Diverse Fracture Properties and Their Impact on Performance in Conventional and Tight Gas Reservoirs, Saudi Arabia: The Unayzah, South Haradh Case Study

The Late Permian-Carboniferous Unayzah Formation in South Haradh, Saudi Arabia includes two major mechanical and petrophysical layers that are separated by shale-rich zones. Open tectonic fracture clusters are rare and not essential for fluid flow in the Unayzah A Zone which has high porosity and permeability. However such fracture clusters are essential to, and impact the production performance in the Unayzah B/C tight gas reservoir. The occurrence of the tectonic fractures in the Unayzah Formation is linked to the rock mechanical properties, which vary with porosity, shale volume, cement type, and texture. The B/C unit is more fractured than the A unit but its layers vary in degree of fracturing. The variation in fracture development within the B/C unit results in differences in fracture-enhanced permeability, based on production profiles where flow is restricted to preferentially fractured mechanical layers that lack effective vertical fluid communication with other layers.

We identify two tectonic fracture systems: an older, subordinate fully mineralized system and a younger, primary mostly open system. Early extensional fractures including joints and faults developed parallel to the basement faults during the opening of the Neo-Tethys. These are fully mineralized and have little or no role as fluid conduits. The younger system includes open fracture clusters that are predominantly parallel or nearly parallel to the regional ENE-WSW maximum horizontal stress of the Zagros (that has been active since late Cretaceous) and is independent of local structures. Therefore these fractures are controlled by remote stresses rather than the basement -rooted forced folds and faults. In this paper we demonstrate that in the Unayzah B/C natural fractures are essential to permeability and in some areas to porosity, and thence to reservoir performance. The results of this study are being implemented in well placement and completion design to optimize intersection of open fracture clusters with positive preliminary results.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California