Identification Of Activated (Therefore Potentially Conductive) Faults And Fractures Through Statistical Correlations In Production And Injection Rates And Coupled Flow-Geomechanical Modelling.
Authors: Heffer Kes J. , Zhang Xing , Koutsabeloulis Nick C. , Main Ian , Li Lun 
Published: December 2007

Long-range, stress-related and fault-related characteristics of correlations in fluctuations in flow-rates are explained conceptually in the context of the lithosphere's near-critical mechanical state and a strong feedback between deformation and local permeability. A more sophisticated statistical model, devised to extract a parsimonious set of flow-rate correlations, has shown similar characteristics. Coupled geomechanical-flow modeling was able to reproduce those characteristics for a generic pattern waterflood perturbed with random noise, but only when loaded to a near-critical state, hence providing strong support for the conceptual model. Coupled modeling of a cross-section representative of the Gullfaks field also demonstrated long-range influences. The matrix of empirical correlations between all well-pairs for a field can be decomposed in various ways. The principal components of the matrix, when interpolated with appropriate spatial correlation functions, have indicated the importance of particular faults in the rate fluctuation history; it is inferred that those faults are mechanically active during the development, and thus are potentially conductive features.

Introduction

It is usually assumed that geomechanical modelling coupled with reservoir simulation is only required in a minor subset of reservoirs that are termed 'stress-sensitive'. This subset is sometimes recognized a posteriori; i.e. during the course of field development, often through a general severe decline in permeability levels with depleting pressure; or perhaps a priori in the case of weak reservoir rocks where compaction drive is considered important to recovery (e.g. unconsolidated sands or weak chalks). Rarely outside such bounds is geomechanical modelling deemed necessary for reliable, unbiased reservoir performance predictions. However, some past analyses of preferred directions of flooding 1,2, and of the correlations in rate fluctuations 3 have suggested that geomechanics may be playing a commercially significant role in many secondary and tertiary floods, if not other development schemes. Key to the interpretation of this field data are the following concepts, which may be novel to many reservoir engineers or geoscientists:Much of the lithosphere is in a near-critical statee.g.4,5. This means that there are percolating paths of faults, fractures and incipient fractures that are near mechanical failure in the prevailing stress states. This concept is supported by direct measurements of stress state 6,7, observations of (micro-)seismicity induced by oilfield development 8, shear-wave splitting observed in most types of rock in the crust 9, and the observations of so-called '1/f' scaling of properties in well-logs 10.

These observations are themselves underpinned by theoretical explanations of the evolution of the earth's lithosphere into a near-critical state. Such models include 'self-organized criticality' (SOC)4,11–15; or 'self-organized sub-criticality'16–19; or in the sense of a spinodal critical point 20. Whilst there is still ongoing discussion as to which of these models best fits lithospheric deformation, common characteristics of these theories, also observed in the real behaviour of rock, are:Strong susceptibility to small perturbation (metastability)Responses often at a distance from perturbing load (long-range correlation)Percolating paths of incipient failure (localization), oriented in association with the modern-day stress state.