RCRL
 
Research Plans for 2000
Outcrop and Subsurface Characterization of Carbonate
Reservoirs for Improved Recovery of Remaining Hydrocarbons
Executive Summary
Database Program
Cretaceous Reservoir Program
Icehouse (Carboniferous) Reservoir Program and Paleokarst Studies
Rock-Fabric, Rock Typing, Petrophysics Program
Permeability from Wireline Logs Program
Subsurface Modeling Program - South Wasson Clear Fork Study
Mentoring Program and Technology Transfer

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:

  • Development of 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.
  • Studies of icehouse reservoir heterogeneity in outcrop and subsurface.
  • Studies of world-class Cretaceous reservoirs and reservoir-analog outcrops.
  • Development of methods for characterizing the fluid-flow properties of large-scale touching-vug pore systems that are characteristic of karsted, reef, and rudist reservoirs.
  • Development of a knowledge-based wireline log system to estimate permeability and original water saturation.
  • Development of new methods for constructing a reservoir model using the South Wasson Clear Fork study as a database.
  • Continuation of our highly successful mentoring program.
  • 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.

    Updated March 2010