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The RCRL program has
run continuously since 1987 and has produced more than 30 outside and
BEG publications on carbonate reservoir characterization, sequence stratigraphy,
petrophysics, geostatistics, and petroleum engineering. We provide research
results to sponsor companies through annual review meetings, CD's,
preprints of publications, short courses on geologic and engineering aspects
of our research, and a mentoring program working hands-on with industry
staff and data sets. In addition, our results are posted in a password-protected
part of our website. (http://www.beg.utexas.edu/rcrl/members/)
The RCRL has maintained
a membership of between 13 and 18 companies per year. The sponsorship
currently has strong interests in Permian and Alberta Basins as well as
Middle Eastern carbonate reservoirs. This enrollment, supplemented by
government grants, supports from three to six professional staff members
and varying numbers of graduate student research associates, plus strong
computer and graphics support. All staff members have extensive industry
experience or have worked closely with industry and are well aware of
the challenges and questions facing development geoscientists and engineers.
We are also very proud of our graduate student staff, which includes several
award-winning students, many of whom are now working in the industry.
Each year we combine
industry input with our own ongoing research plans to develop a set of
key geological and engineering research topics. Plans for 2001 include
the following:
- A new geological
modeling research initiative within the RCRL that has the primary goal
of developing improved methods for distributing rock-fabric facies and
petrophysical properties in reservoir models. An important element of
this work will be seismic modeling of detailed outcrop models and the
use of these images for guiding 3-D seismic interpretation at the characterization
scale. Outcrop facies dimension data and information on petrophysical
variability as determined from outcrop will be integrated with subsurface
field data to advance this important research direction.
- Further development
of the RCRL carbonate reservoir analog data base will take place in
2001. The data base will begin to go online in late 2000, and we will
continue to add both digital and image outcrop data, as well as selective
data from reservoirs that we have studied over the past 10 years.
- Development of
a rock-fabric method for estimating relative permeability is a critical
multiyear effort for the RCRL that will build on our existing knowledge
of the relationship between porosity, permeability, capillary properties,
and rock fabric. The goal is to improve our ability to distribute relative
permeability more intelligently in geologic models by linking it to
properties for which spatial statistics are available.
- Modeling touching-vug
pore systems is still a major issue in carbonate reservoir characterization.
We will approach this issue through an integrated analysis of the SACROC
Unit of the Horseshoe Atoll complex, in cooperation with Kinder Morgan
Company. A preliminary model of this unit conducted in 1999 showed a
predictive nature to the distribution of touching-vug, separate-vug,
and interparticle pore space. A more complete phase of core description,
log interpretation, 3-D model construction, and analysis of production
data will form the basis of our understanding of the role of touching-vug
pore systems.
- Characterizing
fluid flow through large vugs using a combination of CT scanning and
detailed flow modeling will continue in 2001 using large samples of
vuggy rudist facies.
- Modeling fracture
distribution and effective flow properties will be carried out in conjunction
with the DOE Clear Fork Outcrop-Subsurface modeling project.
- Icehouse carbonate
systems remain a focus for the RCRL. Outcrop studies will focus on slope
and toe-of-slope reservoir analogs from Victorio Canyon. Subsurface
studies will highlight the SACROC Unit study mentioned in the touching-vug
section.
- Regional sequence
framework of Shuaiba reservoirs is an ongoing research area for the
RCRL, and additional data will be collected this year to complete this
synthesis.
RESEARCH
DIRECTIONS FOR 2001
Geologic
Modeling
The RCRL’s past
research has established the fundamental link between rock-fabric facies
and petrophysical properties for most carbonate reservoirs. This link
is highly significant in reservoir modeling, because it recognizes the
importance of accurately distributing rock-fabric facies at the high-frequency-cycle
scale in modeling petrophysical properties away from the well bore. The
principal goal of geologic modeling research in the RCRL is to develop
improved methods for distributing rock-fabric facies and petrophysical
properties in reservoir models. Established methods for defining facies
tracts and facies distribution within a reservoir involve the construction
of a sequence stratigraphic framework, followed by the distribution of
rock-fabric facies at the flow-unit scale. The RCRL maintains that the
adequacy of these methods to produce geologically realistic reservoir
models depends on the degree to which the sequence stratigraphic framework
and rock-fabric facies distribution have been conditioned by appropriate
geological models, outcrop analogs, 3-D seismic data, and petrophysical
data.
In 2001 we plan to
construct a 3-D geological model of facies and petrophysical properties
that would be based on one of the better-studied outcrop analogs in the
RCRL’s portfolio. Pseudowells, with synthetic core and well-log data,
and a synthetic 3-D seismic volume will be generated from this 3-D geological
model. These derivative datasets will be used to build a series of new
models using different techniques for comparison with the original synthetic
geological model. This project would provide a testing ground to address
the adequacy of various techniques for building accurate geological models.
