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Alternative Methods of Measuring Strength Anisotropy in the Barnett Shale

Daniel Woodworth, Helge Alsleben, and Milton Enderlin
School of Geology, Energy, and the Environment, Texas Christian University, Fort Worth, TX, 76129

Many layered rocks, particularly shales, display rock strength anisotropy, which is generally controlled by preferential mineral alignment. Strength variations with direction are important when considering how rocks break or respond to hydraulic fracture stimulation. Tests to determine rock strengths are commonly time consuming and often require expensive equipment such as triaxial-load cells. The purpose of this study is to determine the efficacy of two alternative methods of determining rock strength anisotropy, specifically using a point-load penetrometer and micro-rebound hammer. These tools are less expensive and more portable and their use is less time consuming, which are beneficial when testing rock strength anisotropy. With the ultimate goal of ascertaining the effectiveness of these methods in a wide variety of situations, the immediate goal is to determine whether these devices are sensitive enough to detect strength anisotropy on the scale of that found in shale.

In our initial pilot study we used both tools to test 16 locations on a drill core from the Barnett Shale. Additional tests are currently being completed. The percent difference between the results from these two tools was used as an approximation of the percent error in the measurements. This error approximation was subsequently compared to the strength difference between measurements taken parallel and perpendicular to the layering to determine whether the methods in question detected a significant amount of strength anisotropy. Interpretation of results obtained with the micro-rebound hammer suggest an average UCS value of 9,200 psi (63 MPa) and 10,700 psi (74 MPa) parallel and perpendicular to layering, respectively. The results differ by ~16%. Analyzing results from the point-load penetrometer gives values of 7,800 psi (54 MPa) and 10,000 (69 MPa) for the same categories; a difference of approximately 24%. Taking both methods into account, the average anisotropy in the samples is ~20%. The percentage difference between the two methods, for comparison, is 11%, which falls within the expected uncertainty of 10-20%. Although other factors, such as the internal friction angle of the sample, can contribute to different estimates for the UCS values determined via the two methods, it serves as a rough estimate of the error.

In most cases the anisotropy detected is significantly greater than the error estimate obtained by comparing the two methods. Although in a few instances the error approximation is greater than the anisotropy detected, these differences are generally not large, and the error exceeded the average value found for anisotropy in only one case. Based on these data we suggest that both the point-load penetrometer and the micro-rebound hammer are capable of detecting significant amounts of anisotropy in rocks similar to the Barnett Shale.


AAPG Search and Discovery Article #90164©2013 AAPG Southwest Section Meeting, Fredericksburg, Texas, April 6-10, 2013