Complications with Stress Tests - Insights from a fracture experiment in the ultra-deep KTB borehole
Authors: de Pater C.J. Hans , Zoback Mark D. , Wright Chris A. , Engeser Bernard 
Published: December 2007

Complications with Stress Tests - Insights from a fracture experiment in the ultra-deep KTB borehole.

In the main KTB-well (Kontinentales Tiefbohrprogramm der Bundesrepublik Deutschland), we pumped 210 m3 of brine to make a hydraulic fracturing stress measurement and to induce seismic events. We observed a peculiar pressure behavior: a clear breakdown occurred at low flow rate, but at higher flow rate the pressure kept rising until we reached the pressure limit. During the later injection stages, a large number of triggered seismic events occurred. The events mostly came from faults at a depth of 8.6 and 8.9 km, whereas the fluid was injected at 9.1 km depth. A few events came from faults as shallow as 7.2 km. Careful analysis of the pressure record showed that no tensile fracture was created. The seismic events indicated that the injected fluid was most probably going into pre-existing shear faults.

Introduction

Creating a hydraulic fracture and observing the closure pressure of the fracture provides by far the most direct and reliable way to measure in-situ stress at great depth (Ref. 1). However, the interpretation of hydraulic fracture closure rests on the assumption that a dominant tensile fracture is induced perpendicular to the least stress (Ref. 2). In reality, complex fracture geometry may hamper interpretation of stress tests. Instead of a single fracture, we may induce a very tortuous fracture or inject fluid into shear faults. We present evidence for such behavior in a hydraulic fracturing experiment at 9 km depth in the KTB main well (Kontinentales Tiefbohrprogramm der Bundesrepublik Deutschland).

Previously, a stress test in this well at 6 km depth yielded inconclusive results, because the pressure limit was reached when the formation appeared to approach breakdown (see the Discussion Section for a description of this test). In the experiment at 9 km depth, a heavy brine was selected to achieve the highest possible downhole pressure. In this experiment the downhole pressure would be limited by the casing burst strength to 185 MPa. The intention was first to do a microfrac stress test by injecting relatively small volumes of fluid at low flow rates and then to inject a large fluid volume to induce seismicity. The induced seismicity experiment was done to confirm and extend data on stress magnitude and orientation obtained through borehole measurements to 6.8 and 7.6 km depth, respectively (Ref. 6). This study indicated that stresses in the crust were sufficiently high that well oriented, preexisting faults were close to failure. Ref. 5 describes the induced seismicity experiment more fully. A solids free fluid was selected to have the highest fluid leak-off into natural fractures, which would promote seismicity.

In this paper we investigate the pressure behavior to establish the mechanism of the injection process. We can use the seismic observations to support the pressure interpretation. For petroleum applications this experiment is important for stress tests in low permeability formations, with preexisting faults and fractures. With regard to stress test interpretation, we relate the pressure behavior to the stresses at the borehole surface and the stresses on the fault plane, where fluid injection occurred.

Pressure analysis

Wellbore storage and leak-off: The increase in pressure with pumping can be described with a mass balance for the wellbore storage and some (unspecified) leak from the wellbore. The leak may be leak-off towards the formation, a fracture or leak into the casing annulus. The pressure derivative is written as:

(1)

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