Abstract

Laboratory hydraulic fracturing experiments were performed on a single true triaxially confined granite specimen with samples sizes of 15×15×25 cm3and monitored with acoustic emission (AE) to simulate the field development of oil, gas, and enhanced geothermal systems reservoirs. Pump pressure, wellhead pressure, pump flow, applied triaxial stresses, and AE were monitored throughout testing. AE was monitored and extensively analyzed post-testing for source location, AE event attributes, and source mechanism characterization. Mode of failure of each individual microcrack was determined using AE moment tensor inversion techniques. Orientation and direction of crack displacement vector and crack face normal vectors were calculated from an eigenvector decomposition of the moment tensor solution and provided valuable information regarding how individual microcracks contribute to an image of the overall coalesced hydraulic fracture. Further analysis was performed to determine if trends existed between boundary stress conditions and the crack vector information. Several events were observed to have similar microcrack displacement vector orientations compared to principal stress directions. Comparisons between crack displacement vector orientations and mode of failure were also examined. The usefulness of the AE microcrack displacement vector information is apparent when understanding rock damage and possible alterations of the flow behavior near hydraulic fractures.

1. INTRODUCTION

Hydraulic fracturing has become a standard practice in oil- and gas-bearing formations because of the high commonality of nanodarcy permeability source rock in several of these reservoirs. A dense hydraulic fracture and wellbore connected natural fracture network is desired to reach as many isolated hydrocarbon-rich pores and natural fractures as possible. Although dense fracture networks are desired, even if they are obtained, much of the rock between and near the fracture faces contribute very little to production because of the extremely low permeability. It is desirable to understand the microcracking process very near these fracture faces and throughout the macro-scale hydraulic fracture network to determine the affected changes to the reservoir rock, which can alter the near fracture rock permeability.

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