Integrated approach using core analysis and wireline measurement to estimate rock mechanical properties of the Zubair Reservoir, Southern Iraq

摘要

The Zubair Reservoir is a regionally extensive oil- producing sandstone sequence which is part of the petroleum system in Iraq, Kuwait, Syria, Iran, and Saudi Arabia. Rock mechanical properties are extremely useful in optimizing drilling, production, and reservoir compaction. Hence, an accurate technique for estimating rock mechanical properties may significantly improve the economic revenues derivable from the reservoir. However, information about rock mechanical properties along the depth is often discontinuous and limited to core samples taken within the reservoir interval. The present study aims to predict a continuous profile of rock mechanical properties throughout the depth of the Zubair Reservoir directly from wireline porosity logs. Special characteristics of Zubair sandstone were evaluated and described using thin-section photographs, X-ray diffraction analysis, and scanning electron microscope imaging to identify and characterize the mineralogy, texture, and structure of the sandstone unit. Thereafter, both triaxial and triaxial multistage testing techniques were carried out on 130 plug samples from the Zubair sandstone formation to measure the rock's mechanical properties. Valid local empirical correlations were established between the porosity and these mechanical parameters. The obtained results revealed that the rock mechanical properties are primarily functions of porosity and the direct linear expression is more reliable than power functions and exponential functions. The best relationship obtained between UCS, internal friction angle, Young's modulus, bulk modulus, shear modulus, and Poisson's ratio with porosity had a correlation coefficient (R-2) of 0.84, 0.75, 0.80, 0.59, 0.83 and 0.70, respectively. The accuracy of the newly suggested empirical correlations was subjected to statistical analysis. Moreover, these relationships were compared with the existing commonly used correlations reported in the literature on real field data from Zubair sandstone. The results showed that the suggested empirical correlations have a high accuracy and reliability, while the previous correlations could not adequately cover the Zubair sandstone data. This emphasizes the importance of using local correlations to estimate rock mechanical properties. The predicted continuous rock mechanical profile provides a good indication of the strength and stability of the formation around the wellbore. Consequently, it can be used in solving wellbore instability problems; preventing sand production; optimizing drilling processes, which includes the selection of the bit type and drilling parameters; and fracturing operations across the Zubair sandstone reservoir.