Given the ground truth provided by the initial synthetic geological model,
subsequent models built from derivative data could be quantitatively compared
with the ground-truth to provide accurate metrics of model accuracy as
a function of data type and distribution and modeling methods.
Development
of a Rock-Fabric Method for Estimating Relative Permeability
Relative permeability
is a key parameter in the numerical simulation of multiphase fluid flow
in permeable media. Although the industry has expended considerable effort
conducting relative-permeability (kr) measurements on samples
from carbonate reservoirs, no unifying factor allows measurements that
have been made on samples from one reservoir to be transported to another
reservoir. The key to transporting kr data is to develop a
relationship between geologic descriptions and relative-permeability attributes.
Such a relationship would allow kr to be estimated in carbonate
media that have the appropriate geologic description and have few or no
kr data. RCRL has developed a robust system for relating carbonate
rock fabrics to pore-size distribution, porosity, permeability, and capillary
pressure. We propose that a similar system can be developed between carbonate
rock fabrics and attributes of kr.
We propose to
collect kr and capillary-pressure data and samples from sponsoring
companies, describe the rock fabrics, catalog the data in a data base,
and develop relationships between kr attributes and elements
of rock fabric. As a part of this project we plan to develop methods of
characterizing heterogeneity within core samples by using high-resolution
CT scans. We will make the results of this project available to contributing
companies in reports and through the RCRL data base. This information
will be useful for estimating kr attributes in reservoirs where
an appropriate geologic model has been developed but limited or no kr data are available.
Carbonate
Reservoir Analog Database
An important step
in constructing a subsurface reservoir model is the ground-truth verification
of the interpreted model and its essential components against well-studied
analogs, be they from outcrops or closely analogous subsurface reservoir
models. The RCRL is constructing a data base that is designed to provide
access to outcrop and subsurface data collected by the RCRL over the past
12 years in the general area of carbonate reservoir characterization.
Data types that will be accessible include:
- Detailed cross
sections of representative platform architecture from outcrops of Permian
Wolfcampian, Clear Fork, San Andres, Grayburg, and Queen units and the
Cretaceous of Pecos River Canyon.
- Digital foor-by-foot
facies descriptions for outcrop measured sections of all Permian and
Cretaceous study areas. Each measured foot is keyed to such factors
as platform type and geometry, P/A ratio, sequence hierarchy and systems
tract, and other parameters to allow recombination and analysis of data.
- Outcrop and thin-section
photographs of representative facies tied to graphical measured sections.
- Facies object dimensions
from outcrop mapping of more than 400 grainstone, grain-dominated packstone,
and tight mudstone units from a range of stratigraphic ages and accommodation
settings.
- Petrophysical analyses
of rock fabric examples associated with key outcrop data sets.
- Spreadsheets including
all current porosity and permeability data from the various outcrop
study areas.
- Representative
reservoir characterization study results, including digital wireline-log,
core-analysis, and capillary-pressure data, coupled with images of core
slabs, rock fabrics, and interpretive maps and cross sections. Fields
studied over the last 12 years include Permian San Andres, Grayburg,
and Clear Fork reservoirs (Seminole, South Cowden, South Wasson, Fuhrman
Mascho, Dune), Cretaceous reservoirs of the Middle East (Al Huwaisah,
Idd el Shargi, Yibal) and Brazil (Santos Basin), and Jurassic reservoirs
of the Middle East.
- Example flow-simulation
animations showing impact of different heterogeneity styles on sweep
efficiency.
The data base
will run on a local BEG server and will be accessed by a web-searchable
interface available to sponsors only. Large file information (large images
or spread sheets) will be routed to an FTP server. Once RCRL data have
been uploaded, we will begin to consider incorporating outside data.
Modeling
Touch-Vug Pore Systems Within a Sequence Framework; SACROC Unit
Modeling and predicting
reservoir performance from touching-vug (tvug) pore systems remain the
most difficult aspect of carbonate reservoir characterization. Tvug’s
are commonly the source of high-permeability zones (super-k) that cannot
be directly related to depositional facies. The key steps to characterizing
this type of pore space are (1) identifying the high-permeability horizon,
(2) determining the origin of the tvug system, and (3) creating a numerical
model that describes the porosity, permeability, and capillary properties
of the tvug system. BEG has completed several studies of carbonate touch-vug
reservoirs, and we plan to review the results of these studies, as well
as other publications, and report on any generalizations derived from
these studies.
