Craig D. Morgan

Natural CO2 Reservoirs on the Colorado Plateau and Southern Rocky Mountains, USA: A Numerical Model

Publisher Summary Gas reservoirs within the Colorado Plateau and Southern Rocky Mountains region are natural laboratories for studying the factors that promote long-term storage of CO2. They also provide sites for storing additional CO2 if it can be separated from the flue gases of coal-fired power plants in this part of the United States. These natural reservoirs are developed primarily in sandstones and dolomites. In many fields, stacked reservoirs are present, indicating that the gas has migrated up through the section. There is also evidence of geologically young travertine deposits at the surface and CO2-charged groundwater and springs in the vicinity of known CO2 occurrences. These near-surface geological and hydrological features also provide examples of the environmental effects of leakage of CO2 from reservoirs. The confidence in the predictions of numerical simulations by modeling a natural system of CO2 reservoirs and comparing the modeling results with a number of observations has been discussed in the chapter. A numerical model based on the Farnham Dome CO2 reservoir structure located in east-central Utah has been developed. This reservoir is typical of those found on the Colorado Plateau with stacked CO2 reservoirs contained within a Laramide, dome-like structure. Subsequent modeling of the chemical interactions between reservoir brine, CO2 gas and a simplified mineral assemblage using the simulator CHEMTOUGH2 produces results that are generally consistent with the water chemistry observed in basins of Eastern Utah.

MAJOR OIL PLAYS IN UTAH AND VICINITY

Utah oil fields have produced a total of 1.2 billion barrels (191 million m{sup 3}). However, the 15 million barrels (2.4 million m{sup 3}) of production in 2000 was the lowest level in over 40 years and continued the steady decline that began in the mid-1980s. The Utah Geological Survey believes this trend can be reversed by providing play portfolios for the major oil producing provinces (Paradox Basin, Uinta Basin, and thrust belt) in Utah and adjacent areas in Colorado and Wyoming. Oil plays are geographic areas with petroleum potential caused by favorable combinations of source rock, migration paths, reservoir rock characteristics, and other factors. The play portfolios will include: descriptions and maps of the major oil plays by reservoir; production and reservoir data; case-study field evaluations; summaries of the state-of-the-art drilling, completion, and secondary/tertiary techniques for each play; locations of major oil pipelines; descriptions of reservoir outcrop analogs; and identification and discussion of land use constraints. All play maps, reports, databases, and so forth, produced for the project will be published in interactive, menu-driven digital (web-based and compact disc) and hard-copy formats. This report covers research activities for the first quarter of the first project year (July 1 through September 30, 2002). This work included producing general descriptions of Utahs major petroleum provinces, gathering field data, and analyzing best practices in the Utah Wyoming thrust belt. Major Utah oil reservoirs and/or source rocks are found in Devonian through Permian, Jurassic, Cretaceous, and Tertiary rocks. Stratigraphic traps include carbonate buildups and fluvial-deltaic pinchouts, and structural traps include basement-involved and detached faulted anticlines. Best practices used in Utahs oil fields consist of waterflood, carbon-dioxide flood, gas-injection, and horizontal drilling programs. Nitrogen injection and horizontal drilling programs have been successfully employed to enhance oil production from the Jurassic Nugget Sandstone (the major thrust belt oil-producing reservoir) in Wyomings Painter Reservoir and Ryckman Creek fields. At Painter Reservoir field a tertiary, miscible nitrogen-injection program is being conducted to raise the reservoir pressure to miscible conditions. Supplemented with water injection, the ultimate recovery will be 113 million bbls (18 million m{sup 3}) of oil (a 68 percent recovery factor over a 60-year period). The Nugget reservoir has significant heterogeneity due to both depositional facies and structural effects. These characteristics create ideal targets for horizontal wells and horizontal laterals drilled from existing vertical wells. Horizontal drilling programs were conducted in both Painter Reservoir and Ryckman Creek fields to encounter potential undrained compartments and increase the overall field recovery by 0.5 to 1.5 percent per horizontal wellbore. Technology transfer activities consisted of exhibiting a booth display of project materials at the Rocky Mountain Section meeting of the American Association of Petroleum Geologists, a technical presentation to the Wyoming State Geological Survey, and two publications. A project home page was set up on the Utah Geological Survey Internet web site.

Reservoir Characterization of the Lower Green River Formation, Southwest Uinta Basin, Utah

The objectives of the study were to increase both primary and secondary hydrocarbon recovery through improved characterization (at the regional, unit, interwell, well, and microscopic scale) of fluvial-deltaic lacustrine reservoirs, thereby preventing premature abandonment of producing wells. The study will encourage exploration and establishment of additional water-flood units throughout the southwest region of the Uinta Basin, and other areas with production from fluvial-deltaic reservoirs.

CARBON AND OXYGEN ISOTOPIC ANALYSIS: BUG, CHEROKEE, AND PATTERSON CANYON FIELDS, SAN JUAN COUNTY, UTAH

Over 400 million barrels (64 million m{sup 3}) of oil have been produced from the shallow-shelf carbonate reservoirs in the Pennsylvanian (Desmoinesian) Paradox Formation in the Paradox Basin, Utah and Colorado. With the exception of the giant Greater Aneth field, the other 100 plus oil fields in the basin typically contain 2 to 10 million barrels (0.3-1.6 million m{sup 3}) of original oil in place. Most of these fields are characterized by high initial production rates followed by a very short productive life (primary), and hence premature abandonment. Only 15 to 25 percent of the original oil in place is recoverable during primary production from conventional vertical wells. An extensive and successful horizontal drilling program has been conducted in the giant Greater Aneth field. However, to date, only two horizontal wells have been drilled in small Ismay and Desert Creek fields. The results from these wells were disappointing due to poor understanding of the carbonate facies and diagenetic fabrics that create reservoir heterogeneity. These small fields, and similar fields in the basin, are at high risk of premature abandonment. At least 200 million barrels (31.8 million m{sup 3}) of oil will be left behind in these small fields because current development practices leave compartments of the heterogeneous reservoirs undrained. Through proper geological evaluation of the reservoirs, production may be increased by 20 to 50 percent through the drilling of low-cost single or multilateral horizontal legs from existing vertical development wells. In addition, horizontal drilling from existing wells minimizes surface disturbances and costs for field development, particularly in the environmentally sensitive areas of southeastern Utah and southwestern Colorado.