AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Fracture Characterization of the 1st Eocene, Wafra Field, Kuwait-Saudi Arabia
(1) Energy Technology Company, Chevron, San Ramon, CA.
(2) Tengizchevroil LLP, Chevron, Atyrau, Kazakhstan.
We evaluated the natural fracture distribution and orientation within the 1st Eocene carbonate reservoir of the Wafra Field in the Partitioned Zone shared by Kuwait and Saudi Arabia, as part of a larger effort to characterize the reservoir. A total of 37 FMI logs are analyzed throughout the field, including 17 wells located in the Large Scale Pilot (LSP) steam flood area. Preliminary FMI interpretation of the LSP area in the fall of 2009 indicated many natural fractures in each of the wells. Here, we reach different conclusions based on a more detailed study that includes comparison of FMI interpretations to available cores of nine wells for more accurate fracture characterization. Due to the lack of PLT data and lost circulation reports, uncertainty remains as to whether the mapped fractures affect subsurface fluid flow in both the LSP and throughout Wafra field. Thus, using constraints from image log interpretations, a discrete fracture model was generated for the LSP area to investigate the potential impact fractures may have on reservoir connectivity.
In the 1st Eocene reservoir, three fracture sets are identified; NE-SW, E-W, and NW-SE. The in-situ stress direction determined from borehole breakouts and drilling induced tensile fractures in FMI is consistent throughout the 1st Eocene, trending parallel to the dominant NE-SW fracture set. The E-W set is least abundant and oblique to the several sub-parallel anticlinal axes of Wafra field. The NW-SE set is parallel to the axes of the anticlines.
We find that overall, fracture density is low (mean interval density for the LSP = 0.0035 ft2/ft3), which is less than 10% of the amount suggested by the preliminary study. The highest fracture density values occur in the upper 1st Eocene and in a small interval lower in the reservoir. Little correlation was found between intervals of higher fracture density and lithology. A discrete fracture model, with variation in fracture density that corresponds with stratigraphically defined average values, was used to evaluate the potential for fracture interconnectivity between wells. Several realizations suggest small clusters of fractures within the LSP could locally enhance the drainage of some wells, and rare well-to-well fracture connectivity is possible, but widespread well-to-well interconnectivity is very unlikely. Thus, we conclude that the 1st Eocene is a lightly fractured carbonate reservoir where fractures contribute little to reservoir productivity.