Determination of Rock Strength Using Advanced Sonic Log Interpretation Techniques

Abstract

Abstract Analysis and prediction of geomechanical problems is important for the success of drilling, completion, and production operations, and requires knowledge of rock strength, pore pressure and in-situ stress. Geomechanical problems encountered while drilling include wellbore stability and fracturing of the formation, and may lead to financial loss due to loss of drilling fluid, well control events (kicks), stuck pipe, extra casing strings and sidetracks. Examples of geomechanical problems due to reservoir stress changes occurring during production include reservoir compaction, surface subsidence, casing deformation and failure, sanding, etc. The information needed to assess the impact of geomechanical problems on field development can be captured by building a Mechanical Earth Model (MEM) that includes details of the in-situ stresses, rock failure mechanisms, rock mechanical parameters, geologic structure, stratigraphy and well geometry. The workflow for building a Mechanical Earth Model (MEM) along a well trajectory based on logs and drilling data is illustrated for the case of a deviated well drilled through deepwater Gulf of Mexico channel sands. Once constructed, a MEM can be used to identify geomechanical problems and formulate contingency plans for mitigation before future wells are drilled. The stress field in the vicinity of a well is perturbed by the physical operation of drilling the well. This process may result in formation damage or alteration of the rock properties resulting in changes in acoustic velocity in the vicinity of the well. The constructed MEM uses the variation in the fast and slow shear wave velocities as functions of the distance from the borehole to determine the variation in rock strength along the well. The results of mechanical rock properties measured from cores in an independent study are found to be in good agreement with the rock strength determined from the sonic log measurements.