Bureau of Economic Geology, The University of Texas at Austin (www.beg.utexas.edu).
American Association of Petroleum Geologists Annual Meeting, Dallas, April 18-21, 2004
Natural Fracture and Diagenetic Controls on Producibility of Low Permeability,Upper Cretaceous Williams Fork Formation and Cozzette Sandstones, Piceance Basin, Colorado
The Upper Cretaceous Mesaverde Group forms important gas reservoirs in the Piceance Basin. Owing to extensive cement, matrix permeability may be on the order of microdarcys in these units. Natural fractures may significantly increase effective permeability, but inherent problems in sampling subsurface fractures limit effectiveness of core and log analysis for measuring fracture attributes. We are thus investigating indirect measures for predicting the intensity, clustering, connectivity, and porosity preservation of large fractures.
Diagenesis plays a critical role in fracture growth and in the capacity of fractures to conduct fluid. To predict patterns of open and sealed fractures, we used diagenetic information, along with rock mechanical properties, and observed microfracture populations. Rock properties, such as subcritical crack index, reservoir bed height, and rock strain, are measured from core and used as input for a fracture-mechanics-based crack-growth simulator that generates realistic fracture patterns.
Using five microfracture data sets from 35 m (116.4 ft) of subhorizontal Cozzette Sandstone core from the Slant Hole Completion Test (SHCT-1) well, we measured populations of microfractures using a high-resolution SEM-based cathodoluminescence detector. We used microfracture intensities, which are unrelated to proximity to macrofractures, to predict macrofracture intensity by extrapolating microfracture power-law aperture-size distribution functions. Scaling analysis predicts 0.46 large (>1 mm), open fracture per meter of 25/8-inch-diameter core, which is close to the intensity of 0.37 fracture per meter measured in the core. These results suggest that size-cognizant measurement of fracture intensity using microfractures can extend fracture-intensity predictions to areas where conventional methods yield no data.