Fracture Detection Techniques in the Georgetown and Austin Chalk Formations

Abstract

ABSTRACT This study relates the success of detecting and analyzing naturally fractured systems in the Austin Chalk and Georgetown Formations from log evaluations. The Austin Chalk is composed principally of calcareous unicellular algal remains called cocospheres and their disunited, gear-shaped, skeletal remains called cocoliths. The Georgetown Formation is a hard, dense, and finely brittle limestone. Both formations contain matrix porosity, but are dependent on open fracture systems for economical hydrocarbon production. Hence, defining and evaluating the developed fracture systems before setting an expensive casing string is important. The Dual Induction Log and the Compensated Neutron-Formation Density Log offers an economical method of evaluating these fracture systems. Case studies in Robertson, Milam, and Burleson Counties, Texas have shown that fracture systems near the wellbore can be detected. Attempts to determine the existence of hydrocarbons in fracture systems using logs were successful in the Austin Chalk and Georgetown Formations. Full core analyses were also successful in determining the existence of these fracture systems. However, obtaining core samples on all wells as a method of determining fracture systems is not recommended due to the high cost of coring. Emphasis of this study is on the use of logs to determine the existence of economically recoverable hydrocarbons in fracture systems. Although this study concludes that logs were successful in detecting naturally fracture systems near the wellbore, they cannot predict the effects of mechanical fracturing on the formation. Hence geological trends in the area must also be considered.