- Fractures and Fracturing: Hydraulic Fracturing in Jointed Rock
- C. Fairhurst
- Book Title / Journal: Effective and Sustainable Hydraulic Fracturing - Chapter 3
- Year: 2013
- Rock Mechanics
- Keywords: hydraulic fracturing ; fractures ; jointed rock
- Description
- Rock in situ is arguably the most complex material encountered in any engineering disci‐
pline. Deformed and fractured over many millions of years and different tectonic stress re‐
gimes, it contains fractures on a wide variety of length scales from microscopic to tectonic
plate boundaries.
Hydraulic fractures, sometimes on the scale of hundreds of meters, may encounter such dis‐
continuities on several scales. Developed initially as a technology to enhance recovery from
petroleum reservoirs, hydraulic fracturing is now applied in a variety of subsurface engineering applications. Often carried out at depths of kilometers, the fracturing process cannot
be observed directly.
Early analyses of the hydraulic fracturing process assumed that a single fracture developed
symmetrically from the packed off-pressurized interval of a borehole in a stressed elastic
continuum. It is now recognized that this is often not the case. Pre-existing fractures can and
do have a significant influence on fracture development, and on the associated distributions
of increased fluid pressure and stresses in the rock.
Given the usual lack of information and/or uncertainties concerning important variables
such as the disposition and mechanical properties of pre-existing fracture systems and properties, rock mass permeabilities, in-situ stress state at the depths of interest, fundamental questions as to how a propagating fracture is affected by encounters with pre-existing faults, etc., it is clear that design of hydraulic fracturing treatments is not an exact science.
Fractures in fabricated materials tend to occur on a length of scale that is small; of the order
of the ‘grain size’ of the material. Increase in the size of the structure does not introduce new
fracture sets.
Numerical modeling of fracture systems has made significant advances and is being applied
to attempt to assess the extent of these uncertainties and how they may affect the outcome of practical fracturing programs. Geophysical observations including both micro-seismic activity and P- and S-wave velocity changes during and after stimulation are valuable tools to assist in verifying model predictions and development of a better overall understanding of
the process of hydraulic fracturing on the field scale. Fundamental studies supported by lab‐
oratory investigations can also contribute significantly to improved understanding.
Given the widening application of hydraulic fracturing to situations where there is little pri‐
or experience (e.g., Enhanced Geothermal Systems (EGS), gas extraction from ‘tight shales’ by fracturing in essentially horizontal wellbores, etc.) development of a greater understanding of the mechanics of hydraulic fracturing in naturally fractured rock masses should be an industry-wide imperative. HF 2013 International Conference for Effective and Sustainable Hydraulic Fracturing is very timely!
This lecture will describe examples of some current attempts to address these uncertainties
and gaps in understanding. And, it is hoped, it will stimulate discussion of how to achieve
more effective practical design of hydraulic fracturing treatments.
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