Page 232 - Geothermal Energy Systems Exploration, Development, and Utilization

208 4 Enhancing Geothermal Reservoirs
choice of proppants and proppant concentrations to guarantee sufficient fracture
conductivity in sedimentary environments. Experiments have shown a crushing of
proppant pack in addition to no self-propping. To maintain long-term productivity
of the reservoir, in advance to the field experiments different proppant types
should be tested in the laboratory for long-term conductivity under simulated
insitu reservoir conditions as well as for mechanical effects that would lower the
permeability of the proppant pack and the reservoir.
Proppants are used to keep the fracture open after pumping has stopped and
pressure drops below the fracture opening pressure. The proppant pack in the
fracture provides a conductive path from the reservoir rock to the wellbore. Placing
the appropriate concentration and type of proppant in the fracture are critical
parameters for the success of a hydraulic fracturing treatment (Economides and
Nolte, 2000).
Proppant selection must consider hydraulic conductivity at in situ stress condi-
tions. Hydraulic conductivity is influenced by stress on proppant pack, leading to
proppant crushing and embedment as well as to a reduction of fracture width and
fines production. Proppant size and proppant concentration has to be taken into
account. In general, large-diameter proppants yield a better hydraulic conductivity
but they are more sensitive for stress. Small diameter proppants offer less initial
hydraulic conductivity, but the average hydraulic conductivity over the life cycle
of the well is higher. Proppant concentration affects the hydraulic width and is
important for long-term hydraulic conductivity under production conditions (Wen
et al., 2007).
Reinicke et al. (2006) have shown that during fracture closure the majority
of destruction is located at the fracture face, at the rock proppant contact. The
destruction at the fracture face leads to fines production and pore blocking
resulting in a reduced permeability at the fracture face (Legarth, Huenges, and
Zimmermann, 2005) (Figure 4.11).
This reduced permeability can be expressed as fracture face skin (FFS). The FFS
is referred to as an impairment affecting flow normal to the fracture face (Cinco-Ley
and Samaniego, 1977). The FFS (denoted as s ff according to the original publication)
can be described in terms of fracture half-length x f , damage penetration w s and the
ratio of unaffected reservoir permeability to damaged permeability k i /k s :

πw s k i
s = − 1 (4.1)
ff
x f k s
The FFS has a direct influence on the productivity of a reservoir; it can be
used to calculate the dimensionless PI PI D within the pseudo steady state flow
regime:
1
PI D = (4.2)
1
+ s ff
PI D,s=0
with PI D,s=0 representing the dimensionless PI of the well with zero fracture face
skin (Romero, Valk´ o, and Economides, 2003).