Investigation of Shear Wave Anisotropy in Deviated Wells near a Salt Structure in the Gulf of Mexico

Summary

The number of oil and gas wells drilled through and near salt structures continues to increase. These wells are some of the most expensive wells to drill and are prone to numerous drilling problems that drastically increase their associated costs. Better understanding of the geomechanical setting in proximity to salt results in more successful drilling and completion of these wells. One promising area of study for investigating the perturbation of the stress field in and around salt bodies is numerical modelling. However, in order to apply the results of the simulated stress field from a numerical model to a well drilled near a real salt body with confidence, it is necessary to compare the modelled results to data observations. Therefore, it is important to have independent observations of the stress state near the salt body. One technique for determining principal stress directions uses cross-dipole shear wave velocity anisotropy data. However, shear wave velocity anisotropy can be induced by structural mechanisms as well as stress-related mechanisms. In this study, we investigate a technique to identify structure-induced velocity anisotropy and to characterize possible stressinduced velocity anisotropy. The investigation uses cross-dipole sonic data from three deep water, sub-salt wells in the Gulf of Mexico. First, we determine the parameters necessary to ensure the quality of the fast azimuth data used in our analysis. We then characterize the quality controlled measured fast directions as either structure-induced or stress-induced by predicting the apparent structure-induced fast direction the dipole sonic tool should measure for known bedding planes. We find that this technique supplements the use of dispersion curve analysis for characterizing anisotropy mechanisms. We also find that this technique has the potential to provide information on the stresses that can be used to validate a numerical model of the salt-related stress perturbations.

Key words::

• shear wave anisotropy
• stress
• structure
• salt