The project goal was to characterize heterogeneities in sedimentary rocks that influence fluid flow at the reservoir/aquifer scale using innovative geographic information systems (GIS)-based analyses. Heterogeneities of interest include fractures, faults and solution cavities.

Program
This project was selected under DOE's Historically Black Colleges and Universities Program.

Florida International University
Miami, FL

U.S. Geological Survey
Miami, FL

Occidental of Elk Hills, Inc.
Tupman, CA

Many important oil and gas reservoirs are naturally fractured, deriving the bulk of their production from a network of highly permeable fractures of various dimensions. However, direct observation of fractures (and solution cavities for the case of karst limestone) in the subsurface is inherently limited by the dimensions of the wellbore and the resolution of available borehole and seismic techniques. This project addresses the fundamental scaling problem faced by petroleum geoscientists and water resource hydrologists: how to characterize the reservoir/aquifer scale heterogeneities that impact fluid flow.

Project Results
The project developed new techniques to characterize the spatial distribution of fractures and solution cavities in sedimentary rocks. Fractures mapped from bedrock exposures can be analyzed in terms of fracture type (joints or faults), intensity, orientation and dimensions within the framework of a GIS database. In addition, a new method was developed to quantify porosity in karst limestones from borehole image logs. As a consequence, zones of high porosity are correlated to subsurface conduits that localize fluid flow.

Benefits
One outcome of the geospatial analysis of fracture networks is the capability to identify through-going fracture zones as linear trends of high fracture intensity. These fracture zones often serve as preferential pathways for fluid flow in the subsurface and therefore are of great interest to the oil and gas industry because they provide hydraulic connectivity among populations of smaller fractures. Quantifying the distribution and dimensions of these multi-layer, through-going fractures may lead to a better understanding of fractured reservoirs.

Freshwater supplies for many municipalities in the United States are derived from karst limestone aquifers. The high productivity of these aquifers is a direct consequence of the numerous solution cavities in the limestone bedrock. This study provides new methods to quantify the distribution and dimensions of solution cavities in karst aquifers using high-resolution borehole imagery. Despite their impact on aquifer-scale fluid flow, large solution cavities and conduits have been difficult to quantify in the past. Results from this study may help municipalities and regulatory agencies monitor and protect these valuable resources through more accurate characterization of the aquifer medium.

Project Summary
Project researchers have developed:

• Geospatial analyses to characterize the distribution of fractures in layered rocks, focusing on fracture zones and fracture type.
• A new GIS-based method to identify solution conduits in karst limestone from borehole image logs.
• A new GIS-based method to estimate the representative elementary volume (REV) for porosity in karst limestone aquifers.

Fracture zones are geologic structures consisting of numerous, closely spaced fractures, localized within narrow linear zones. They serve as major pathways for fluid flow, especially in rocks with low matrix permeability, and hence represent zones of enhanced weathering and preferential pathways for contaminant migration. Using outcrop surveys, air photos, and published maps, reseaerchers developed a method to identify fracture zones based on the spatial distribution of fracture intensity.

The project resulted in the development of a method to estimate the 2-D porosity of karst limestone using a combination of remote sensing and GIS techniques. When applied to high-resolution borehole imagery, the method can yield an estimate of the medium's REV, thus providing the means to quantify the heterogeneity of these complex aquifers.

(August 2005)
The project is nearly completed, with two months remaining in the no-cost extension.

Publications
Finn, M.D., Gross, M.R., Eyal, Y., and Draper, G., Kinematics of through-going fractures in jointed rock: Tectonophysics, V. 376, 2003, pp. 151-166.

Manda, A.K., and Gross, M. R., Estimating aquifer-scale porosity (the representative elementary volume, REV) for karst limestones using geospatial (GIS) analysis: in Harmon and Wicks, eds., Geological Society of America Special Paper on Advances in Karst Research honoring Derek Ford and William White (in press).

Manda, A.K., and Gross, M.R., Identifying and characterizing solution conduits in karst aquifers through geospatial (GIS) analysis of porosity from borehole imagery: an example from the Biscayne aquifer, south Florida (U.S.A.): Advances in Water Resources (in press).

$199,004

In-kind support (matching) equivalent to ~$50,000 (~25% of total)

Other Government Organizations Involved: U.S. Geological Survey, U.S. Army Research Office

NETL - Purna Halder (purna.halder@netl.doe.gov or 918-699-2083)
Florida International U. - Michael Gross (grossm@fiu.edu or 305-348-3932)