Keynote: Geomechanical Aspects of Fractured Reservoirs

Geomechanical concepts and approaches are associated with many aspects of fractured reservoirs. It is convenient to consider two main contexts: those where the goal is to understand the formation of the trap and its fracture system, and those where the goal is to predict or explain production performance. The practical justification for efforts aimed at understanding the pre-production history of the reservoir is that some or most of the learnings from this are to be used to inform the production evaluation/prediction efforts. The geological evolution of the trap considers aspects such as the tectonic cause of the reservoir’s shape, and how deformation processes operated to create fracture networks. These considerations often lead to the recognition that fractures may terminate against other fractures, but often do so at bedding contacts, leading to a conclusion that the pre-deformation vertical arrangement of rock types and their properties is an important control, along with the deformation-caused curvature or distortion of the initially-planar rock succession. The use of fracture observations for predicting reservoir performance can take several forms: from their use to estimate spatial variations of a static equivalent permeability, to the creation of explicit fracture network models with the potential for production-related changes in fracture opening, to methods that aim to calculate fully-coupled interactions between the mechanical state and the porefluid changes. Natural fracture arrays, as seen in outcrop analogues, can range from simple patterns composed of one or two sets, to complex networks whose genesis is not simply explained. The common concepts used to explain fracture formation are based on stress states, but this conceptual framework leads to difficulties in explaining complex networks, which may show overlapping creation and movement on the discontinuities belonging to many fracture sets (Fig. 1). An alternative conceptual framework focuses on fracture arrays in terms of the strain states that they define, and relegates stress to being a local indicator of instantaneous load-carrying arrangements that evolve during fracture creation and trap growth. This framework, with its emphasis on strain accumulation by fractures, provides the understanding needed to inform reservoir prediction.