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Petroleum Technology News
TORP and KGS Expand Coalbed Methane Research and Services
With the recent publishing of the Tertiary Oil Recovery Project's (TORP) manual "Fundamentals of Coalbed Methane Production", and current field research and testing by the Kansas Geological Survey (KGS), the state's coalbed methane industry is being provided with a new and continuing source of engineering and geological support and information. The TORP manual is intended to provide the operator of midcontinent coalbed methane wells with a practical document which provides information concerning:
- the properties of coal
- the occurrence of natural gas in coal,
- reservoir engineering characteristics of coalbed methane production,
- drilling, completing and equipping coalbed methane wells,
- developing and managing the coalbed methane prospect from defining the geology to marketing the gas.
The manual is available for $20 and an order form is included in this newsletter.
The Kansas Geological Survey in cooperation with companies active in the area has undertaken a research project to evaluate the coals of eastern Kansas. The project is a broad based resource assessment of coalbed gas in eastern Kansas and western Missouri.
Since the early 1980s, natural gas has been intentionally produced from coalbeds. Spurred on initially by significant tax credits, this sub-industry has grown rapidly in recent years, now contributing in excess of 5% (1 tcf) of the nation's natural gas production. The term "coalbed methane" (CBM) is derived from the coal mining industry, describing the gas which is a common hazard in coal mining operations. Methane typically comprises 95 to 100 percent of the hydrocarbon constituents present in coalbed gas, although it is common for the mixture to contain a number of different hydrocarbon gases, plus minor amounts of nitrogen and carbon dioxide.
In the early Twentieth century, "shale gas" was commonly produced in the midcontinent oil and gas fields. The common source of this gas was later proven to be the coalbeds, which are prevalent in the region. Coalbed methane reservoirs of the midcontinent are high, volatile bituminous A, B, and C coals. Gas can be produced from coals of nearly every rank; however, some of the less attractive coals (e.g., lignite) may require substantial bed thickness to develop adequate reserves. The industry continues to grow, and is active in nearly every sedimentary basin in the United States, primarily the San Juan, Black Warrior, and Raton basins.
Coal, a very complex mixture of organic and inorganic compounds, differs from other sedimentary reservoirs as the gas is adsorbed within the matrix of the rock rather than compressed in pore spaces. A typical one-foot thickness of coal six hundred feet deep is capable of containing as much gas as a typical sandstone reservoir five thousand feet deep. Another unique characteristic of coalbed production is its producing behavior. In most cases, initial production of gas is quite low while water production may be high. As the water is withdrawn, and the bottom-hole pressure decreases in the reservoir near the wellbore, gas production gradually increases. During the first few producing months the water-producing rate will continue to decrease accompanied by an increase in the gas-producing rate, until a pseudo-steady state occurs for both phases.
Due to its unique characteristics, coalbed evaluation requires methods not prevalent in other oil and gas producing operations. Coal rank is determined by measuring the light reflected from the surface of coal samples - vitrinite reflectance. Drill cuttings or core samples are analyzed for gas-in-place content by means of a canister test. Proximate analysis of coal, the same method utilized by the mining industry to determine coal quality, is a usable tool to estimate gas absorbing potential of various coals. The plot of a laboratory measurement of a coals gas-absorbing capability, the isotherm, is useful in determining the gas-content of coals, critical pressures, and estimating residual gas at abandonment.
The gas storage capacity of a coal is a complex function of reservoir temperature and pressure, composition, micropore structure and molecular properties of the adsorbed gas. In addition the geology of the coal deposit and the surrounding rocks can affect gas content and deliverability (e.g., cleat structure). Core samples are being collected from continuous cores using the KGS's coring rig and wells of opportunity. Samples are analyzed to determine the gas-in-place value, gas composition, and gas storage capacity. Selected samples of key coals are characterized as to cleat structure and by proximate analysis to determine ash content, moisture, volatile matter and fixed-carbon content.
Studies are being carried out to evaluate the regional geology and CBM production potential of coal deposits throughout eastern Kansas. The stratigraphic architecture of the coal-bearing Middle Pennsylvanian strata is being studied by mapping CBM fields and characterizing production, constructing correlation sections, structure, isopach and derivative maps, and digital datasets.
Completions of coalbed wells require unconventional methods in most cases. Chemical treatments, cementing, and hydraulic fracturing require a knowledge of the coal, otherwise substantial reservoir damage may occur. Casing installation should be designed to prevent damage and allow adequate space for phase separation and pumping equipment.
Coalbed gas reserves, not unlike conventional reservoirs, may be estimated by analogy, volumetric methods, material balance calculations, or decline curve extrapolation.
The end products of the KGS effort will include a comprehensive regional study and digital database of the distribution and quality for the significant coals in the Cherokee and Forest City basins. Limited data is currently available but is increasing rapidly. As data is synthesized it will be made easily accessible through the Internet and KGS Website.
Please send me ____ copies of "Fundamentals of Coalbed Methane Production". I have enclosed $20.00 for each copy.
Name: ______________________________________________
Company: ___________________________________________
Address: ____________________________________________
____________________________________________________
City: ______________________________________________
State: ______ Country: _________________
Postal Code: _________ Phone: ________________________
Mail: order form and check or money order payable to "The
University of Kansas" to:
Tertiary Oil Recovery Project, 1530 W. 15th St., 4008 Learned
Hall, University of Kansas, Lawrence, Kansas 66045-7609
One of the many coal samples obtained by KGS coring rig in southeast Kansas. Core sample is placed in a desorption canister and maintained within a controlled temperature environment. The quantity and quality of desorbed gas is measured to determine the gas-in-place value, gas composition, and gas storage capacity.