New Methods For Detecting And Characterizing Primary Flow Paths, The Coupling Among Stress, Fracturing, Seismic Anisotropy, And Reservoir Flow
William D. Rizer
W. D. Rizer Consulting, Katy, Texas
In recent years considerable progress has been made in acquiring and interpreting log, core, and seismic data for fractures and stress. We now know how to distinguish natural from induced fractures in cores and on logs, and how to interpret the induced fractures and breakouts observable on image logs in terms of the orientation and at least the relative magnitudes of the principal stresses. We also know that surface fracture systems do not necessarily reflect subsurface systems, even in the same formations in similar structural settings. Perhaps more importantly production data confirms that faults and fractures, when favorably aligned to stress, can dominate flow in hydrocarbon reservoirs. As a result, there is now growing recognition and acceptance that the current in situ stress field (ij), through its effects on fracture permeability, commonly exerts a major control on flow anisotropy at the field and reservoir scale. Stress is now widely recognized as a critical uncertainty in reservoir characterization. This new understanding, along with our ability to characterize stress and better identify and locate faults with 3-D seismic, offers considerable promise for improving many aspects of production, such as designing more efficient primary and secondary reservoir strategies, and locating regions of bypassed oil and gas.
Fractures are the most compliant elements of rock masses. Under the influence of stress, they impart to the rock mass an elastic anisotropy and therefore a seismic anisotropy that is roughly aligned with the stress. Recent data confirm a strong coupling between seismic anisotropy and stress, fracturing, and flow, indicating considerable potential for remotely imaging fractures at the well to the field scale using image logs and multi-component and/or high frequency seismic techniques.
Currently available technology can detect and characterize fractures and aspects of ij at the well bore scale (centimeters to meters) and seismic scale (10’s to 100’s of meters). However, while the higher frequency and multicomponent VSP and/or single well seismic technologies required to characterize stress and structure at sub seismic scales exist, they are not yet offered as standard services. Despite this limitation, the recent advances provide a new way of looking at reservoir characterization.
Results from a series of controlled seismic and flow experiments at a geologically very well characterized multi-well test facility illustrate these coupled relationships. In addition, these experiments, along with considerable data from producing fields, indicate that flow in a given reservoir volume can be completely dominated by the larger structural elements, such as faults and fractures, that are properly aligned to the stress field in that volume. Numerous studies, particularly in areas of complex structure, aggressive production, and/or active tectonics, also indicate that the stress field is dynamic and can change significantly over relatively short length and time scales.
Despite the obvious complexities presented by these new advances and data, they represent a new and most likely lucrative way to look at reservoirs as they evolve through production history. And they offer promising new ways to locate the important flow paths and to predict the likely areas of the field with bypassed production.
Copyright © 2004. The American Association of Petroleum Geologists. All Rights Reserved.