Research Plans for 2004
Outcrop and Subsurface Characterization of Carbonate
Reservoirs for Improved Recovery of Remaining Hydrocarbons
Executive Summary
The Reservoir Characterization Research Laboratory (RCRL) for carbonate studies is an industrial research consortium run by the Bureau of Economic Geology (BEG), John A. and Katherine G. Jackson School of Geosciences, The University of Texas at Austin (UT). RCRL’s mission is to use outcrop and subsurface geologic and petrophysical data from carbonate reservoir strata as the basis for developing new and integrated methodologies to better understand and describe the 3-D reservoir environment.
The RCRL program has run continuously since 1987 and has produced more than 40 external publications, as well as BEG publications, on carbonate reservoir characterization, sequence stratigraphy, petrophysics, geostatistics, and petroleum engineering. We provide research results to our 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 in which we work hands-on with industry staff using their datasets. In addition, our results are posted in a password-protected part of our Website (www.beg.utexas.edu/indassoc/rcrl/index.htm).
RCRL has maintained a membership of between 13 and 18 companies per year (see list of 2003 sponsors at end of proposal). The sponsorship currently has strong interests in the Permian Basin as well as Middle Eastern carbonate reservoirs. This enrollment, supplemented by government grants, supports between three and six professional staff members and varying numbers of graduate student research associates, plus strong computer, editing, and graphics services.

Three geologists, a geological engineer, two reservoir engineer/modelers, and a geophysicist/interpreter form our core group:

Dr. Charles Kerans, Geologist
Dr. Xavier Janson, Geologist
Mr. Jerome A. Bellian, Geologist
Mr. F. Jerry Lucia, Geological Engineer
Dr. James W. Jennings, Jr., Reservoir Engineer/Modeler
Dr. Fred Wang, Petroleum Engineer
Dr. Hongliu Zeng, Geophysicist
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 has included 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 2004 are focused on the following areas:
Mapping, characterizing, and modeling touching-vug pore systems.
3D modeling of geologic facies and petrophysical rock-fabric elements using outcrop and subsurface data
CT imaging for characterizing heterogeneity at the core-plug scale
Limestone recrystallization and its effects on petrophysical properties
Expansion and integration of the rock-fabric and core databases
Continuation of field-based studies of carbonate slope systems

Initiation of regional sequence analysis of the Arab-D

If you have any questions on any aspects of the RCRL Carbonate Reservoirs Research Program, please contact Charlie Kerans (512-471-1368 or charles.kerans@beg.utexas.edu) or Jerry Lucia (512-471-7367 or ).
With this proposal, we invite you to participate in the continuation of the RCRL Carbonate Reservoirs Research Program. A list of sponsors during 2003 can be found at the end of this proposal. In 2004 the annual RCRL Industrial Associates contribution to the program will continue to be $45,000 per year. Our industrial sponsors will continue to receive research results at annual review meetings, in short courses, during mentoring activities, in publications and CD’s, and on our Website.
Research Directions for 2004
Mapping, Characterizing, and Modeling Touching-Vug Pore Systems

One of the single most pressing issues in the characterization and modeling of carbonate reservoir strata that remains to be effectively addressed is the geologic and petrophysical characterization of touching-vug pore systems, and the integration of these data into fluid-flow models. We have outlined two areas of research where touching vugs of depositional and diagenetic origin are seen to be significant and are planning to model these datasets as a contribution to this important area of research.

Multi-Scale Characterization and Flow Modeling at Pipe Creek
Outcrop exposures of Cretaceous rudist mounds and their associated deposits near the town of Pipe Creek in Central Texas exhibit a variety of heterogeneities throughout a range of scales including (1) decimeter-scale separate-vug pores within mound-core caprinid fossils standing upright in porous sediment, (2) decimeter-scale touching-vug pores within and between caprinid fragments in steeply dipping flank beds, (3) an unknown scale and degree of connectivity between these pore systems, (4) a heterogeneous arrangement of mounds, debris deposits, and intermound sediment capped by discontinuous beaches at lateral scales of tens of meters and larger, but too small to be mapped between wells, and (5) the organization of these features in kilometer-scale shore-parallel belts stacked in depositional cycles as much as 10 m thick.

