The RCRL program has
run continuously since 1987 and has produced more than 27 outside and
BEG publications on carbonate reservoir characterization, sequence stratigraphy,
petrophysics, geostatistics, and petroleum engineering, including papers
winning the 1994 and 1995 AAPG Wallace Pratt awards for best publication
in the Bulletin for that year. We provide research results to sponsor
companies through annual review meetings, executive slide sets, 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
portion of our Web site (www.utexas.research.beg).
The RCRL has maintained
a membership of between 14 and 18 companies per year. The sponsorship
currently has strong interests in Permian Basin, Middle Eastern, and Alberta
Basin carbonate reservoirs. This enrollment, supplemented by government
grants, provides support for three to six professional staff and varying
numbers of graduate student research associates, plus strong computer
and graphics support. Two geologists (Kerans, Ruppel), a geological engineer
(Lucia), and a reservoir engineer/modeler (Jennings) form our core group.
All staff 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 that 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 geologic and engineering research topics. Our
research efforts in 2000 will be directed towards the following projects:
Database
Program
The RCRL has accumulated
a large body of information ranging from sequence stratigraphy to fluid-flow
simulation on a wide variety of reservoirs. In 2000 the RCRL plans to
develop an integrated database on carbonate reservoir heterogeneity styles
using facies, sequence stratigraphy, petrophysics and rock types, and
geostatistical analysis and flow simulation as data types. An HTML environment
will be used to rapidly link a range of data types including photomosaics,
photomicrographs, core descriptions and related cross sections, graphs
and illustrations, raw data including petrophysical data and facies data
in Excel and ASCII format, movie files of simulations, and other material.
The hierarchical database
will group reservoirs in terms of basic carbonate depositional settings
and diagenetic styles. Geologic search parameters will include (1) age,
(2) Milankovitch setting, (3) margin/platform type, (4) sequence setting
(systems tract and facies tract position), and (4) diagenetic environment.
Rock fabric categorization will also be a key sorting parameter so that,
for example, a series of examples with separate-vug porosity regardless
of age could be examined in succession. The database format is still evolving,
and we welcome comments and input on organization from interested sponsors.
Cretaceous
Reservoir Program
The RCRL has a continuing
program studying world-class Cretaceous reservoirs and reservoir-analog
outcrops. To date the reservoirs studied are within the upper Aptian Shuaiba
Formation in (1) Idd el Shargi, offshore Qatar (in conjunction with OXY
International and Q.G.P.C.), (2) Al Huwaisa, Oman (in conjunction with
P.D.O.), and most recently and still under way for 2000, (3) the Yibal
field in central Oman (again in conjunction with P.D.O.). All three reservoirs
provide important clues to a larger general model of Shuaiba production
that we are currently working on. These studies feature advances in understanding
the reservoir correlation framework and petrophysical description of these
fields, and schemes range from very basic to very complex, depending on
the Shuaiba field or facies being considered.
The Yibal study is
being carried out during 1999-2000 with data and special assistance from
PDO. This study will be an integrated study that will feed into work on
fracture modeling being carried out simultaneously by Shell EP Technology
and Research (Rijswijk). Relevant results will be presented at the 2000
meeting.
In conjunction with
this subsurface work the RCRL maintains a parallel effort to describe
and utilize outcrop analog data to provide the conceptual basis for advanced
stratigraphic frameworks and geostatistical insights into characterization
and model building. The past 5 years have led to development of an outcrop
research laboratory in upper Albian strata of the Pecos River Canyon.
This outcrop has been the site of extensive detailed outcrop descriptions
and some petrophysical and geostatistical analysis.
In 2000 the RCRL plans
to expand this program by conducting a collaborative effort with Elf Aquitaine
Research. This program will test a carbonate reservoir modeling method
developed by Elf. The modeling will involve (1) collection of petrophysical,
paleobathymetric, and facies data from the outcrop, (2) construction of
accommodation and paleobathymetric models, (3) correlation of paleobathymetry
to facies and petrophysics, and (4) testing the resulting models by comparison
with mapped outcrop properties.
New areas in southwest
Texas are currently being examined as analogs for Middle Eastern Cretaceous
reservoirs. We are searching for isolated buildups similar to those we
believe many of the Shuaiba fields represent.
Icehouse
(Carboniferous) Reservoir Program and Paleokarst Studies
In 1999 the RCRL began
to collect data on icehouse reservoir heterogeneity models. Outcrop data
were collected from the Big Hatchet Range of the Pedregosa Basin, New
Mexico. The giant SACROC Unit of the Horseshoe Atoll, Permian Basin, Texas,
serves as our first reservoir data set for this style of carbonate. This
research element is still in the early stages but has as its goal the
development of a set of general reservoir heterogeneity models for Carboniferous-Early
Permian carbonate platform reservoirs, such as the Carboniferous reservoirs
of the North Caspian that developed under conditions of high-amplitude
high-frequency eustatic settings.
