Nano-indentation tests to characterize hydraulic fractures

A method comprising:

• obtaining energy density of a portion of a subterranean zone in a hydrocarbon reservoir;
• determining an energy density of a rock sample that is substantially smaller than the portion of the subterranean zone by:
• nano-indenting the rock sample to a depth less than a thickness of the 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;
• and
• determining an indentation volume responsive to nano-indenting the rock sample;
• determining that the energy density of the portion of the subterranean zone is substantially equal to the determined energy density of the rock sample;
• and
• assigning physical properties of the rock sample to the portion of the subterranean zone in response to determining that the energy density of the portion of the subterranean zone is substantially equal to the determined energy density of the rock sample.

A method comprising:

• performing a plurality of nano-indentation experiments on a plurality of rock samples, each nano-indentation experiment comprising nano-indenting a 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;
• obtaining, responsive to the plurality of nano-indentation experiments, a plurality of load-depth curves, each load-depth curve identifying, for each rock sample, a plurality of loads on the rock sample measured during penetration from the surface to the specified depth;
• determining, from the plurality of load-depth curves, a plurality of pop-ins experienced by the plurality of rock samples, each pop-in associated with a respective sample, each pop-in representing a rate of change of loads over a unit depth that is different from a threshold rate of change of loads over the unit depth;
• determining a plurality of energies associated with the plurality of pop-ins, each energy released in response to a corresponding pop-in in the respective sample;
• determining, for the plurality of rock samples, a corresponding plurality of nano-indentation volumes, each nano-indentation volume representing a volume of the rock sample indented responsive to a corresponding nano-indentation experiment;
• determining a plurality of energy densities for the plurality of rock samples from the plurality of energies and the plurality of nano-indentation volumes;
• and
• assigning physical properties of one or more of the rock samples to one or more portions of a subterranean zone based on a result of comparing the plurality of energy densities for the plurality of rock samples with a plurality of energy densities of a corresponding plurality of portions of the subterranean zone.