Predicting Stress Sensitive Productivity of Naturally Fractured Reservoirs in Low Strain Environments
Authors: Myers R. D. , Crawford B. R. , Barron J. W. , Huang H. 
Published: February 2017

Many fine-grained reservoirs require additional permeability associated with open natural fractures to achieve economic flow rates. Away frome borehole calibration, natural fractures are challenging to both characterize and predict, especially in low strain environments where natural fractures can be abundant but often lack clear causes or indicators. Additionally, properties of natural fractures (i.e. dynamic aperature) can be modified through production or injection activity, further modifying reservoir performance in typical production settings. To optimize performance in such naturally fractured reservoirs, this study focuses on improving the prediction of: (i) subsurface natural fractures associated with low strain tectonic environments and; (ii) their potential stress-sensitivity resulting from production-induced changes in reservoir pressure, where both objectives (i) and (ii) are achieved using a combination of routinely acquired core and log data. Burial and exhumation of heterolithic reservoirs in otherwise unstructured settings can result in stress variations and development of local tension and the creation of open natural fractures. To address this behavior, a methodology has been developed that incorporates mechanical stratigraphy from wireline logs, burial and thermal histories, and mechanical modeling to predict the intensity, vertical connectivity and mechanical aperture of these natural fractures. The natural fracture predictions compare very favorably with observed natural fractures in outcrops and conventional cores from the subsurface, over a range of lithologies from areas with very mild to no discernable tectonic deformation. Fracture characteristics such as intensity, vertical connectivity and aperture form the basis for predicting static fracture network connectivity and permeability in these relatively undeformed reservoirs. Coupled with these predictions of natural fracture characteristics, predictions of pressure sensitivity, based on fracture mechanical properties derived from laboratoryl testing, are generated to account for dynamic changes in fracture aperture associated with pressure depletion. It has been determined through laboratory geomechanics testing of natural fractures, subjected to realistic subsurface loads, that fracture compressibility and transmissibility multipliers are primarily a function of host lithology stiffness and initial unstressed fracture aperture. By incorporating fracture compressibility and transmissibility into reservoir simulation models, it has been recognized that dynamic apertures induced by production or injection activity can be of sufficient magnitude to impact reservoir performance and sweep efficiency in typical production settings. This paper shows a novel technique for prediction of opening-mode natural fractures associated with burial and exhumation of heterolithic sequences using wireline log-based data. It describes new experimental geomechanics results of fracture compressibility and stress-sensitive permeability reduction in a wide range of host lithologies and previews a procedure for incorporating such dynamic fracture behavior in subsurface models for improved reservoir performance prediction.