The Big Hatchet Range
is well suited for examination of icehouse platform reservoir facies because
it spans a complete range of Pennsylvanian Stephanian-Sakmarian deposition
ranging from platform interior grain-dominated pelmatozoan sheets, to
platform margin cyclic algal mound complexes, and platform margin to slope
breccias. Multiple occurrences of late diagenetic dolomitization and its
relationship to stratigraphically and structurally controlled permeability
pathways are also present and will be the focus of one of the key styles
of reservoir heterogeneity in these limestone-dominated reservoirs.
The SACROC Unit data
set provided by PennzEnergy (now Devon Energy) is a classic example of
stratigraphically and diagenetically complex icehouse reservoir systems.
A spectrum from laterally continuous platform-top upward-shallowing cycles
to complex transgressive debris/reef wedges illustrates some of the possible
depositional heterogeneities. Simple interparticle pore types such as
in pelmatozoan grain-dominated packstones are common, but moldic porosity
in ooid shoal complexes is also important related to the high-frequency
high-amplitude glacio-eustatic falls and associated meteoric leaching.
Paleokarst-generated fractures and vuggy porosity are also important elements
of the reservoir system, and their impact is still being assessed.
The RCRL plans to
continue this icehouse reservoir heterogeneity research direction in 2000.
Facies stacking patterns, facies dimensions, sequence architecture, diagenetic
patterns associated with sequence boundaries and late diagenetic events,
and related information will be collected from both outcrops and reservoirs.
The Big Hatchet and SACROC studies will continue, and new outcrop areas
with more prevalent meteoric diagenetic overprints in ooid shoal complexes
will be sought.
Significant production at SACROC is from the karst-modified Cisco section.
Developing and understanding the impact of paleokarst-related fracture
systems on production will be a part of the SACROC study. This link will
provide an opportunity for us to continue our research on paleokarst systems
with direct application to subsurface reservoirs and production characteristics.
We will continue to look for good outcrop analogs of this style of paleokarst
in icehouse carbonates.
Rock-Fabric,
Rock Typing, Petrophysics Program
Over the past 10 years
the RCRL has gathered rock-fabric and petrophysical data from a wide variety
of carbonate reservoirs and integrated these data into a robust system
for relating geologic descriptions and petrophysical parameters. This
system is based on a number of assumptions including (1) cement and compaction
effects are near-uniformly distributed, (2) the carbonate is either limestone
or dolostone, (3) well-connected pore space can be distinguished from
poorly-connected pore space by distinguishing between interparticle and
separate-vug pore space, and (4) interconnected vuggy pore space can be
distinguished from interparticle and separate-vug pore space by distinguishing
between fabric-selective and nonfabric-selective vugs. We have tested
this system and find that these assumptions apply in many reservoirs.
However, the past few years we have studied reservoirs where some of these
assumptions do not apply. For example, partial dolomitization can improve
petrophysical properties of mud-dominated fabrics if the dolomite crystal
size is large enough. If the cement has a patchy distribution the permeability
for a given porosity is much higher than expected from the rock-fabric
petrophysical class. The patchy distribution of cement reduces porosity
but does not change the pore size.
A major difficulty
in applying this system to carbonate reservoirs is the identification
of touching-vug pore systems, systems composed of fractures, solution-enlarged
fractures, vugs, and breccias. Our research has shown that large-scale
dissolution can produce a touching-vug pore system that dominates reservoir
performance. However, we have not been able to quantify the impact on
performance because we lack the proper size sample on which to conduct
flow experiments. We have been able to quantify the impact of microfractures
on matrix permeability in several reservoirs and find that microfractures
can improve matrix permeability by a factor of five. Distinguishing between
poorly connected vugs and touching vugs is a problem of scale. If the
vugs are clearly fabric selective they are most likely poorly connected
to other vugs on the scale of the rock-fabric body. However, if the vugs
are not clearly fabric selective, they may be interconnected, but the
scale at which they are connected is difficult to determine. For example,
the vugs may have a large impact on the permeability of a 1-inch plug
or a 3-inch core, but the scale at which the vugs are interconnected maybe
too small to have any impact on reservoir scale fluid flow.
In 2000 we will continue
to test the rock-fabric system with new reservoir data and develop modifications
as required. We have collected data from Permian dolostone reservoirs
of West Texas, karsted reservoirs from the Lower Ordovician and the Permian
carbonate reservoirs of West Texas, Jurassic limestone and dolomite reservoirs
of the Middle East, Cretaceous reservoirs of offshore Brazil, and Cretaceous
Shuaiba reservoirs of the Middle East. We will continue to collect data
from both domestic and international reservoirs 2000 for the purpose of
improving our rock-fabric system.
In 2000 the RCRL will
develop a dual outcrop-subsurface approach to understanding the fluid-flow
properties of large-scale pore systems that are characteristic of karsted,
reef, and rudist reservoirs. We will collect large outcrop samples, image
the pore structure using an industrial high-resolution X-ray CT scanner
at the UT Department of Geological Sciences, and conduct high-resolution
3D flow modeling using high-performance parallel computers at the UT Institute
for Computational and Applied Mathematics. We also plan to collect image
logs from analog reservoirs and calibrate these logs with the flow characteristics
found in the outcrop samples.
