Applications of Acoustic Measurements in Shale Stability Research

by
James Alexander Davidson, Ph.D.

University of Texas at Austin, 1999
Supervisors: Steven L. Morriss
Augusto L. Podio

Shales make up over 75% of the formations encountered during oil and gas drilling operations. Shale related problems cost the industry in excess of $500 million per year. Hole washouts, wall sloughing and hole collapse can cause the drill pipe to become stuck, resulting in costly delays or the complete loss of the hole. Invasion by water from the drilling fluids resulting from chemical potential gradients is believed to be the major cause of shale instability. Increases in water content decrease shale strength.

Acoustic measurements are routinely used to estimate the values of various mechanical properties of rocks. Advances in technology have made it possible to make acoustic measurements during drilling operations. Data from pulse-echo transducers is used to estimate the borehole diameter, and data from transmitter-receiver pairs is used to determine the velocity of sound through the formations. This study was undertaken to develop methods to use such transducers to monitor changes in the mechanical properties of shales during drilling operations. If the onset of instability can be identified, changes in the drilling fluid chemistry or casing program can be made to avert costly wellbore failures.

Experiments were conducted to study the changes in shale properties with water content. Outcrop samples as well as subsurface shales from oil wells were used. Measurements were made at surface and simulated downhole stress conditions. Significant changes in the mechanical properties were found to occur with small changes in the water content.

Problems with the preservation and handling procedure that have been routinely used over the years were identified. One of the major problems has been the incomplete removal of entrapped air from conventionally prepared samples. Methods to identify the presence of air in laboratory samples have been developed as well as improved preparation procedures to minimize air entrapment.

Transient phenomena occurring within the samples influenced many of the acoustic measurements. The transients usually resulted from perturbations caused by laboratory handling. The characteristics of these transients varied with water content. This behavior may offer a means to monitor, on a real time basis, the movement of water and ions within shales during hydration.

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