Understanding and modeling spatial variability and fluid flow in systems such as this presents numerous challenges. Perhaps the most important of these challenges are those having to do with flow in the touching-vug pore network at multiple scales. Advances in this area will of course have significant applications to analogous reservoirs in the Middle East, but the methodologies developed to address this problem will probably have even more important applications to other touching-vug systems as well.
In past years we have studied touching-vug pores in a roughly 25 cm ? 25 cm ? 35 cm caprinid debris sample by using CT scanning (in collaboration with Richard Ketcham and coworkers at the UT High-Resolution X-ray CT Scanning Facility), numerical flow modeling (in collaboration with Todd Arbogast and coworkers at the UT Institute for Computational and Applied Mathematics), and physical experiments (in collaboration with Steve Bryant and coworkers at the UT Department of Petroleum and Geosystems Engineering). The results of this study to date suggest that there is a connected pathway spanning a large fraction of the sample that can sustain very large flow rates equivalent to permeabilities on the order of 100 Darcys or more. However, the behavior of the sample is highly dependent on the boundary conditions, and experiments on larger samples are needed to adequately characterize the flow patterns.

In 2003 we constructed a stochastic simulation of mound cores, debris, inter-mound sediment, and beach deposits each having realistic shapes in a 1-km-scale fluid-flow model using data from the outcrop and several cored wells. In 2004 we may be able to refine the geometries in that model by using ground-penetrating (GPR) data and interpretations. However, the most important factor in fluid-flow prediction seems to be the unknown magnitude, scale, and orientation of the high-permeability connections in the debris deposit.

Therefore, in 2004 we hope to conduct a field experiment flowing air with gas-phase tracers between newly drilled shallow wells to gain some insight on effective permeability at the 1- to 10-m scale.
We plan to continue this approach by studying large pore systems in other large outcrop samples and improving our methods of visualization, numerical modeling, and laboratory experimentation to obtain a better understanding of the magnitude, scale, and orientation of the high-capacity flow paths.

Geology, Petrophysics, and Modeling of Touching-Vug Pore Systems
We have made considerable progress toward converting wireline logs into matrix permeability profiles and distributing these data within a sequence stratigraphic model using geostatistical methods. A key aspect in this conversion is linking rock fabrics to petrophysical properties through petrophysical class and rock-fabric number. In the past we have presented methods for determining the petrophysical class and rock-fabric number from porosity, acoustic, and resistivity logs. In 2003 we demonstrated how the rock-fabric method could be used to construct permeability models when only porosity logs are available by integrating rock-fabric numbers with stratigraphic information.

Characterizing and estimating permeability of touching-vug pore systems remains a critical problem that is unresolved. Touching-vug systems are formed by a complex combination of processes including dissolution, fracturing, and cavern collapse, as well as regional tectonism, which in part accounts for the difficulty in constructing a predictive model of this important pore type. We have gathered a considerable volume of information on touching-vug pore systems and have concluded that vug interconnection is the most important character to model and that the fracture aperture and surface roughness is less important. Vug interconnections are best studied at the outcrop scale because the important scale of interconnection is typically larger than core scale.

Equally important is the problem of constructing flow simulations of touching-vug pore systems with or without matrix permeability. We already have experience modeling coupled flow between a permeable matrix and thin fractures in standard finite difference simulators using both explicit fracture representation and nonneighbor connections. Flow in larger vugs can be simulated with coupled Stokes flow for the vugs and Darcy flow for the matrix using specialized code developed by Todd Arbogast and coworkers at the UT Institute for Computational and Applied Mathematics. Complicated systems having fractures and vugs at several scales may require a combined approach.

In 2004 we propose to construct a touching-vug model of a Cretaceous road cut located in Kerr County, Texas, and use this model in flow simulation studies. An initial description of this road cut has been made by Susan Hovorka and others as part of a study of the Edward aquifer, and that study will be used to place the road cut in a stratigraphic and structural context. The road cut contains a variety of touching-vug types, including fracture porosity, solution-enlarged fracture porosity, cavernous porosity, a small cave, and numerous dissolved anhydrite nodules. It is located just north of the Balcones Fault Zone. We intend to (1) map the touching-vugs on both walls of the road cut using photo pans and ILRIS imaging, (2) extend fractures and caves from one side of the road cut to the other where possible, and (3) distribute other features stochastically within the 3D volume. Although modeling matrix petrophysical properties will not be a prime consideration, we have core plug measurements that indicate a matrix having an average porosity of 12 percent and permeability of about 5 md. After this static model is constructed we plan to (1) study methods of flow-model construction for various sizes of touching vugs and (2) use the resulting models to study the effect of fracture connectivity on fluid flow.
3D Modeling of Carbonate Reservoir Analogs

