LATEST GENERATION LOGGING WHILE DRILLING SONIC TOOL: MULTIPOLE ACOUSTICS MEASUREMENTS IN HORIZONTAL WELLS FROM OFFSHORE WEST SOUTH AFRICA

摘要

Logging while drilling (LWD) technology has progressed swiftly in recent time to address the need for saving rig time, making real-time informed decisions for drilling efficiency and risk management, and accurate well placement. LWD sonic measurements provide information for timely analysis of borehole stability, drilling optimization, and assisting with pore- pressure prediction and well-to-seismic tie. Most of these applications have relied on the measurements of formation compressional slowness due to the difficulty in obtaining shear slowness in the LWD environment, especially in unconsolidated formations. Formation shear slownesses, measurable by logging while drilling sonic technology, can complement compressional measurements for advanced applications (AVO analysis, geomechanics, completion, lithology, and hydrocarbon detection). This paper will demonstrate advances that have been made in new LWD sonic tools with the evaluation of both shear and compressional velocity measurements; the new technology was used for two horizontal wells in West South Africa. A new LWD sonic tool has been developed to address the challenges in obtaining shear measurements in unconsolidated formations. The new technology designed for robust measurements of compressional and shear slownesses, regardless of the formation velocity, enables advanced applications from sonic data. The tool can acquire multiple acoustic modes (monopole and quadrupole) with 48 sensors (4 azimuthal measurements at 12 axial positions). Its large acoustic aperture and short inter-receiver spacing enhances the quality and resolution of processed slownesses. With its powerful computing capability, the tool enables more complex processing downhole, leading to broader applications in real time. Field test data acquired in two horizontal wells in West South Africa show that the tool can acquire high-quality waveforms over a wide frequency band, providing reliable compressional and shear slownesses in the formations penetrated by the boreholes. Formation compressional and shear slownesses from the monopole waveforms were transmitted in real time. Shear data are present in the monopole dataset, and formation shear slowness was extracted from the monopole waveforms in addition to the quadrupole waveforms, when possible. Monopole shear measurements are not continuous throughout the logged interval because of variation in formation types (slow or fast relative to mud speed) but quadrupole waveforms enabled a continuous shear measurement. When possible to extract shear slownesses from both monopole and quadrupole data, some differences were discovered between the two datasets. The difference between measured shear slownesses from monopole and quadrupole waveforms suggests acoustics anisotropy. In these environments, shear anisotropy in the horizontal well is expected from the large difference between vertical and horizontal stress, as well as layering (from the horizontal or near horizontal shale layering). With high-quality waveform recording, real-time data delivery, and advanced processing, this new LWD sonic technology provides robust compressional and shear slowness data, opening new frontiers for LWD sonic applications.