Abstract:

A laboratory method is devised to study acoustic emission induced by critically stressed planes and to understand field micro-seismic activity upon minor pore pressure increase, much below the hydraulic fracturing injection pressure. The experiment relies on generating a shear failure plane, re-loading the sample to the near-failure axial stress that closes/narrows the fracture zone, and finally reopening the fracture with a pore pressure increase. The entire process is controlled based on the acoustic emission response, which determines the reopening early enough to prevent the sliding. A change of the emission event energy distribution exponent is observed during fracture generation/activation and fracture sliding, combined with an increase of acoustic emission activity upon the dislocation. The results indicate that even small pore pressure increases, up to 2 MPa and in extreme cases as low as 0.4 MPa, are sufficient to activate a pre-existing fracture and finally to fail the outcrop and field sandstone sample. This lab observation supports the field findings of micro-seismicity even for low pore pressure increments due to fluid injection below the frac gradient. The field response may arise from geologically created critically stressed planes, which are not sliding at the current stresses, but are suddenly activated with minimal pressure increase.

Introduction

Reopening or reactivation of an existing fracture/fault in an underground reservoir has important consequences on the field operations during CO2 injection and storage phases [1, 2, 3]. Sometimes a fracture/fault reopening can help the CO2 absorption process inside porous rocks by creating larger surface areas for CO2-fluid interactions. On the other hand, it can trigger a CO2 leakage path by activating existing fracture networks in the cap-rock. This fracture/fault reopening scenario is equally important to petroleum production phase, where permeability increases once an isolated conducting fracture becomes connected to the main flow path – as a result the productivity is enhanced. Therefore, a close monitoring of fracture/fault reopening processes is a necessity. It can help the operators to select injection pressure, mud-weight window and other related parameters for a definite goal, either to stop fracture reopening or to allow it. A good control on the in-situ processes during fracture reopening should lead towards better planning of the well treatment, improve productivity and avoid risk.

The CO2 geological storage pilot projects [4, 5] have identified some important scientific aspects for advanced research: How will the injected CO2 be distributed within the storage reservoir? What is the mechanism behind the injection-induced micro-seismicity? How can we control and predict it?

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