In this study, an attempt has been made to develop a new theoretical model that can be used to predict the fracture spacing/density that develops in a single competent layer and in multilayers as a result of folding. The work is based on earlier analyses concerned with the fracturing of unfolded strata subjected only to layer-normal compression. Such a stress state exists in the upper crust in any tectonically relaxed region where the principal cause of stress is the overburden.
Unlike previous studies on theoretical fracture-spacing modelling that are mainly designed for a layer-parallel horizontal-extension system, this study has introduced a new theoretical model for the predicting and modelling of fracture spacing/density in ‘folded’ reservoirs, which contain > 85% of the world's oil and gas traps.
This theoretical model is an integrated model: that is, it takes into account both rock mechanical and geometrical properties of the reservoir.
The big advantage of the theoretical model developed in this study is that it provides well- and reservoir-scale estimates of the fracture spacing, for both axial and cross-axial fractures (i.e. the dominant fracture sets in folded reservoirs), which can be used for predicting fracture density (the reciprocal of fracture spacing), fracture aperture, the Rock Fracture Potential Index (RFPI), fracture porosity, fracture permeability, the shape factor (sigma) and for optimizing the drilling (i.e. the Optimum Drilling Direction (ODD) and the Optimum Drilling Angle (ODA) to maximize the fracture intersection in the wells) in three dimensions in folded, single layer and multilayer fractured reservoirs.
In addition, new approaches are described for quantifying the mechanical bed thickness (MBT) or mechanical unit thickness (MUT), estimating the fracture aperture (w), estimating the distance from the neutral surface (a) and determining the RFPI data that are essential for implementing the theoretical model presented in this paper related to subsurface, folded, fractured reservoirs.
The expressions derived for fracture spacing/density, for both axial and cross-axial fracture sets, involve data that are always available for every field development (i.e. seismic, well and core data). An understanding of the distribution of the fracture spacing/density, fracture aperture and the RFPI at an early stage in the development of a fractured reservoir is crucial in selecting a proper field development strategy, managing well placement and for monitoring production from the reservoir.
In summary, based on several case studies, one of which is presented in this paper, it can be confirmed that the theoretical model, expressed in equations given in this paper, predicts what has been observed in the folded clastic reservoirs of the study area. It is concluded that curvature alone cannot reveal the location of natural fractures in a reservoir and that the mechanical properties of the reservoir rock play a significant role in the development of a natural fracture system. Rock can accommodate strain by fracturing (i.e. if its RFPI is high) or (if its RFPI is low) use its internal strain storage capability (associated with its mechanical properties: e.g. porosity collapsing, grain sliding and the formation of intra-grain hairline fractures) to consume the stress without the need to accommodate strain by fracturing.