Abstract

Hydrocarbon extraction is relying progressively more on hydraulic fracturing stimulation of shale. Enhanced Geothermal Systems (EGS) also rely on hydraulic fracturing to create fractures through which water is circulated to recover heat. While hydraulic fracturing has been extensively used in field applications, the fracturing processes involved in this method are still not well understood. Since data obtained from field hydraulic stimulations may be very difficult to interpret, laboratory testing can play a major role in understanding the way fractures initiate, propagate and interact when hydraulically stimulated.

This paper describes a test setup developed at MIT, which allows one to apply hydraulic pressure to flaws, or existing fractures, leading to the initiation and propagation of fractures. The test setup consists of (1) an enclosure designed and built at MIT in which water pressures up to 10 MPa can be generated, (2) a high speed camera that captures the last seconds of a test at 14,000 frames per second, (3) a high resolution camera that captures frames every 2 to 5 seconds throughout a test, and (4) an acoustic emission system that monitors the micro-seismic activity throughout the test.

The setup has been successfully used in several tests on granite. An example of the type of results obtained in a test is shown.

1. INTRODUCTION

Hydraulic fracturing has been extensively used in field applications, but the fracturing processes involved in this method are not completely understood. Because data from field hydraulic stimulations may be difficult to obtain and interpret, laboratory testing plays a major role in understanding the way fractures initiate, propagate and interact when hydraulically stimulated.

For this purpose, the fracturing mechanisms occurring in rocks with prefabricated flaws under uniaxial compressive loading have been extensively studied by several authors. Bobet and Einstein [1] studied the fracturing mechanisms of gypsum, while Miller [2], Wong and Einstein [3] and Morgan et al. [4] used a high-speed video camera to better capture the fracturing mechanisms in gypsum, marble and granite. Zang et al. [5], in turn, created new fractures by uniaxially compressing granite cores without prefabricated flaws, while Mayr et al. [6] propagated fractures in sandstone cores by applying a confining stress and increasing the pore pressure; both measured acoustic emissions in order to identify newly-formed fractures.

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