Mitch D. Pavlovic1, Gerald J. Kuecher2, Al. G. Pena3
(1) Baker Atlas, Houston, TX
(2) Baker Atlas Geoscience, Spring, TX
(3) Swift Energy, Houston, TX
ABSTRACT: The Distribution of Natural Fractures in the Austin Chalk Formation Utilizing Borehole Image Logs
The ability to drill horizontally through fractured rock of otherwise tight oil and gas reservoirs has stimulated renewed interest in naturally fractured reservoirs worldwide. Perhaps the most spectacular results have been experienced in the Austin Chalk trend were horizontal boreholes were drilled with production up to 10 times that of vertical wells. However, high initial production rates were quite frequently followed by rapid production decline probably due to completion in low-porosity, low-permeability zones of insufficient natural fracture density.
The use of borehole image data to locate fractures and determine the geometry of the existing fractures, lateral and vertical distribution of productive fractures, fracture quality and hydrocarbon potential prior setting casing is an essential element in evaluating the economics of any given well. The objectives of fracture study are twofold: 1) to accurately map fractures over the drilled interval and provide a structural synthesis; and 2) to propose a model that relates fracture intensity to the distribution of the most brittle rocks in the section, the chalks.
Mapped fractures exhibited a dominant strike azimuth of N85E. Most of the fractures are steeply dipping, closed to only partially open and they terminate vertically at chalk-bed margins, or less commonly, within chalk beds. Borehole image analysis indicates that natural fractures in the Austin Chalk are not uniformly spaced. Fracture intensity increased as a function of chalky electro-facies and with increasing bed thickness. Intervals of thick, homogeneous chalk beds are most likely to have tall, vertically connected fractures that make good horizontal wells targets.
AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado