INVESTIGATING THE RELATIONSHIP BETWEEN FAULT PERMEABILITY, PORE PRESSURE AND EFFECTIVE STRESS USING CONSTRAINTS FROM RESERVOIR INDUCED SEISMICITY

Understanding the role of geological faults in fluid flow and chemical transport is critical for oil and gas, waste disposal and deep storage industries. Faults can traverse many lithological sequences, forming large-scale structures that span several kilometres laterally and over depth; their sheer physical extent implies that their hydraulic properties have a major influence on deep flow systems. This research uses observations of reservoir-induced seismicity (RIS) beneath Açu Reservoir, NE Brazil, to investigate the damage zone permeability of geological faults in crystalline basement rocks. High-resolution digital seismic monitoring of the reservoir has provided detailed information on the locations of seismic events. The temporal distribution of these events shows them to be directly related to annual fluctuations in the reservoir level. Model simulations, using a decoupled hydromechanical formulation (i.e. a static permeability field decreasing exponentially over depth due to increased confining pressure), show that pressure-diffusion is a hydrogeologically consistent mechanism for RIS, if preferential flow occurs within 2D fault planes embedded in a 3D low permeability matrix. Predictions of the maximum pressure change in these faults at hypocentral depths indicate <0.05 kPa is required to trigger seismic events. Further, the observed spatial and temporal variability of earthquakes indicate that these faults must have heterogeneous permeability fields with significant spatial structure; pockets of high and low permeability of the order of 0.5 - 1.5 km in diameter.

This research demonstrates for the first time that microseismicity data can be successfully employed to image spatial and temporal evolution of fault permeability. Further analyses of the Acu data are on-going to improve the accuracy of hypocentral locations and to investigate earthquake co-location and healing rates. A fully coupled hydro-mechanical model is being applied to simulate pressure-diffusion within the fault plane and hence to provide a more detailed understanding of permeability evolution.