3-D Modeling of Mt. Simon Gas Storage Fields Could Enhance Winter Gas Availability from the Illinois Basin

Formations Hold 60% of Illinois Gas Storage Inventory

A two-year research study partnership between the Illinois State Geological Survey (ISGS), natural gas storage companies and the Department of Energy's Federal Energy Technology Center will develop 3-dimensional computer models for natural gas storage wells in the Mt. Simon Gas Storage Fields in the Illinois Basin.

Underground, water-filled sand/sandstone aquifers in the Mt. Simon area in Northern Illinois are used extensively to store natural gas for peak seasonal usage in the Midwest. In Illinois alone, the Mt. Simon formations hold 60% of the total gas storage inventory, an important supply for peak winter deliveries for northern Illinois cities, particularly Chicago.

Illinois is ranked fourth in the nation in natural gas storage with between 1.2 and 1.6 trillion cubic feet (tcf) of gas in storage. It has 600 billion cubic feet (bcf) of working gas. That is the gas stored in reservoirs for use in the winter months. The remainder is cushion gas, which stays in the reservoir year round.

The physical structure of the Mt. Simon reservoirs, however, is geologically complex, making it difficult to predict their performance as natural gas suppliers.

By developing 3-dimensional computer models of the reservoirs using geological and geophysical data collected from 11 storage reservoirs, the DOE project will help gas storage companies raise the amount of working gas that can be produced economically from the gas storage fields.

This will help ensure that customers have uninterrupted and affordable gas supplies especially during the winter months.

The Energy Department, through its Office of Fossil Energy, will provide a total of $180,000 ($100,000 this fiscal year) to the University of Illinois for use by the Illinois State Geological Survey.

The 3-D models will be based on full characterizations of four storage reservoirs. When the operators of gas storage wells improve their understanding of the wells' geological and petrophysical reservoir characteristics, reservoir performance and management are improved. The proposed work is consistent with the objectives of the U.S. Department of Energy to develop technologies for more reliable, safe, cost-effective and efficient gas storage, which will help ensure an adequate, uninterrupted gas supply for residential, commercial, and industrial end users.

The geological models can be used as the fundamental framework for subsequent fluid-flow simulation studies. Although production histories have been gathered over the years, simulations to increase deliverablilty or storage capacity strategies require fundamental knowledge of the reservoir geology and geometry.

This study will define and link the sedimentary facies (referring to rock units as rock type, mode of origin, composition, fossil content, or environment of deposition) and their geometry, mineralogy, and pore characteristics to fluid-flow potential.

Technically, this geologic reservoir characterization study will provide the following regional and field specific products:

• Core descriptions of the physical wells and interpretations of the rock-type (facies)

• Geologic and isopach (which detail the thickness and distribution of the sandstone) maps and cross sections showing facies and porosity distribution

• Mineral composition and character of sediments and diagenetic (the process of turning sediment into rock) cements that comprise the reservoir and its internal, natural clay compartment seals

• Relations of the well porosity to the well characteristics database (well log)

• Geologic structure maps for northern Illinois and specific fields

• 3-D models of facies and porosity distribution

• Salinity map of Mt. Simon formation in Northern Illinois.

All data integration, mapping, and modeling will be performed with Landmark software, utilizing digital wireline log data, lab measurements of porosity and permeability, core descriptions, and facies interpretations. Wireline logs are databases of information gathered by a series of instruments, lowered on a cable, that characterize well material structure and send this information to a surface computer via a digitized wireline. ISGS will supply individual contributing companies with digitized wireline logs for their wells if they desire. ISGS has been offered previously recorded digital logs, core data, and much other support by the contributing companies.

The study will assist many companies that are investigating the possibilities of expanding the capability or capacity of existing storage facilities to take advantage of FERC Order 636. FERC 636 is an effort to unbundle the pipeline services into individual parts, pipeline transport, storage, production etc., which can be separately contracted by the user. Before the Federal Energy Regulatory Commission ruling, pipeline companies charged for all of the handling from wellhead to the user and charged what was essentially an uncompetitive fee for the service. Users can now separately contract for the lowest gas, storage and transport prices.

Dr. David G. Morse (morse@geoserve.isgs.uiuc.edu) and Bryan Huff (huff@geoserv.isgs.uiuc.edu), petroleum geologists at the Illinois State Geological Survey (217-244-5527 or 217-244-2509) will be the principal investigators.

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Technical Program Contact:
Tom Mroz, DOE Federal Energy Technology Center, 304/285-4071, e-mail: tmroz@fetc.doe.gov