Evaluation of a Potential Borehole Televiewer Technique for Characterizing Lost Circulation Zones

Abstract

An experimental study is described that evaluates the potential for using an acoustic borehole televiewer technique to measure fracture dimensions in lost circulation zones encountered in geothermal drilling. A simulated wellbore was constructed of sandstone blocks with adjustable, inclined fractures, stacked in a barrel filled with water. A slim hole televiewer was used to log the wellbore, and a computer was used to digitize and analyze the data. Televiewer signal perturbations caused by fractures were studied to evaluate the effects of fracture thickness and signal amplification on the width of the signal perturbation in the digital record. It was found that the signal perturbation width can be correlated with the fracture thickness if the correct signal amplification is applied. Two techniques for determining the correct amplification are described. With the televiewer used in the present study, fractures thicker than 0.15 inch in a 5.1 inch wellbore can be measured to within an accuracy of 5-15%. Fractures as small as 0.031 inch can be detected but cannot be accurately and reliably measured.