Page 206 - Geothermal Energy Systems Exploration, Development, and Utilization

182 4 Enhancing Geothermal Reservoirs

Water pressure
S 1
S 1 S 1

S 2 S 2
S 2
S 2 S 2 S 2

S 1
S 1 S 1
Fracture Opening, shearing Self propping
acoustic emissions
Figure 4.4 Potential self-propping mechanism after water frac treatment (from Jung, 1999).

Enhancing the treatment design comprises adding some abrasive agent such
as sand or proppants in the fluid during the high flow rates. This will help etch
conductivity into the fractures created, and using a proppant suspending agent
which gives the proppant mechanical suspension while travelling through the frac,
will increase the height that will be etched and allow the proppant to travel to the
end of the fracture. It can be considered using a friction reducing agent in the fluid
as opposed to using a guar-based gel in case pH values do not correspond between
the fluids injected and the cross-linked gel.

4.5.1.3 Gel-Proppant Treatments
Gel-proppant treatments are used to stimulate reservoirs with cross-linked gels in
conjunction with proppants of a certain mesh size (Figure 4.5). These treatments
can be applied in a wide range of formations with varying permeabilities and
a good control of stimulation parameters. The produced fractures have a short
length of about 50–100 m, but a higher aperture of up to 10 mm compared to
the waterfracs. It is especially used to bypass the wellbore skin in high permeable
environments. In general, this kind of treatment is more expensive than a waterfrac
treatment.
Typically, the gel-proppant treatments start with a Data FRAC to obtain infor-
mation about friction and tortuosity of the perforated interval. In this DataFRAC,
one would first pump an uncross-linked gel which would give an indication if any
near-wellbore problems exist which could potentially adversely effect the placement
of the frac treatment. This would then be followed by pumping a cross-linked fluid
which would give an idea of leakoff as well as help to predict closure pressures,
the frac geometry, and whether there is any indication of pressure dependent
leakoff.
The MainFRAC treatment that followed after these pretesting measures is an
injection of gel-proppants with a stepwise increase of proppant concentration