The second area we see as a major barrier to building realistic 3D models of carbonate reservoirs is in populating our models with petrophysically significant, geologically reasonable facies distributions. We believe that our workflow and methods for constructing the basic chronostratigraphic framework have advanced greatly and are reliable. However, “filling the space” with facies distributions that represent realistic 3D depositional systems is still a major problem. Toward improving this process, we continue to build the new generation of 3D laser-based outcrop analog models to better understand the process of realistic facies modeling. The Dry Canyon dataset will be our major thrust, but we will attempt to pull into the 3D world two previously generated datasets from Lawyer Canyon and Painted Canyon.

Dry Canyon 3D Model
We are planning in 2004 to complete a 3D stratigraphic model of the Dry Canyon outcrop. This effort will entail (1) completing the field mapping, (2) acquiring lidar for the main Beeman Canyon and associated side canyons, (3) generating detailed descriptions of the three EPR cores and interpreting the associated well-log suites, and (4) collecting additional sample profiles in Yucca Canyon and Beeman Canyon to provide fabric/property data for population of properties within the 3D model. We will refine our initial sequence stratigraphic framework by incorporating the stratigraphic architecture exposed in Beeman Canyon. By integrating the stratigraphy exposed in these multiple canyons in a single 3D geologic model we hope to answer the following questions:

What is the exact timing and spatial distribution of mound growth within the sequence stratigraphic framework?
Is mound size and position on the shelf predictable within a sequence stratigraphic framework?
What are the time-stratigraphic and spatial relationships between carbonate and siliciclastic facies?
What are the controls on their position in time and space, extent, and relative importance? Antecedent topography? Position within the large scale sequences? Higher frequency accommodation changes?

Our initial results suggest that buildup distribution is controlled by paleotopography and changes in accommodation. These parameters also control where and when siliciclastic and carbonate sediment are accumulating on the shelf. We will test these observations against the stratigraphy exposed in Beeman Canyon. In addition, we are planning on using GPR to fill the gap between exposures where stratigraphic relationships between sedimentary features such as mound and intermound areas are poorly exposed.

Building a 3D model of the Dry Canyon area will improve our workflow and methodology for populating facies and rock properties within the 3D stratigraphic framework. We will build on knowledge gained from the Victorio Canyon example and apply new algorithms, particularly for modeling discrete buildup patterns. We expect to use a combination of stochastic and deterministic methods to properly model the distribution of small phylloid-algal and leopard-rock mounds. A range of deterministic and object modeling methods will be used to recreate the siliciclastic channels.

To build a realistic reservoir model, we will collect data from subsurface analogs where mechanical and petrophysical properties can be extrapolated. We have been actively studying a series of Canyon-Cisco-age reservoirs along the Horseshoe Atoll trend that are partly time-equivalent and have numerous facies that compare favorably with the outcrop facies. In addition to building the 3D model, we will calculate 3D synthetic seismograms and compare the seismic signature of the Dry Canyon model to these subsurface analogs.

3D Digital Documentation of Classic Outcrops
Addition of the ILRIS 3D and associated tools to the RCRL workflow has opened up capabilities that were previously not available for characterizing classic carbonate outcrop analogs. A workflow of capturing outcrops using lidar, Global Positioning System, and limited GPR, coupled with standard sequence stratigraphic and petrophysical approaches will permit us to do a better job of creating realistic analog models for facies modeling and reservoir simulation experiments.
Two areas previously studied that will benefit greatly from this 3D model construction approach are Lawyer Canyon and the Pecos River (Painted Canyon). Both these areas have detailed 2D geological models, have varying amounts of site-specific petrophysical data, and have undergone some level of reservoir simulation experimentation using 2D windows. Lawyer Canyon still serves as the reference section for understanding detailed flow unit layering and simulation in shallow-water carbonate reservoirs. Petrophysical data collected for Lawyer Canyon comes from a series of 3D canyon walls and then is collapsed onto a single 2D plane. Placing data in a more realistic 3D setting will most likely improve our understanding of flow by moving this 3D problem (reservoir heterogeneity and sweep efficiency) into a 3D context.

