Estimating Rock Mechanical Properties of the Zubair Shale Formation using a Sonic Wireline Log and Core Analysis

The Zubair Formation is the most prolific reservoir in Iraq, which is comprised of sandstones interbedded with shale sequences. Drilling boreholes in this formation has always been a challenge due to the weak nature of the shale sequence. Historically, over 90% of wellbore problems in the Zubair Formation are due to shale instability. To solve this problem, it is necessary to understand the rock mechanical properties and the response of shale. The main objective of this study is to develop local empirical correlations of rock mechanical properties that can be used to estimate a continuous profile of these properties throughout the depth of the Zubair shale formation directly from a wireline sonic log. Well-preserved core samples of Zubair shale were run through extensive testing, including a number of shale characterization and rock mechanical tests. Special characteristics of shale were measured and described, including the porosity, structure, texture, and mineralogy, using the free water content method, a scanning electron microscope image, a thin section photograph, and X-ray diffraction analysis. Consolidated undrained triaxial tests were conducted to determine the static rock mechanical properties. Local empirical correlations were established, with the acoustic compressional velocity as a primary input parameter. Thus, sonic wireline logs can be used directly to obtain a continuous profile of the rock mechanical properties through the entire interval of the Zubair shale formation. The accuracy of the newly developed empirical correlations was examined using statistical analysis. Moreover, these correlations were compared with previous correlations from the literature. The results showed that the suggested empirical correlations are highly accurate and reliable, in contrast to those in the literature, which did not adequately fit the Zubair shale data. This highlights the importance of using local correlations to estimate rock mechanical properties. The predicted continuous rock mechanical profile gives a good indication of the strength and stability of the shale around the wellbore. Consequently, it can be used to solve shale instability problems, optimize drilling processes (i.e., the selection of bit type and drilling parameters), seal integrity evaluation, and otherwise improve fracturing operations across the Zubair shale formation