INTEGRATED TEAMS WITH MODERN COMPUTER METHODS ARE KEY TO RESERVOIR CHARACTERIZATION

Based on a workshop sponsored by PTTC’s Rocky Mountain Region and the American Association of Petroleum Geologists on July 21-22, 1998, in Denver, CO, 

BOTTOM LINE

Embracing computer technologies can increase efficiencies and provide a positive economic impact through integrated reservoir characterization studies.

PROBLEM ADDRESSED

Many oil and gas fields in mature basins have not benefited from interdisciplinary characterization studies. These reservoirs provide an opportunity for innovative independent operators to maximize the economic potential through integrated teams using low-cost PC-based software packages.

KEY WORDS:

Reservoir Characterization, Computer Mapping, Database Management, Interdisciplinary Studies

SPEAKERS

Geological Heterogeneities and Compartmentalization:
Roger Slatt, PTTC Rocky Mountain Resource Center

Data Handling for Reservoir Characterization:
Sandra Mark, Consulting Geologist

Reservoir Simulation:
Lanny G. Schoeling, Shell CO₂ Company Ltd.

TECHNOLOGY OVERVIEW

Reservoir heterogeneity, often due to compartmentalization, has a profound impact on oil and gas recovery. Heterogeneities occur at the wellbore, interwell, and on field-wide scales Types of compartmentalization can include: stratigraphic/sedimentologic, structural, petrophysical/fluid, diagenetic, pressure, and combinations. The use of integrated, multidisciplinary teams has proven to be an effective way to increase recovery and profits. Experience confirms that putting the right people with the right skills (and tools) in the right projects— with firm management support— is the key to success.

Computer-related data acquisition, manipulation, and interpretation techniques must be used to develop efficient reservoir management schemes that will maximize economic potential. Routine map, production, and log data are available from a wide variety of sources, along with mudlogs and borehole imaging logs. In addition, Internet resources, such as state oil and gas commission websites, provide a wealth of useful information. In general, digital data allow easier data manipulation, the choice of display orientations, a variety of features, and interactivity.

Computerized data manipulation is highly efficient. Database management (DBM) software has also become more user friendly with the implementation of easier interfaces. Today, large databases can export to applications designed for specific analyses. Raster data can be input using scanners; vector data is input using a digitizing tablet or stylus. The average cost of digitizing well data in-house is $42 to $57 per well.

FIELD RESULTS

Field studies have demonstrated several ways to use this digitized data. Examples of integrated reservoir characterization are described below for two fields:

Sorento Field in Colorado produces primarily from fluvial sandstones deposited in incised valley fills cut into marine mudstones of the Morrow Formation (Pennsylvanian) on the eastern flank of the Denver Basin. To understand compartmentalization of the field, a reservoir characterization study was conducted integrating geology, geophysics, and petroleum engineering information.

Sequence stratigraphy was used to define the development of reservoir architecture. The fluvial architecture is highly heterogeneous and compounded by several minor fluctuations of sea level that stacked and partially eroded individual depositional stages. High-frequency 3-D compressional seismic data were used to distinguish the incised valley outline and lateral barriers to flow. Lowpermeability carbonate-cemented zones and floodplain deposits are associated with stacked channel sequences within the valley. These deposits created baffles and barriers to fluid flow that were mapped using high-frequency 3-D compressional data.

When the distribution of diagenetic features was com bined with sandstone geometry, the reservoir compartments became apparent. Production data were used to corroborate geological and geophysical interpretations. It was found that the reservoir is comprised of four flow units that display separate fluid contacts but maintain a baffled pressure relationship. This implies that, in developing similar fields, drilling a location structurally lower than a known oil-water contact may be justified. A large 3-D seismic survey may pay for itself in a complex reservoir setting if it can decrease the number of dry holes. Based on a dry hole cost of $115,000 per well and a seismic cost of $50,000 per square mile, a 3-D survey of five square miles that prevents two dry holes would cost the same as drilling them. If more than two dry holes were prevented, which is likely in a large field, the 3-D survey is even more economic. Also, the operator gains understanding that should result in more efficient development and management of the field.

Slattery Field in Wyoming contains approximately 65 wells and produces primarily from aeolian sandstones of the Minnelusa Formation (Permian) on the eastern flank of the Powder River Basin. The study of South Slattery sought to: maximize oil recovery, demonstrate an integrated approach to reservoir characterization, and create a “cookbook” that independent operators could use to develop their own methodologies for reservoir characterization.

Geological work for this study was done using a desktop computer loaded with GeoGraphix GES (well data base manager, mapping and cross section software) and GeoQuest QLA2 and RIS LESA (log analysis software programs). Digital log data was used, even though it tends to be expensive to acquire.

The geologist and engineer were physically located in facilities 1,000 miles apart and relied upon e-mail for communication and file transfer. This arrangement illustrates that operators can use remote expertise without having to hire additional staff to complete reservoir characterization work.

Most of the time was spent mapping, correlating, and working on cross-sections. While the geologist had a working knowledge of the software at the beginning of the project, nearly 25% of the time was spent learning even more about the software. If the same software were used in future projects, the learning curve should decrease.

If digitized log data had been purchased, about 20% of the geologist’s time could have been conserved. The amount of time the geologist spent preparing for simulation (grid preparation) was small, suggesting that simulation would not add a significant time commitment.

Based on this study, a realistic estimate for the time a geologist needs to characterize a reservoir is approximately one well per day. Using the mean salary for a petroleum geologist ($ 300 per day according to the American Geological Institute), the cost of a geologist’s time for reservoir characterization is $300 per well. The cost of software (amortized over the first year, with one well per work day and 250 work days/year) is about $82 per well. The cost of data works out to be $27 per well. Thus, the total cost of developing the geological model for reservoir characterization is approximately $400 per well.

The highest cost consideration most likely is the geologist’s time, which is three times that of the data and software combined. Efforts to save money should, therefore, be directed at equipment, software, and other methods to make the time spent by the geologist more productive. Simple suggestions for increased geological efficiency include purchasing data and using geotechnical assistants.

CONNECTIONS:

Sandra Mark, Consulting Geologist
26452 Wolverine Trail, Evergreen, CO 80439
Phone 303-674-1682, Fax 303-273-3859, E-mail smark. 95@alum.mines.edu

Roger Slatt, Department Head
Geology/Geological Engineering
Colorado School of Mines, Golden, CO 80401-1887
Phone 303-273-3822, Fax 303-273-1859, E-mail slatt@mines.edu

Lanny G. Schoeling, Senior Reservoir Engineer
Shell CO ₂ Company, Ltd.
200 N. Dairy Ashford, Houston, TX 77079
Phone 281-544-4856, Fax 281-544-3841, E-mail lgschoeling@shellus.com

For information on PTTC’s Rocky Mountain Region and its activities contact:
Roger Slatt, Department Head, Geology/Geological Engineering
Colorado School of Mines, Golden CO, 80401-1887
Phone 303-273-3822, Fax 303-273-3859, E-mail rslatt@mines.edu

Disclaimer: No specific application of products or services is endorsed by PTTC. Reasonable steps are taken to ensure the reliability of sources for information that PTTC disseminates; individuals and institutions are solely responsible for the consequences of its use.

The not-for-profit Petroleum Technology Transfer Council is funded primarily by the US Department of Energy’s Office of Fossil Energy, with additional funding from universities, state geological surveys, several state governments, and industry donations.

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