Fracture Analysis: Methods and Applications to Coal-Bed Methane and Devonian Shale
LOTTMAN-CRAIG, LINDA, GeoMet Incorporated, Washington, DC, and PHIL MALONE, GeoMet Incorporated, Birmingham, AL
When used exclusively, conventional remote sensing inadequately defines underground fracture systems for detailed study areas. A combination of mapping surface joint features and interpretation of remote sensing data provides an improved method of identifying zones of higher fracture intensity. Identification of these zones defines optimal location and spacing of coal-bed methane wells and interpretation of production trends. Although developed for the coal-bed methane industry, this fracture analysis has other applications including the evaluation of Devonian shales.
To consistently define linear features and joint patterns, a method has been developed to define them. This mapping program classified the strike, dip, development, and spacing of joints observed in the field. Joints are then grouped into categories based on joint plane development. Data on faults, folds, and coal cleat are also recorded. This joint classification system is relatively simple so as to be consistent and reproducible. These data are then entered into a computer to generate fracture maps and rose diagrams. This method allows evaluation of large areas, as well as defining detailed structures within a specific area. The linear study consists of interpreting and classifying linears from a variety of remote sensing imagery, including low-altitude photos, SLAR, high altitude, nd thematic mapper. The
resulting fracture analysis uses data from both the joint field mapping program and the linear study.
Detailed study of production trends vs. fracture zones in the Oak Grove degas field of Alabama confirmed the importance of structure; wells connected to fracture systems yielded higher production. Although data obtained from the Oak Grove degas field established the existence of subsurface communication between seams via fracture systems, extensive field mapping reveals fracture avenues from subsurface coal seams to the surface. The evidence for this is continuous streams of methane bubbles observed in water in creek beds.
Knowledge of local structure optimizes well spacing, increases methane production, and minimizes cost. Although conventional well spacings of 40 to 60 ac are commonly used in degas fields, optimum grid spacing for an individual field is highly dependent on fracture patterns. Because fracture zones greatly increase the communication between wells horizontally and between seams vertically, increased well spacing in highly fractured areas does not decrease early production, yielding a longer production period. Structural highs such as anticlines and faults also may favorably affect production. Several large-scale degas projects in the Warrior basin have successfully used this combined method of joint mapping, linear study, and structural analysis to optimize well placement and spacing, t us increasing production.
AAPG Search and Discovery Article #91005 © 1991 Eastern Section Meeting, Pittsburgh, Pennsylvania, September 8-10, 1991 (2009)