The prograding grainstones of Painted Canyon on the Pecos River provide a superb example of Cretaceous prograding highstand grainstones. The complex architecture of caprinid debris rudstones and associated grainstones is well displayed in the 90-degree arc of the main Pecos Canyon, as well as in the side canyon (Painted Canyon). These exposures combined illustrate differences in development of rudist reservoirs in environments from beach-foreshore to lower shoreface. As yet, this outcrop is represented only by a simplified 2D panel. Greater control from lidar will allow a full 3D model to be constructed that will aid in better understanding of this important facies association.
Products from these studies will be full Gocad models that will allow examination of these classic outcrop analog datasets in a range of workstation and VR settings.

Additional Areas of Research
Quantification of Internal Core-Plug Variability with CT Scanning
Understanding the flow properties of core samples has always been hampered by inadequate characterization of their internal heterogeneities. Thin sections provide critical insight into the internal structure of a sample, particularly at the pore scale, but the information is often unrepresentative because of the small volume of a thin section relative to that of the sample as a whole. High-resolution X-ray CT scanning may bridge the gap. Although the resolution of these instruments does not yet equal that of thin sections, they can quantify bulk density on a tightly spaced 3D grid filling the entire sample volume. The cost of scanning a sample is small enough to consider making it a regular part of routine plug analysis, and large quantities of these data may become available in the coming years. Methods to analyze, summarize, and relate the data to other petrophysical measurements will be desirable.

We have CT scans of 50 plugs samples from the South Wasson Clear Fork reservoir and another 30 from the Fullerton Clear Fork reservoir. In addition, we have routine porosity, permeability, and grain density measurements for all these samples, thin sections for most of them, and various special core analyses for many of them. In 2002 we used detailed flow modeling for one of these samples to study the effect of patchy anhydrite on permeability. In 2004 we plan to use this collection of CT scan data to develop methods for routine quantification of internal plug variability, and to relate that variability to porosity, permeability, rock fabric, and if possible, other special core analysis parameters. Porosity should be related directly to the average bulk density measured by each CT scan, whereas rock-fabric, permeability, and other flow-related properties may be related to the variances, correlation lengths, and correlation anisotropies of the internal heterogeneities.

Limestone Recrystallization: Effects on Petrophysical Properties
Petrographic/petrophysical studies of ooid grain-dominated fabrics from the Sacroc and Cogdell fields of Pennsylvanian age and the Fullerton Clear Fork field of Permian age have shown relationships that are very different from predictions based on previous work. Oomoldic grainstones typically are characterized by low permeability and poor capillary properties compared with ooid grainstones, Archie m values in the range of 2.5 to 4 compared with an average value of 2 for ooid grainstones, and total porosity values much larger than porosity derived from acoustic logs. The classic explanation for these characteristics is that (1) oomoldic porosity contributes much less to permeability than intergrain porosity, (2) pore space is composed of large oomolds connected by very small pores found in the matrix and ooid rings, and (3) electric and sonic waves are controlled more by the small matrix pores than by the large oomolds in an oomoldic grainstone.

Petrographic studies completed in 2003 revealed that some oomoldic grain-dominated limestones have petrophysical characteristics more like intergrain porosity than moldic pore space. There appears to be complete spectrum from grain molds to recrystallized ooids with pore space between calcite spar of varying sizes, and only grainstones with pure grain molds appear to have characteristics typical of ooid grainstones. However, most of the “ooid grainstones” in these three fields have a minimal volume of grain molds and are best described as recrystallized grainstones, and the petrophysical properties are related to the size of the calcite spare rather than the size of the ooid grains.

In 2004 we proposed to further investigate the processes by which an ooid grain-dominated sediment is recrystallized and how the resulting changes in pore structure affect the petrophysical properties. Key questions include 1) What is the porosity history of an ooid? (2) How are the changes related to original mineralogy and diagenetic environment? and finally (3) Can the diagenetic changes be linked to depositional environment, paleotopography, and sequence of events? Most of the reservoir data will come from the three fields mentioned above. We will gather fabric and porosity data from ooids of different ages starting with modern ooids to investigate the history of diagenetic change.

