Research Plans for 2001
Outcrop and Subsurface Characterization of Carbonate
Reservoirs for Improved Recovery of Remaining Hydrocarbons
Executive Summary
Geologic Modeling
Development of a Rock-Fabric Method for Estimating Relative Permeability
Carbonate Reservoir Analog Database
Modeling Touch-Vug Pore Systems Within a Sequence Framework; SACROC Unit
CT Scanning and Flow Modeling of Vuggy Rocks
Modeling of Fracture Networks and Their Fluid-Flow Effects — South Wasson Clear Fork Reservoir
Sequence Stratigraphic Setting and Reservoir Heterogeneity Styles of Icehouse Slope and Toe-of-Slope Facies
Regional Sequence Framework of Cretaceous Shuaiba Reservoirs

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.


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.

Updated March 2010