Print this pageFracture and In-Situ Stress Characterization of the Unayzah-B/C Tight Gas Reservoir, South Haradh, Saudi Arabia
We analyzed fractures in the Unayzah reservoir (Upper Carboniferous-Permian), South Haradh, Saudi Arabia, using a total of 15,000 ft imaged and cored sections, 3D seismic and dynamic data to facilitate exploration and development, particularly in the lower tight part of the formation (Unayzah-B/C). The Unayzah-A is a non-brittle, sparsely fractured unit. The Unayzah-B/C is a brittle unit in which meso- and micro-tectonic fractures are dominant and include stylolite-related tensile fractures and non-stylolite fractures (tensile fractures and faults). Most fractures are barren with clay mineral or quartz coating or partial mineralization and dead hydrocarbon. The fractures aperture is of micron scale. The non-stylolite fractures are most effective permeability conduits when they occur in clusters. The clusters are steep dipping (angle of dip ≥70°) with dominant NE-SW to E-W strikes, up to 3 ft width (thickness perpendicular to zone vertical boundaries) and up to 16 ft minimum vertical extent (height). The clusters true spacing ranges from 20 ft to 60 ft. The fracture clusters consist of a scale-independent system, where meso-fractures are connected via microscopic fractures.
The maximum horizontal in-situ stress is oriented NE-SW to ENE-WSW nearly parallel to the dominant open fractures. The parallel orientation of the fracture clusters to the plate-tectonic stress regime in the region indicates that they are controlled by such stresses rather than the underlying basement rooted forced folds and faults (predicted from seismic data). Open tectonic fractures are essential to productivity in the Unayzah-B/C and their role is a function of aperture, size, density, clustering and connectivity. Stimulation by hydraulic fracturing is used to enhance connectivity and productivity.
Early diagenitic structures (faults, dewatering and slump structures) are preferentially oriented parallel to the basement faults and forced folds axes, probably due to unstable slopes and seismic activities related to the basement structures.
AAPG Search and Discover Article #90100©2009 AAPG International Conference and Exhibition 15-18 November 2009, Rio de Janeiro, Brazil