We plan to concentrate
on a study of the SACROC reservoir of Pennsylvanian age in the Permian
Basin, West Texas. Previous investigations by the RCRL have shown this
reservoir to have a significant karst overprint, as well as interparticle
and separate-vug porosity. The pore types appear to be arranged in a predictable
stratigraphic pattern. A key goal of this study will be to relate the
extensive production history from this field to the stratigraphic arrangement
of the pore types. This will be accomplished by first constructing a reservoir
model using cores, logs, and 3-D seismic and then integrating the operational
history and production performance with the reservoir model. The SACROC
study will provide an excellent opportunity to model a buildup-dominated
shelf margin and isolated platform interior, and it will provide an opportunity
to integrate production and engineering data into geologic models of rock-fabric
facies and petrophysical properties.
CT
Scanning and Flow Modeling of Vuggy Rocks
We have completed
CT scanning (in collaboration with the UT X-ray CT lab), image analysis,
flow-model construction, and preliminary flow modeling (in collaboration
with the Texas Institute for Computational and Applied Mathematics) for
a 10-inch ´ 14-inch
vuggy rudistid sample collected from the Pipe Creek Reef, Texas. We have
also completed CT scanning of one of three subsurface 4-inch-diameter
core samples supplied by Petroleum Development Oman from a Cretaceous
carbonate oil reservoir. The preliminary flow-modeling results from the
Pipe Creek sample indicate that the vugs do not form a connected network
across the entire sample. The calculated effective permeabilities are
in the 1- to 10-Darcy range.
In 2001 we will complete
CT scanning, image analysis, and flow-model construction of the other
two subsurface samples. We also plan to collect additional petrophysical
data from the scanned samples via plug analysis and minipermeametry. We
will continue our flow-modeling work to refine the effective permeability
calculations while investigating issues of pixel-scale permeability modeling,
scaleup, boundary conditions, and Navier-Stokes flow in the vugs. Although
this flow-modeling work will be concentrated on the Pipe Creek sample,
it will include fluid-flow simulations on the three subsurface samples
as well. We also plan to begin constructing generalized petrophysical
models for porous media having large vugs on the basis of insight we are
obtaining in this research.
We also plan to demonstrate
the effects of high-permeability vuggy intervals on reservoir performance
using a simplified model of the Al Huwaisah reservoir. Construction of
this model will incorporate a geological model developed by Petroleum
Development Oman, identification of vuggy intervals made from image logs,
and effective permeability estimates made from the Pipe Creek sample.
Modeling
of Fracture Networks and Their Fluid-Flow Effects — South Wasson
Clear Fork Reservoir
As a component of
our DOE-funded project to conduct an integrated outcrop-subsurface study
of the South Wasson Clear Fork oil reservoir, we have obtained measurements
(with the help of Jon Olsen’s rock mechanics laboratory at the UT
Department of Petroleum Engineering) of the subcritical crack index from
South Wasson core samples. In 2001 we will use this information to construct
fracture-network simulations for use in fluid-flow calculations to estimate
effective flow properties. These effective properties will be incorporated
into our ongoing South Wasson simulation study. In our final report on
this project, due October 2001, we will discuss the merits of this fracture-modeling
approach and the effects of fracture flow on South Wasson reservoir behavior.
Sequence
Stratigraphic Setting and Reservoir Heterogeneity Styles of Icehouse Slope
and Toe-of-Slope facies
Reservoirs developed
in slope and toe-of-slope carbonates represent an entirely separate and
unique style of heterogeneity, unlike any other studied by the RCRL. These
facies are known to be important reservoirs in the Wolfcampian and lower
Leonardian of the Permian Basin, the Cretaceous of Mexico (for example
Poza Rica) and must recently in the rims of some of the North Caspian
carbonate giants, including Tengiz. Slope and toe-of-slope carbonates
are the least well studied of ancient carbonate systems, particularly
in terms of the composition of grains, the dip and strike dimension of
fundamental flow units, and the sequence controls on slope formation.
The Wolfcampian
outcrops along the east face of the Sierra Diablo Range and in Victorio
Canyon on the west side of the Delaware Basin will provide an ideal setting
for studying slope and toe-of-slope carbonate reservoir facies in terms
of both facies architecture and sequence stratigraphic framework and controls.
These outcrops present continuous shelf-to-basin floor transects, which
will allow a direct link between slope deposits and their equivalent sections
on the platform. Tracing this platform-to-basin transition will help clarify
the relative partitioning of slope deposits into LST, TST, and HST systems
tracts and will allow mapping and documentation of the dip and strike
dimensions of flow units on the slope. Work was initiated on this project
in late 2000 and will carry through 2001 with the extablishment of the
geologic framework. Petrophysical and reservoir modeling efforts beginning
in 2002.
Regional
Sequence Framework Of Cretaceous Shuaiba Reservoirs
During the last 4
years the RCRL has conducted studies of several major Shuaiba reservoirs
in the Bab intrashelf basin area. We hope to continue this effort in 2001
by adding detailed descriptions of other major Shuaiba fields and by focusing
on regional correlation of these data. As with previous efforts, all core-description
and log-correlation data will be accessible through the data base and
as PDF files on CD's. |
