Abstract:

Laboratory hydraulic fracture (HF) tests on Ruhr sandstone have been conducted with cubes of intact as well as pre-fractured rock. The cubes had an edge length of 15 cm and the applied external stresses were in the order of 5 MPa. The hydraulic fracture was achieved by pumping oil into the sealed space between the packers at a pressurization rate of 0.5 MPa/s. Both, continuous pressurization and cyclic pressurization was applied to induce hydraulic fractures. The breakdown pressures as well as displacements along faces were recorded. Two data acquisition systems were used to sample acoustic emission (AE) signals. Individual features of AE signals such as amplitude, duration and risetime may be used to identify different processes during a hydraulic fracturing experiment. The tests showed that, with cyclic pressurization, stable fracture propagation may be achieved and identified by AE. It appears that cyclic pressurization weakens the rock wall around the sealed space between the packers and allows for a lower breakdown pressure. This pressurization strategy may be promising for achieving safe reservoir stimulation. New fractures may be induced in already pre-fractured rock when the proper external stresses are applied.

1 INTRODUCTION

A series of laboratory hydraulic fracture (HF) tests on Ruhr sandstone has been performed. The tests lead to the generation of new hydraulic fractures within a fractured and partly anisotropic rock subjected to differential stresses of up to 5 MPa. Ruhr sandstone has been used a model rock for HF-tests by Rummel and Winter (1983), Rummel (1987), to name a few researchers and a significant body of information exists on its properties. In our experiments we additionally use two types of acoustic emission systems to listen to the fracture processes. One system is a basic data acquisition system capable of recording peak amplitudes.

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