Gale, Julia F. W.1, Leonel Gomez1, Randall Marrett1,
Jon E. Olson1, Stephen E Laubach1, Jon Holder1, Peggy
Rijken1, Stephen P. Cumella2
(1) The University of Texas at Austin, Austin, TX
(2) Williams Production RMT Company, Denver, CO
ABSTRACT: 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.
AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.