Sonic velocities from wireline/LWD logs have been used to establish relationships with rock mechanical properties. Because sonic logs frequently are not available, there has been great interest in relating rock mechanical properties and other logs (e.g., porosity logs). Resistivity logs, are commonly acquired logs. The relationship between sonic and resistivity logs is often masked by factors, such as fluid salinity and hydrocarbon saturation. The evidence indicates that filtering these factors out will increase the probability that the resistivity log would correlate well with the sonic log, and ultimately, with rock mechanical properties. This paper presents a methodology to minimize background noise from the resistivity log data and, moreover, to select and categorize the interest points in levels of rock strength. Overall, it involves establishing a baseline and offset lines in the resistivity log and the sonic log. These lines are compared quantitatively to the original log curves to identify and filter out data points associated with factors that perturb the trend between sonic slowness and resistivity. Techniques used to derive the resistivity baseline (curve smoothing techniques) and to provide the reasonable ranges for the factors that cause resistivity offsets (based on resistivity models from the literature) are presented in the paper. The proposed methodology is demonstrated using case studies from wells drilled in the Gulf of Mexico.
Mechanical properties of sub-surface rocks for geomechanical studies in oil/gas exploration are usually obtained from rock mechanical testing, if conditions are favorable. For shaly lithologies, the rock testing encounters some difficulties related to the integrity of the specimen when brought to surface, and test results include a high degree of uncertainty. Furthermore, rock testing uses the samples from the cores that cannot cover the whole interval of the well. For these reasons, rock mechanical properties have been obtained primarily from correlations with wireline and LWD data, especially from sonic velocity data. Many of the correlations have been reviewed by Khaksar et al. [1]. The use of correlations between sonic log and rock mechanical properties has been proven to be successful and useful in analyses performed along the interval of a well, such as the safe mud window from shear failure gradient and fracture gradient. However, sonic logs are frequently unavailable. There has been great interest in establishing relationships between rock-mechanical properties and other geophysical logs (e.g., porosity logs) [2]. Porosity logs may also be unavailable or affected by borehole quality effects. Resistivity logs, on the other hand, are the most commonly acquired type of wireline/LWD logs and are not affected by borehole effects so that the quality of resistivity logs is usually high. Establishing a physical relationship between resistivity and sonic log data would be highly useful for deriving rock mechanical properties directly from resistivity logs. A simple cross-plot between sonic and resistivity logs can, in most of cases, reveal that there is a natural trend of decreasing sonic travel time (slowness) for increasing resistivity, but with a poor correlation perceived by a large spread of data points.
