Predicting Weathered Rock Properties Using Integrated Geophysics and Laboratory Testing Approach

Abstract

ABSTRACT Reliable estimation of the mechanical properties (e.g., strength) of weathered rocks is critical for taking proactive measures, such as adjusting mud weight density and/or drilling parameters, to achieve safe and efficient infill drilling. Currently, the common approach to estimating rock properties is to use its regression with geophysical logs, particularly the sonic velocity. Such an approach was challenged when applied to shallow and heavily weathered bedrocks accompanied by high porosity and water saturation and low density. The preserved drill cores from the previous drilling of adjacent wells offer an opportunity to correlate laboratory results with borehole logging data to estimate rock properties and identify high-risk drilling horizons. In this work, 196 cylindrical drill cores with an average diameter of 89.25 mm from the depth between 35 m and 396 m were prepared for uniaxial compressive strength (UCS) and Brazilian tensile strength tests. Sample characterization on each sample was also conducted, including density, porosity, P- and S-wave velocities, and static and dynamic elastic moduli. Compared with the logging P-wave velocities, the laboratory P-wave values are consistently lower by 32.7% on average, especially for shallow rock cores, which likely reflects the impact of water moisture, further weathering and in-situ compaction stress. The characterisation results were then used as inputs to correlate with rock UCS and tensile strengths, which were found to be well correlated with the depth, density, P- and S-wave velocities, and elastic modulus. This work provides useful experimental data sets of shallow weathered bedrocks and highlights the importance of including rock depth in the regression model for rock strengths. The developed correlations could be directly employed to forecast the mechanical properties of shallow rocks as well as being a guideline to identify high-risk weak rock facies, where are severely weathered and expect significant drilling challenges. INTRODUCTION The mechanical properties of in-situ reservoir rocks are key input parameters for geomechanical assessment of geoengineering applications such as well drilling and completion and wellbore stability. Accurate site investigation, including assessing rock strength, is essential prior to drilling to plan the most efficient and cost-effective activities (Kolapo, 2021). However, weathering constantly affects the strength of rock masses, particularly near the ground surface, posting a significant challenge for conducting engineering activities, such as drilling (Gupta and Seshagiri Rao, 2000; An et al., 2020). It is therefore crucial to reliably predict the mechanical strength of each rock layer, particularly the weathered zones, to achieve a safe and successful drilling operation.