TY  - JOUR
A1  - Zang, Arno
A1  - Stephansson, Ove
A1  - Stenberg, Leif
A1  - Plenkers, Katrin
A1  - von Specht, Sebastian
A1  - Milkereit, Claus
A1  - Schill, Eva
A1  - Kwiatek, Grzegorz
A1  - Dresen, Georg
A1  - Zimmermann, Günter
A1  - Dahm, Torsten
A1  - Weber, Michael
T1  - Hydraulic fracture monitoring in hard rock at 410 m depth with an advanced fluid-injection protocol and extensive sensor array
JF  - Geophysical journal international
N2  - In this paper, an underground experiment at the Aspo Hard Rock Laboratory (HRL) is described. Main goal is optimizing geothermal heat exchange in crystalline rock mass at depth by multistage hydraulic fracturing with minimal impact on the environment, that is, seismic events. For this, three arrays with acoustic emission, microseismicity and electromagnetic sensors are installed mapping hydraulic fracture initiation and growth. Fractures are driven by three different water injection schemes (continuous, progressive and pulse pressurization). After a brief review of hydraulic fracture operations in crystalline rock mass at mine scale, the site geology and the stress conditions at Aspo HRL are described. Then, the continuous, single-flow rate and alternative, multiple-flow rate fracture breakdown tests in a horizontal borehole at depth level 410 m are described together with the monitoring networks and sensitivity. Monitoring results include the primary catalogue of acoustic emission hypocentres obtained from four hydraulic fractures with the in situ trigger and localizing network. The continuous versus alternative water injection schemes are discussed in terms of the fracture breakdown pressure, the fracture pattern from impression packer result and the monitoring at the arrays. An example of multistage hydraulic fracturing with several phases of opening and closing of fracture walls is evaluated using data from acoustic emissions, seismic broad-band recordings and electromagnetic signal response. Based on our limited amount of in situ tests (six) and evaluation of three tests in Avro granodiorite, in the multiple-flow rate test with progressively increasing target pressure, the acoustic emission activity starts at a later stage in the fracturing process compared to the conventional fracturing case with continuous water injection. In tendency, also the total number and magnitude of acoustic events are found to be smaller in the progressive treatment with frequent phases of depressurization.
KW  - Geomechanics
KW  - Fracture and flow
KW  - Broad-band seismometers
Y1  - 2016
SN  - 0956-540X
SN  - 1365-246X
VL  - 208
SP  - 790
EP  - 813
PB  - Oxford Univ. Press
CY  - Oxford
ER  -