Page 207 - Geothermal Energy Systems Exploration, Development, and Utilization

4.5 Definition and Description of Methods (Theoretical) 183

Figure 4.5 Proppants ready for gel-proppants treatments.

with a high viscous cross-linked gel into the fracture. The result of the treat-
ment, that is, the propagation of the fracture, mainly depends on the slurry rate
and the concentration of proppants added and their variation as a function of
time.
An adjustment during the treatment is possible and often necessary to avoid a
screen-out of the well. One can adjust the treatment by varying the flow rate and
the proppant concentration in case the pressure progression suspects a failure of
the treatment.

4.5.1.4 Hybrid Frac Treatments
In hybrid frac treatments, slickwater is pumped first to generate a fracture. Then, a
gel-pad with cross-linked gel is injected, followed by proppants or sand of a certain
mesh size with a cross-linked gel to fill the fracture. This method can be applied to
low permeable reservoirs and provide sustainable production rates.

4.5.2
Thermal Stimulation

While thermal stimulation as a phenomenon is known to occur spontaneously at
injection wells in hydrocarbon production (see e.g., Charlez et al., 1996 or Gadde
and Sharma, 2006) and although it has actively been used in high enthalpy fields
in volcanic and metamorphic settings to increase the well productivity, the detailed
process is not yet satisfyingly understood. The basic mechanism of thermally
stimulating new fractures was delineated by, for example, Clifford, Berry, and Gu
(1991). The injection of cold water into a hot well, leads to a cooling of the rock
surrounding the wellbore, or adjacent to existing natural or induced fractures.
The cooling rock matrix contracts and thus induces a tensile component of stress
(thermo elastic stress) near the injection well or adjacent to the injection surface.
The value of this thermally induced tensile stress depends on the shape of the
cooled region, the thermal and elastic rock properties, the difference between the
downhole and surface water temperatures, as well as the injection rate. It may
−1
exceed a value of 100 kPa C . In case the water bottomhole pressure exceeds the
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