Natural Pyrometamorphism: Relevance to Underground Coal Gasification

Daniel J. Daly, Robert F. Stevenson, Gregory J. McCarthy

Although 28 underground coal gasification (UCG) tests have been conducted since the early 1970s in the United States, only limited information is available concerning the nature and formation of high-temperature, low-pressure alteration products in adjacent noncoal geologic materials--information basic to the evaluation of key process and environmental questions. A comprehensive literature search, conducted as part of the Gas Research Institute's program to develop an understanding of UCG-related thermal effects, was initiated to determine the relevance of natural fuel-combustion products and events to UCG.

Natural fuel-combustion events, typically associated with high-volatile fuel deposits, are confined to the unsaturated zone and require oxygen, typically supplied from the surface through fractures, for combustion. Noncoal materials overlying the burn undergo physical and mineralogical alteration through pyrometamorphism, a type of metamorphism characterized by elevated temperatures, low pressures (up to a few atmospheres), and essentially anhydrous conditions. With increasing temperatures, materials undergo calcination (decarbonation, oxidation, and dehydration), sintering, and fusion.

Although differences in setting and reaction dynamics exist between UCG and natural fuel-combustion pyrometamorphism (e.g., UCG is typically conducted under saturated conditions at depths of a few hundred feet and combustion rates are generally higher and more uniform), the character of the UCG alteration products is similar to that found in nature. Such deposits, therefore, represent an important source of data for the development of predictive models for cavity growth, development, and stability; geothermometry of the process; and the potential for adverse impacts on ground water flow and quality.

AAPG Search and Discovery Article #91033©1988 AAPG Rocky Mountain Section, Bismarck, North Dakota, 21-24 August 1988