Abstract
Knowledge of in-situ stress orientation and magnitude is important for calibration of geomechanical models in low permeability reservoirs and bounding formations. The analysis of azimuthal anisotropy from cross-dipole acoustic data and study of drilling induced wellbore failures from acoustic borehole image logs provides critical information about the in-situ stress magnitude and orientation of the field: A thorough understanding of which has direct application in solving the problems related to reservoir production, hydrocarbon migration, wellbore stability and well stimulation optimisation.
Cross dipole wireline acoustic slowness and acoustic image data were acquired in an offshore well of Sarawak Malaysia, to provide input for the geomechanical model for the development a low permeability reservoir. The cross dipole acoustic data was processed to obtain compressional and shear velocity and azimuthal shear anisotropy. Acoustic image data was processed to generate amplitude and travel time images, which were used to identify and characterize the natural fractures and differentiate them from drilling-induced wellbore failure. Stress induced wellbore breakouts are clearly visible in acoustic amplitude and travel time images due to the large contrast in acoustic impedance between a fluid filled fracture and the matrix. The difference in fast and slow shear wave velocities and the azimuth of the fast shear from anisotropy analysis indicate the maximum horizontal stress azimuth as well as providing the key inputs to characterise the minimum horizontal stress contrast between reservoir sandstones and bounding shales. Integrating this result with the breakout analysis from the acoustic borehole images provides the comprehensive state of the in-situ stresses.
Meaningful differences in the fast and slow shear velocities, in orthogonal directions, is observed in this well indicating a potential imbalance in in-situ stress. The fast shear wave azimuth of NNE-SSW is orthogonal to the breakout direction of ESE-WNW observed from the acoustic Image log. Integrating both results provides the framework of the geomechanical model with a maximum horizontal stress azimuth of the field identified to be NNE-SSW. This is critical information in planning the drilling of infill wells and hydraulic fracturing design and optimisation for the next phase of the development campaign.
Integrating acoustic anisotropy with the wellbore breakout analysis observed on acoustic borehole image logs provides assertive determination of in-situ stress azimuth and magnitude. This constitutes the key elements of the geomechanical model, which is applied to support future well placement, infill horizontal wellbore stability, hydrocarbon reservoir development and hydraulic fracturing optimisation.
