Summary
In clastic reservoirs in the Oriente basin, South America, the rock-quality
index (RQI) and flow-zone indicator (FZI) have proved to be effective
techniques for rock-type classifications. It has long been recognized that
excellent permeability/porosity relationships can be obtained once the
conventional core data are grouped according to their rock types. Furthermore,
it was also observed from this study that the capillary pressure curves, as
well as the relative permeability curves, show close relationships with the
defined rock types in the basin. These results lead us to believe that if the
rock type is defined properly, then a realistic permeability model, a unique
set of relative permeability curves, and a consistent J function can be
developed for a given rock type. The primary purpose of this paper is to
demonstrate the procedure for implementing this technique in our reservoir
modeling.
First, conventional core data were used to define the rock types for the
cored intervals. The wireline log measurements at the cored depths were
extracted, normalized, and subsequently analyzed together with the calculated
rock types. A mathematical model was then built to predict the rock type in
uncored intervals and in uncored wells. This allows the generation of a
synthetic rock-type log for all wells with modern log suites. Geostatistical
techniques can then be used to populate the rock type throughout a
reservoir.
After rock type and porosity are populated properly, the permeability can be
estimated by use of the unique permeability/porosity relationship for a given
rock type. The initial water saturation for a reservoir can be estimated
subsequently by use of the corresponding rock-type, porosity, and permeability
models as well as the rock-type-based J functions. We observed that a
global permeability multiplier became unnecessary in our reservoir-simulation
models when the permeability model is constructed with this technique.
Consistent initial-water-saturation models (i.e., calculated and log-measured
water saturations are in excellent agreement) can be obtained when the proper
J function is used for a given rock type. As a result, the uncertainty
associated with volumetric calculations is greatly reduced as a more accurate
initial-water-saturation model is used. The true dynamic characteristics (i.e.,
the flow capacity) of the reservoir are captured in the reservoir-simulation
model when a more reliable permeability model is used.
Introduction
Rock typing is a process of classifying reservoir rocks into distinct units,
each of which was deposited under similar geological conditions and has
undergone similar diagenetic alterations (Gunter et al. 1997). When properly
classified, a given rock type is imprinted by a unique permeability/porosity
relationship, capillary pressure profile (or J function), and set of
relative permeability curves (Gunter et al. 1997; Hartmann and Farina 2004;
Amaefule et al. 1993). As a result, when properly applied, rock typing can lead
to the accurate estimation of formation permeability in uncored intervals and
in uncored wells; reliable generation of initial-water-saturation profile; and
subsequently, the consistent and realistic simulation of reservoir dynamic
behavior and production performance.
Of the various quantitative rock-typing techniques (Gunter et al. 1997;
Hartmann and Farina 2004; Amaefule et al. 1993; Porras and Campos 2001;
Jennings and Lucia 2001; Rincones et al. 2000; Soto et al. 2001) presented in
the literature, two techniques (RQI/FZI and Winland’s R35) appear to be used
more widely than the others for clastic reservoirs (Gunter et al. 1997,
Amaefule et al. 1993). In the RQI/FZI approach (Amaefule et al. 1993), rock
types are classified with the following three equations: [equations]
© 2007. Society of Petroleum Engineers
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History
- Original manuscript received:
13 July 2005
- Meeting paper published:
9 October 2005
- Revised manuscript received:
6 November 2006
- Manuscript approved:
11 August 2007
- Version of record:
20 December 2007
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