Natural Fractures Enhancing Hydrofracs in a Dense Reservoir, a Case Study from Abu Dhabi
Authors: Noufal Abdelwahab , Obaid Khalid , Bregar Udo 
Published: May 2017

Natural fractures represent planes of weakness within tight dense reservoirs in Abu Dhabi fields. It is likely that the fractures types, fracture density and orientation will influence hydraulic fractures initiation and propagation. Thus, the interaction between natural fractures and hydraulic fractures is the key objective of this paper. Limited studies have been conducted on natural fractures in Abu Dhabi, from this perspective and this represents a clear gap in our understanding. This paper addresses a workflow to predict the natural fractures that will enhance the hydrofrac initiation and propagation.

There are, however, several concerns related to the dense zones exploration and production, as they are associated with hydraulic fracturing. There is also a debate on the natural fractures impact on the hydrofrac initiation and propagation. In addition, which fracture types, if they will slip or reactivate, and bedding planes corridors. Accordingly, there is a need for understanding the behavior of natural fractures, and their critical stress state. Therefore, there is a need to assess the interactions between natural fractures from the borehole images and core, and their presence on seismic attributes.

The workflow integrates multidisciplinary data aimed to optimizing our understanding of hydraulic fractures and their relations to the natural fractures. Seismic (interpreting key seismic horizons and generating attributes), Bore Hole Images (BHI), Core, wireline logs, CT scan, and Rock Mechanics testing (RMT) were used to characterize the fracture system and build a 1D-MEM (Mechanical Earth Model). On top of that, reservoir heterogeneity and relation to tectonic deformation events were established.

Critically stressed fractures are opened when exposed to additional stress, and can act as pathways for fluid flow. The studied reservoirs showing critically stressed fractures that influence the hydraulic fracs. Most of the reservoir units are dominated by the NW, WNW, NE to E-W fracture and microfault trends, but show the dominance of the NW trend. The spatial distribution of microfaults, hybrid fractures and pure opening mode fissures with wide variations in strike appears to be related to their location with respect to the crestal axis of the main folded fields and in the faulted zones relative to the location between the strike-slip segments. Only fractures with certain dip and dip azimuth could be critically stressed. From the stereoplot, it can be observed that four groups are considered as critically stresses, these are: N75°W: The strain character of this group is of horizontal stretching along the NW-SE. The data sets collected from the studied wells have fracture populations of open mode-extensional as the principal stress ratio increases.N45°W: The group dominated by dilational opening mode fractures. The strain is of horizontal shortening stretching along the NE.E10°E This group is represented by shearing mode fractures. The strain is of horizontal shortening and subhorizontal stretching along the NE.W45°E: This group is represented by rotated shearing fractures opening mode fractures. The strain is of horizontal shortening and subhorizontal stretching along the WSW.

Technical contributions and implications of these results include hydraulic fracturing initiation and propagation enhancement through the presence of critically stressed fractures for optimum production and correctly hydraulic fracs design.