Azimuthal variations in sonic logs have been observed for many years, both in wireline and LWD data. Rudimentary reactive geosteering methods have been used in the past, including drilling until real-time sonic log detected a change in formation velocity and subsequently altering the wellbore trajectory. We propose taking sonic geosteering a step further by providing real-time azimuthal images showing the velocities as they vary around the wellbore. Images can also be transmitted at both shallow and deep depths of investigation to determine how close an approaching boundary may be. While more traditional geosteering measurements, such as resistivity and gamma ray, are highly suitable in many cases, there are instances where the resistivity or gamma ray contrast between beds may be low or the depth of investigation very shallow, making geosteering problematic. However, in such environments, there may well be a porosity contrast, detectable as velocity differences by the sonic tool, which are more suitable to geosteer with. In addition, sonic logs are an ideal way of determining gas contact points in a reservoir, as compressional velocity measurements are highly sensitive to gas. For unconventional shale reservoirs, sonic anisotropy and its relationship to rock mechanical properties are a principle determinant of a well-placement strategy. In this paper, we explore some key factors of sonic geosteering, such as depth of investigation, azimuthal resolution, and compressional vs. shear velocity responses. A workflow for integrating azimuthal sonic measurements into existing visualization and geosteering software is described. Field data examples are presented, showing the feasibility of sonic geosteering as well as its current limitations.
展开▼
