Published: 04/13/2015
Published: 04/13/2015
An operator with two appraisal wells in the North Malay basin was concerned about wellbore stability. The shallow formations, at 2,000 to 4,000 ft, are characterized by weak, soft sediments in which leakoff tests did not return reliable data for calibrating the fracture gradient and determining the casing point for reaching the deeper reservoir targets. The pore pressure significantly ramps up to the deep overpressured reservoirs below 8,000 ft, which require mud weights up to 7 lbm/galUS higher than for a normal pressure environment.
To measure the minimum horizontal stress for determining the fracture gradient, a wireline formation tester was deployed in dual-packer configuration to conduct a minifrac test in a clean water-bearing sand. Measurement of the stress required to break down the formation would be used to calibrate log and core measurements. But even with pump pressure as high as 4,500 psi—the limit of the packers—the formation did not break down. Without sufficient stress data from the leakoff and minifrac tests, the MEM could not be calibrated to be certain of the minimum horizontal stress or fracture gradient.
The Sonic Scanner acoustic scanning platform uses multiple monopole and dipole transmitters to accurately measure at multiple depths of investigation. In addition to conventional compressional and shear slownesses, the Sonic Scanner platform obtains axial, radial, and azimuthal waveforms for characterizing both the near-wellbore and far-field formation volumes. For relatively soft rocks, such as the shallow reservoirs in the North Malay basin, near-wellbore stress concentrations cause deformation that is noticeable in the compressional and shear measurements. By combining the nearwellbore and far-field radial profiles with a nonlinear elastic wellbore stress model, the horizontal stress regime can be estimated for zones that are sensitive to stress.
The stress magnitudes determined with the Sonic Scanner platform's measurements in a stress-sensitive sand formation were calibrated to core measurements to explain why the minifrac and leakoff test pressures had not reached breakdown pressure. The fracture initiation pressure was recalculated to be 5,500 psi, nearly 40% higher than the existing estimate and 1,000 psi higher than the pressure applied during the minifrac test.
Beyond the essential pore pressure and fracture gradient calculations, calibrated rock strength and elastic properties were constructed with data from the Sonic Scanner platform, petrophysical logging, and core measurements. Based on this accurate, continuous dataset, the MEM can be used to predict wellbore stability at different well deviations for calculating the safe mud-weight window in drilling both these initial wells and subsequent wells, as production depletes the reservoir pressure. This predictive modeling for shallow reservoir pressure management is essential for successfully reaching deeper reservoir targets.
Challenge: In the absence of leakoff and minifrac tests, resolve uncertainty in the fracture gradient and mud-weight window required for the drilling design to a deeper targeted reservoir.
Solution: Run the Sonic Scanner acoustic scanning platform, which measures sonic velocities at multiple depths of investigation to provide a full 3D characterization from which stress magnitudes and the stress regime can be calculated for direct input to the drilling design and model calibration.
Results: Accurately specified mud weights for different hole sections and offset well locations based on continuous elastic properties and a calibrated mechanical earth model (MEM) calculated with increased confidence by using a fracture gradient profile based on stress estimations from Sonic Scanner platform measurements.
