Abstract: Faulting, Fracturing and In-Situ Stress Prediction In Hydrocarbon Reservoirs: a Finite Element Approach

Beekman, F. and J. D. van Wees - Vrije Universiteit

Natural fracture and fault systems are important contributors to reservoir permeability in many hydrocarbon reservoirs. Low-efficiency hydrocarbon reservoirs that have very low matrix permeabilities are productive largely because delivery of fluids to well bores is controlled by natural fractures. Faults play an important role in creating hydrocarbon traps and in the formation of sealed compartments in hydrocarbon reservoirs. And in addition to the enhancement of flow and storage of fluids in low-permeability rocks and low-porosity rocks, fractures and faults may be necessary to allow primary migration of hydrocarbons from source rocks and to rupture seals in pressure cells.

Basic information on faults and fractures, including reliable data on fracture strike, is often exceedingly sparse, thus hindering efficient development through optimized well placement or directional drilling. As exploration and development move into increasingly challenging and deeper reservoirs where natural fractures are key to successful completion, accurate and low-cost information on fault and fracture information and other fracture attributes will be critical in reducing well costs and increasing well recoveries. Modern advanced numerical modeling techniques may provide such information.

In a case study we used the finite element method to compute in-situ stresses and rock deformation in a hydrocarbon bearing fault-propagation-fold structure (Fig. 1a), and to predict reservoir fracturing and fracture properties as a function of material properties, structural position and tectonic stress. Two-dimensional plane-strain end-member models show that the presence of a thick shale layer (Fig. 1b) leads to a mechanical decoupling of the structural deformation of the shallower sediments from the underlying sediments and basement. All models predict rock fracturing to initiate at the surface and to expand with depth under increasing horizontal tectonic compression. The stress regime for the formation of new fractures changes from compressional to shear with depth (Fig. 2a). If pre-existing fractures exist, only (sub) horizontal fractures are predicted to open, thus defining the principal orientation of effective reservoir permeability. In models which do not include a blind thrust fault in the basement, flexural amplification of the initial fold structure generates additional fracturing in the crest of the anticline. The folding-induced fracturing expands laterally along the stratigraphic boundaries under enhanced tectonic loading. Models incorporating a blind thrust fault correctly predict the formation of secondary syn- and antithetic mesoscale faults in the basement and sediments of the hanging wall (Fig. 2b). The faults divide the structure in compartments with different levels of pressure, stress and rock failure. Some faults cut reservoir and seal layers, and thus may affect seal integrity and influence effective reservoir permeability. The model outcomes may assist classic interpretation of seismic and well bore data in search of fractured and overpressured hydrocarbon reservoirs.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil