SwRI geophysicists have created a new algorithm that uses cross-dipole sonic data to estimate formation properties around the borehole Reading the Rocks

etroleum exploration companies rely on sophisticated analyses of the geologic formations from which they hope to produce oil and gas. The subsurface geol-ogy of a hydrocarbon reservoir typically consists of sedimentary rock such as shale, limestone or sandstone. In large part, the productive potential of a well drilled into the reservoir depends not just on the permeability and porosity of the formation in which the hydrocarbons occur, but also on the amount and distribution of fractures into which the hydrocarbons tend to gather and flow. Thus, it is important to know in what direction from the borehole these elements of non-uniformity, or anisotropy, occur in greatest numbers, and also whether they are oriented horizontally, vertically or at an angle relative to a well. The answers offer valuable clues for production-improving strategies, such as directional drilling from the original borehole outward into the formation. In particular, reservoirs formed by sand and shale sequences are characterized by mineral distributions and cracks. Anisotropy can occur in different axes. In layered sedi-mentary formations, the effective stiffness properties in the vertical plane (perpendicular to the earth’s surface) are differ-ent from the properties as seen at more horizontal planes. The axis of symmetry in this case assumes vertical transverse isotropy (VTI), where the same effective material properties exist in any azimuth direction that can represent a well inter-cepting a horizontal shale sequence.Also, naturally fractured reservoirs, like those typically formed by carbonate or limestone, exhibit a different type of anisotropy called fracture-induced anisotropy, which is related to fracture permeability or porosity. Because of stress exerted by overlying formations, naturally fractured reser-voirs can be formed by vertical or tilted fractures, or both. For example, the vertical fractures are oriented parallel to the well, and the axis of symmetry is perpendicular to the bore-hole axis. In this case the anisotropy is horizontal transverse isotropy (HTI).Sonic well logging provides an electronic profile of a geologic formation by reading the refracted patterns of sonic waves sent through it. The process can involve a simple uni-pole technique that is delivered in the well, in which a single sensor measures a compressional (or primary) wave as it travels through the formation from an acoustic source in the borehole. More sophisticated data can be gathered using the cross-dipole method, so named because it sends dual, or dipole, pressure signals outward directionally, at right angles from each other, such that their pathways cross at the center of the borehole. These signals are detected as shear waves, so-called because, as they encounter non-uniform subsurface features, such as vertical fracture, they split into faster and slower waves. Shear waves naturally travel slower than the compressional wave but are more sensitive to anisotropy. The paths and arrival times of the shear waves are detected and