Slope Carbonates
Carbonate slope and toe-of-slope depositional settings have received far less investigation compared with extensively studied shallow-water carbonate deposits. Our understanding of the processes and mechanisms involved in carbonate slope sedimentation, as well as the resulting sedimentary geometries and stratigraphic architecture, is limited compared with similar deposits in siliciclastic settings. Yet there is a significant opportunity to leverage learnings from the past 10 years of extensive research in deep-water clastic systems to better understand carbonate systems.

The RCRL just finished an integrated study of the classic Albian slope/toe-of-slope Poza Rica field in Mexico. In addition, last year we completed a study of Permian toe-of-slope to basin redeposited carbonates in Victorio Canyon. Though many of the pore types are similar to those associated with the shallow-water, many other aspects differ significantly. We have begun to develop an initial conceptual model for the stratigraphic architecture of these redeposited sediments in the lower slope, toe-of-slope, and basinal depositional environment. We see a critical need for comprehensive investigations that will provide the fundamental depositional mechanisms and the time/space architecture of redeposited carbonate sediments in deep-water settings. Though some degree of prediction has been gained through better understanding of the sequence framework of the slope systems, a great deal is yet to be understood with regard to lateral continuity of turbidites and debris flows and what controls this continuity.

We are searching for additional outcrops that expose platform to basin deposits in order to better understand the sequence controls on depositional styles, and to start to quantify flow-unit continuity along both strike and dip. Last year we visited the spectacular Gorbea platform outcrop in northwestern Spain that shows good exposures of Lower Cretaceous (Albian) platform-margin clinoforms and toe-of-slope megabreccia. In 2004, we plan to investigate other potential candidates for a 3D outcrop-based reservoir modeling. Possible candidates include the northern part of the Maiella platform in Italy, the slope of the Triassic isolated platforms in the Dolomites, Cretaceous outcrops in the Spanish Pyrenees and in the Vercor of Southeastern France, and the Jurassic outcrop in Oman. We are particularly interested in potentially interesting slope/toe-of-slope outcrop suggestions from sponsor companies.

Rock Fabric and Core Descriptions Database
In 2003 we initiated work on an Access database to include core descriptions from the numerous fields the RCRL has studied by compiling core descriptions from Shuaiba fields in Oman. In addition, the rock-fabric database that was presented in 2002 was improved. In 2004 we plan to continue this effort by (1) adding more core and rock-fabric descriptions and (2) combining the core and rock-fabric databases where feasible.

Mentoring and Projects within the RCRL Umbrella

Direct contact with the technical staff of our sponsoring companies is an important ongoing aspect of our program. This interaction allows us to test our concepts and methods on real problems while assisting sponsors in developing new reserves. Sponsors are encouraged to contact us with projects that could be mutually beneficial.

In 2003 we worked with Oxy Permian constructing a geologic and porosity model of the Cogdell (Pennsylvanian) field in West Texas. We plan to continue this work into 2003 integrating matrix and touching-vug permeability into the model. Matrix permeability will be based on rock-fabric/porosity transforms, and touching-vug permeability will be based on production and injection profiles as well as other production data.

In 2004 we will initiate a feasibility study for carrying out a core-based study of the Arab Formation throughout Saudi Arabia, Qatar, and the U.A.E. The Arab Formation is the world’s most prolific hydrocarbon system. It is well developed in several countries and states throughout the Middle East, including Saudi Arabia, Qatar, and the U.A.E. This study would last for the next 3 years and would focus on developing an understanding of the larger scale basin patterns and sequence hierarchy of the Arab. Countries contributing data will get a strong say in where the study will be focused and the number of cores examined in particular regions. Other RCRL members will receive digital core descriptions and log suites for selected wells, with full documentation of key facies and sequence/cycle analysis. It is planned that over the course of the next 3 years a sufficient database will be collected to add significantly to our understanding of the regional stratigraphic architecture of the Arab. Recruiting QP and ADCO to the program and engaging the temporary assistance of a biostratigrapher will be important to the success of this project.

List of 2003 Sponsors
Great Western Drilling
Kinder Morgan

Occidental Petroleum
Petroleum Development Oman
Shell International

Updated March 2010