The work will initially
be conducted on vuggy Cretaceous caprinid limestones, and other types
of large-scale pore systems will follow. The first example, a 14-inch-long,
10-inch-diameter limestone sample from a Cretaceous caprinid buildup in
the Glen Rose Formation near the town of Pipe Creek, Texas, has already
been collected and scanned. In addition, we will continue to gather subsurface
information on touching-vug pore systems as part of the SACROC reservoir
study. Icehouse ages are characterized by large sea-level fluctuations,
which should produce extensive karsting and associated touching-vug pore
systems. We have identified one such system in SACROC and will continue
to investigate its impact on performance 2000.
Permeability
from Wireline Logs Program
Estimating permeability
from wireline logs is a principal focus of the RCRL program. We have developed
a knowledge-based method for estimating permeability from resistivity,
porosity, and acoustic logs. Basic elements are (1) a global permeability
transform that relates interparticle porosity and rock fabric petrophysical
class to permeability, (2) a method of calculating the petrophysical class
using crossplots of water saturation and porosity, and (3) a method of
calculating interparticle porosity by subtracting vuggy porosity estimated
from acoustic-porosity crossplots from total porosity. The estimation
of petrophysical class from resistivity and porosity logs requires knowledge
of the change in saturation with reservoir height (height above the zero
capillary pressure level). We have been unsuccessful at integrating the
reservoir height parameter into the equation so that, at present, this
method can be applied only above the transition zone.
We are using reservoir
data to model the relationship between porosity, saturation, and rock
fabric above the transition zone. We plan to use mercury capillary pressure
data to develop a model that includes reservoir height. We have been collecting
data to accomplish this task; however, we need to collect more data on
petrophysical class 1 fabrics in order to develop a robust model. In 2000
we will continue to collect capillary pressure data and to test this method
in carbonate reservoirs.
Subsurface
Modeling Program—South Wasson Clear Fork Study
The RCRL has an ongoing
reservoir characterization project in the South Wasson Clear Fork reservoir
in West Texas funded in part by the U.S. Department of Energy. Last year
the RCRL completed the study of the outcrop analog in Apache Canyon, Sierra
Diablo Mountains, and successfully constructed an initial geological/petrophysical
model of this reservoir. In 2000 the RCRL plans to construct a fluid-flow
simulation model for part of the reservoir, incorporating the geological
model, well log data, the petrophysical model, insight on spatial permeability
statistics and fluid flow from the outcrop studies, and an outcrop and
subsurface study of the fractures.
Outcrop petrophysical
studies in Apache Canyon and elsewhere in West Texas and New Mexico demonstrate
a consistent pattern of spatial permeability statistics with 2 to 5 orders
of magnitude variability, most of which occurs within distances of a few
feet or less in single rock-fabric units. This short-range variability
is present in every outcrop and composes most of the overall variance
in each case. It has weak spatial correlation that can be modeled with
semivariograms having asymptotic power-law behavior at small lags. A variety
of longer range features are also present, including (1) vertical trends
within grainstone bodies, (2) vertical average permeability contrasts
between grainstone bodies, (3) 140180-ft lateral periodicities within
high-frequency cycles, and (4) lateral trends at scales from several hundred
to several thousand feet in high-frequency cycles.
The longer range features
compose a much smaller fraction of the overall variability and, thus,
require careful analysis to quantify and model. However, fluid-flow simulations
indicate the longer range features can have a significant effect on fluid
displacement and special attention should be given to modeling them in
reservoir performance predictions. The short-range variability contributes
mainly to dispersion of waterflood fronts and enhancement of injectivity
that can be modeled with effective properties.
In 2000 the RCRL will
develop methods to quantify and model the longer range trends and apply
them to the South Wasson Clear Fork reservoir model. These methods will
take advantage of the relatively dense well spacing typical in West Texas;
however, the resulting models will provide data sets for testing subsequent
methods developed for reservoirs with wider spaced well control. The effects
of the smaller scale variabilities will be incorporated via effective
properties applying our ongoing research on scaleup and additional data
from our DOE-funded special core analysis program. In addition, the BEG
fracture research group has completed an outcrop and subsurface study
of the fractures in this field, and the results of this study will be
incorporated into the fluid-flow simulation model.
Mentoring
Program
Direct contact with
the technical staff of our sponsoring companies is an important aspect
of our program. This allows us to test our concepts and methods on real
problems while assisting sponsors in developing new reserves. This is
an ongoing effort, and several activities are planned for work between
RCRL and sponsors for 2000. Sponsors are encouraged to contact us with
projects that could be mutually beneficial. Projects anticipated for 2000
include the following:
Exxon-Mobil Oil Company
Slaughter Levelland Reservoir (Permian of West Texas)
Southwestern Energy Diamond M Field (Permian of West Texas)
Production Development Oman Yibal Shuaiba Reservoir (Cretaceous of Oman)
Technology
Transfer
Transfer of research
results to sponsoring organizations is a vital activity of the RCRL. Technology
transfer is accomplished by a fall annual review meeting, technical reports,
short courses, mentoring programs, and personal discussions, all of which
are ongoing activities that will continue into 2000. We anticipate that
the database will be a major avenue for the transfer of research results
to RCRL sponsors. |
