Summary
The extent to which fractures affect fluid pathways is a vital component of
understanding and modeling fluid flow in any reservoir. We examined the Wafra
Ratawi grainstone for which production extending for 50 years, including recent
horizontal drilling, has provided some clues about fractures, but their exact
locations, intensity, and overall effect have been elusive. In this study, we
find that a limited number of total fractures affect production characteristics
of the Ratawi reservoir. Although fractures occur throughout the Wafra field,
fracture-influenced reservoir behavior is confined to the periphery of the
field where the matrix permeability is low. This work suggests that for the
largest part of the field, explicit fractures are not necessary in the
next-generation Earth and flow-simulation models.
The geologic fracture assessment included seismic fault mapping and fracture
interpretation of image logs and cores. Fracture trends are in the northeast
and southwest quadrants, and fractures are mineralized toward the south and
west of the field. Pressure-falloff tests on some peripheral injectors indicate
partial barriers, and most of these wells lie on seismic-scale faults in the
reservoir, suggesting partial sealing. A few wells show fractured-reservoir
production characteristics, and rate-transient analysis on a few producers
indicates localized dual-porosity behavior. Producers proximal to dual-porosity
wells display single-porosity behavior, however, to attest to the notion of
localized fracture response. The spatially restricted fracture-flow
characteristics appear to correlate with fracture or vug zones in a
low-permeability reservoir.
Presence of fracture-flow behavior was tested by constructing the so-called
flow-capacity index (FCI), the ratio of khwell (well
test-derived value) to khmatrix (core-derived property).
Data from 80 wells showed khmatrix to be consistently higher
than khwell, a relationship that suggests insignificant
fracture production in these wells.
Introduction
The Wafra field is in the Partitioned Neutral Zone (PNZ) between Kuwait and
Saudi Arabia, as shown in Fig. 1. The field has been producing since the 1950s
and has seen renewed drilling activity since the late 1990s, including
horizontal drilling and implementation of peripheral water injection (Davis and
Habib 1999).
The Lower Cretaceous Ratawi formation contains the most reserves of the
producing intervals at Wafra. The Ratawi oolite (a misnomer--it is a
grainstone) reservoir has variable porosity (5 to 35%) and permeability that
ranges from tens to hundreds of md (Longacre and Ginger 1988). The main Wafra
structure is a gentle (i.e., interlimb angle >170°), doubly plunging
anticline trending north-northwest to south-southeast, which culminates near
its northern end. The East Wafra spur is a north-trending branch that extends
from the center of the main Wafra structure. As seen in Fig. 1, relief on the
Main Wafra structure exceeds that on East Wafra.
The Ratawi oolite in the Wafra field has been studied at length, and various
authors have reported geologic and engineering elements, leading to reservoir
characterization and understanding of reservoir performance. Geologic studies
are those of Waite et al. (2000) and Sibley et al. (1997). In contrast, Davis
and Habib (1999) presented implementation of peripheral water injection,
whereas Chawathé et al. (2006) discussed realignment of injection pattern owing
to lack of pressure support in the reservoir interior.
Previous studies considered the reservoir to behave like a single-porosity
system. But recent image-log fracture interpretations indicate high fracture
densities, suggesting that the implementation of a dual-porosity model may be
necessary because the high impact of fractures during field development has
been recognized in some Middle East reservoirs for more than 50 years (Daniel
1954). Static and dynamic data are required to characterize fracture reservoir
behavior accurately (Narr et al. 2006). Geologic description of the fracture
system, by use of cores, borehole images, seismic data, and well logs, does not
in itself determine whether fractures affect reservoir behavior. While seismic
and some image logs were available to locate fractures in the Wafra Ratawi
reservoir, no dynamic testing with the specific objective of understanding
fracture impact has occurred. So, to determine whether fractures influence oil
productivity significantly, we used diagnostic analyses of production data and
well tests of available injectors. The assessment of fracture effects in the
Ratawi reservoir will be used to guide the next generation of geologic and
flow-simulation models.
Dynamic data involving pressure and rate have the potential to reveal the
influence of open fractures in production performance. Unfortunately,
pressure-transient testing on single wells does not always provide conclusive
evidence about the presence of fractures with the characteristic dual-porosity
dip on the pressure-derivative signature (Bourdet et al. 1989). That is because
a correct mixture of matrix/fracture storativity must be present for the
characteristic signature to appear (Serra et al. 1983). In practice,
interference testing (Beliveau 1989) between wells appears to provide
more-definitive clues about interwell connectivity, leading to inference about
fractures.
In contrast to pressure-transient testing, rate-transient analysis offers
the potential to provide the same information without dedicated testing. In
this field, all wells are currently on submersible pumps. Consequently, the
pump-intake pressure and measured rate provided the necessary data for
pressure/rate convolution or rate-transient analysis.
We provide the Ratawi-reservoir case study primarily as an example of the
integration of diverse geologic and engineering data to develop an assessment
of fracture influence on reservoir behavior. It illustrates the use of
production-data diagnostic tests to determine fracture influence in the absence
of targeted fracture-analysis testing. The workflow can be applied to similar
static/dynamic problems, such as fault-transmissivity determination. Secondly,
this analysis illustrates the process of deciding that fractures, although
present throughout the reservoir, may not lead to widespread
fractured-reservoir characteristics (e.g., Allan and Sun 2003).
© 2008. Society of Petroleum Engineers
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History
- Original manuscript received:
15 February 2007
- Meeting paper published:
11 June 2007
- Revised manuscript received:
18 August 2008
- Manuscript approved:
1 September 2008
- Version of record:
29 December 2008
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