Hydraulic fracturing is a standard technology in the hydrocarbon industry for more
than 30 years to overcome effects of formation damage and low rock permeability and
to increase the productivity of a reservoir. Two different concepts exist to
stimulate a reservoir hydraulically depending on rock, formation and fluid
properties. 1: Waterfracs (WF): Low viscous gels (~ 10 cP) without proppants or with
a small proppant concentration (50 - 200 g/l) are used to create long fractures (up
to 250 m half length) with a small width (~ 1 mm). 2: Hydraulic Proppant Fracturing
(HPF): High viscous gels (> 100 cP) with high proppant concentrations (200 -  1000
g/l) are used to create highly conductive but short (compared to WF) fractures.
In 2002 a HPF treatment was applied in the Rotliegend sandstone aquifer of the
geothermal research well Gross Schoenebeck 3/90. The site is located near Berlin. The
initial productivity of this well was significantly lower than expected from core
measurements due to near wellbore damage. Therefore it was stimulated with 11 tons of
ceramic proppants and over 200 m³ of high viscous gel.
Simulations had predicted a productivity increase by a factor of 7-8, but the
productivity was only doubled due to different reasons. One possible cause for
missing the productivity goal is proppant pack damage due to crushing, compaction and
embedment of proppants. To investigate this issue in more detail a new laboratory
equipment was set up which allows determination of permeability development of
proppant pack and proppant rock interface under increasing effective stress. During
an experiment Acoustic Emission (AE) is recorded and the damage events can be 
located.
First tests show that fracture creation as well as proppant and rock crushing lead to
considerable generation of fines and a clear permeability reduction. The AE events
indicate that grain crushing and proppant embedment start at low effective stress (~
5 MPa) at the fracture face. In this area fines are generated and flow paths get
blocked. This leads to a reduced permeability at the fracture face.  During long-term
production this damaged zone (so called Fracture Face Skin (FFS)) acts as a filter
for migrating fines resulting in a continuously increase of the thickness of the FFS
and an increasing reduction of productivity.

CH-8532 Warth TG
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