In Canada and many exploration and production areas worldwide, a better understanding of fractured reservoirs is essential to efficient exploration and production. New enhancements in software allow improved prediction and modeling of fractured reservoirs. Combined application of structural restoration software (3DMove), an associated fracture generator based on geologically oriented rules, and stress modeling (3Dstress) open new opportunities for examining and understanding fractured reservoirs. A workflow that utilizes these tools can be applied to various aspects of fractured reservoirs of interest in exploration, production, reservoir/flow modeling, drilling and field management. The workflow may contain combinations of three parts: 1) validation of the model with restoration and balancing tools, followed by forward modeling to calculate strain on surfaces and volumes, 2) creating models of the fracture network, utilizing strain and curvature attributes, for export to reservoir modeling software, 3) investigating dilation and leakage tendency of fractures and faults under a regional stress field.
The initial deformed state interpretation is first validated by structural restoration (balancing). Several geometric algorithms that describe deformation in distinct tectonic settings allow validation of models from all tectonic settings. Strain can be determined from subsequent forward modeling of the horizons back to their original deformed state, and in cases may provide a proxy for relative fracture density and a guide to probable fracture directions. Current or cumulative surface strain is determined by changes in area of the triangles that define a horizon. Finite strains are determined from the change in shape and volumes of tetrahedra that define volumes in the model. The particular geometric algorithm used in the forward modeling controls the measured strains. The algorithms mimic well-documented natural deformation processes, such as flexural slip or inclined shear. Curvature can also provide clues to areas of increased fracture density. Other data sources such as well and flow/pressure data can provide calibration and conditioning of the model. Case studies from well-exposed surface folds and from subsurface examples suggest that this approach can in certain cases provide effective predictive tools for fractures/faulting. The software provides a set of tools that can be used empirically to assist with fracture prediction away from existing well control.
In fractured reservoirs, a geologically sensible fracture network is necessary to any attempt to model flow characteristics. In reservoir modeling, the fracture models typically used are entirely statistical in nature and do not represent, convincingly, natural fracture systems as documented in many outcrop settings. The new fracture generator aims to produce more geologically viable fracture sets for use in reservoir modeling packages. Swaby and Rawnsley (1996) published a set of rules to better define fracture networks, and their system is the basis for the modeling of fracture networks in 3DMove. Strain and curvature attributes, amongst others, can be mapped onto the surface and volumes in the model. These, or any other attribute, can be used to control aspects of the fracture network. Multiple fracture sets can be created, with distinct characteristics of interaction. Well and log constraints can be used to improve the resultant fracture model before exporting to reservoir simulators. Maximum curvature can be used to condition the probability of seeding of fractures, so that more fractures will “grow” where the curvature is higher. A direction perpendicular to maximum stretching can be used to control the fracture direction. Other sets of fractures can also be created; for example a set could be oriented relative to an assumed regional tectonic stress. The resultant fracture model is stochastically derived, but according to defined rules that appear to do a much better job of reproducing natural mode one fracture systems.
Fractured reservoirs: software and workflow advancements
The third aspect of fracture analysis now available with these software tools is analysis of shear stress/normal stress ratios in various ways to get at slip tendency, dilation tendency and leakage tendency of fault or fracture networks in regional stress conditions. The dilation tendency can be viewed along with an equal area net view of the regional stresses. The effect of changes in the regional stress directions can be seen interactively with a map view or 3D-model view of faults. In case examples, the regional tectonic stresses are primary in controlling the flow paths in the fault or fracture networks. 3DStress yields a quick analysis of the stress ratio, and various displays can quickly suggest the connectivity and flow paths in the fracture network. The combined application of these new software tools can reduce exploration risk in fractured reservoirs, assist in guiding development of fields, managing drilling in complex fractured reservoirs, and managing fields from the perspective of reservoir modeling.
About the Author(s)
David Richards graduated from the University of Arizona in 1995 with a Ph.D. in Structural Geology and Tectonics, following undergraduate degrees from the University of Colorado. He has done extensive post-doctoral research on the structural geology of Bolivia, as well as having over 5 years experience in hydrocarbon exploration and production. Currently he is employed by Midland Valley Exploration, a structural geology consulting and software company based in Glasgow, Scotland.
Swaby and Rawnsley, 1996. An interactive 3D fracture modeling environment. Society of Petroleum Engineers, SPR 36004.