Fit For Purpose Integrated Structural Identification Using Azimuthal Monopole Sonic Waveforms and Vertical Seismic Profiling: The First Case Study From The Vertical Unconventional Well in North Sumatera Basin – Indonesia

Abstract

The uncertainty in understanding unconventional plays has encouraged more studies and technological advancement in the oil field industry. A complementing expertise to dive into the process of understanding reservoir structure has been one of the most essential aspects in the appraisal phase of an exploration well. This paper elaborates a case study that uses an integrated solution to understand structural continuity of a formation in various resolutions and scale. For a vertical onshore exploration well, CMA-001, located in North Sumatera Indonesia an advanced acoustic wireline data set was acquired to assess the degree of anisotropy and the formation elastic properties for input to the stimulation model. Our study focuses on extracting the transversely isotropic shale characteristic from the Baong Formation and following the success story of its predecessor, MLC-001, aims to support an unconventional hydrocarbon exploration of the new vertical well. Nonetheless, the wellbore image log’s quality that is processed and commonly used as inputs in the anisotropy processing have associated uncertainties because of the development of mud cracks that hindered the interpretation of true dip and azimuth. To overcome this complexity, the azimuthal acoustic waveforms are processed and identified for reliable reflectors attributes providing an understanding of reflector dips at a distance from the wellbore. Direct confirmation from zero-offset vertical seismic profiling (VSP) provides broader understanding of reflector’s continuity far from the wellbore. With modern sonic imaging techniques, filtered monopole waveforms are explored to identify coherent reflected energy. The ray-tracing inversion and 3D slowness time coherence (STC) processes take place to confirm for a low misfit between the ray path type and the time pick and act as the ultimate validator on the attributes which translates to the reflector’s true dip and azimuth. The result in the logged section of the well identifies reflectors that correspond to a combination between PP (compressional to compressional), PS (compressional to shear), and SP (shear to compressional) raypath types. The reflectors indicate the presence of a dip angle below and above 30°. Identified reflectors span from 0.02 m to 4.56 m laterally away from the wellbore primarily orienting towards northwest-southeast. The azimuthal monopole sonic migration result visualizes finer layer images of the formation that show excellent agreement with what is investigated by the VSP image. The integration of azimuthal monopole sonic imaging and zero offset VSP has delivered valuable insight and has become a differentiator, providing the ability to connect the dots between two stand-alone measurements.