);

imaging nano-indenting the rock sample while nano-indenting the rock sample;

determining an energy associated with a pop-in representing a rate of change of nano-indentation loads over a unit depth that is different from a threshold rate of change of nano-indentation loads over the unit depth;

determining an indentation volume responsive to nano-indenting the rock sample; and

determining an energy density of a rock sample based on the energy and the indentation volume.

15. The method of claim 14 , wherein the thickness of the rock sample is between seven and ten times the depth to which the rock sample is nano-indented.

16. The method of claim 14 , wherein the thickness of the rock sample is substantially 1 millimeter, and wherein nano-indenting the rock sample comprises:

positioning a nano-indenter on the surface of the rock sample; and penetrating the surface of the rock sample using the nano-indenter by a distance that is at least equal to 10 nanometers and less than a thickness of the rock sample.