This paper gives some of the issues present in the hydraulic fracturing design,
application and evaluation procedures in the hydrocarbon industry.
Depending on the reservoir, hydraulic fracture treatments have as a goal either
to bypass damaged permeability close to the well, or to create additional
contact area between the reservoir and the well. While for the first goal the key
is to maximize the fracture conductivity and thus the fracture width in the
vicinity of the wellbore, the second goal requires large fractures, preferably
connecting to an already existing network of natural fractures. Typically, such
massive hydraulic fractures are placed in low-permeability reservoir.
The successful placement of a propped hydraulic fracture depends critically on
the quality of the design input data. Such data contain knowledge about the in-
situ stresses, the reservoir permeability, the elastic parameters, and the
fracture propagation criteria. Minifrac tests are designed to disclose such
parameters. To this end, the time-dependent behaviour of the pressure after a
short injection test above the fracture pressure is analysed with specially
designed software. Further important knowledge is the containing capacity of
different layers in the subsurface, as these determine the height / length ratio
of the fracture. A profile of the parameters, required to assess this, however, is
often difficult to obtain.
Even with good design input data and a properly operated fracturing treatment,
the results are not always in line with the predictions. Knowledge is usually
built up in specific areas during subsequent hydraulic fracturing treatments and
their careful evaluation. Method that can help considerably in this evaluation
are tiltmeter mapping and microseismic monitoring, by which the dimensions of
the created fracture can be estimated. Microseismic monitoring has proven
particularly helpful in deep, low-permeability fractured reservoir, where the main
goal of the fracturing campaigns was to connect the well to an already existing
fracture network.
The experience with hydraulic fracuring in Geothermal applications like the one
on the drillsite in Gross Schönebeck and the GeneSys project in Hannover will
be put in the context of hydraulic fracturing in the oil and gas industry.
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