New Resistivity-Logging Tool Helps Resolve Problems of Anisotropy, Shoulder-Bed Effects
Authors: Leveridge Robert Mark 
Published: February 2015

Technology Update

Three-dimensional, colocated resistivity measurements at multiple depths of investigation (DOI) can quantify even low-resistivity, laminated pay zones to reduce uncertainty and refine reservoir models. Obtaining measurements of this type has made it possible to resolve formation anisotropy problems and shoulder-bed effects in a broad range of wells.

For decades, well-log interpretation classes began with the caveat, “Assume an isotropic, homogeneous formation.” Indeed, this was the only way a systematic approach could be developed to calculate such fundamental parameters as porosity and water saturation in reservoir rocks. Logging tools of the day lacked the spatial resolution to make precise measurements in any but the most basic formation/borehole geometries. Because most wells were vertical, or nearly so, and since many wells penetrated strata that resembled layer-cakes, tools were designed using the principle of radial symmetry. That is, they worked optimally when the borehole axis was perpendicular to the formation’s bedding planes.

Even when we knew that formation isotropy was rarely the case, we were powerless to do anything about it because of tool limitations. It could be postulated that the introduction of horizontal drilling provided the impetus to develop new tool systems that could accurately characterize formations and reservoirs regardless of formation/borehole geometry.

Today, it is possible to resolve even extremely tough interpretation problems through the application of the Rt Scanner, a triaxial-induction resistivity-logging tool that

calculates both vertical and horizontal resistivity (Rv and Rh), respectively, from direct-induction measurements while simultaneously solving for formation dip at any well deviation. By making measurements at multiple DOI in three dimensions at the same depth in the tool, true 3D resistivities are derived. Subsequently, enhanced hydrocarbon and water-saturation estimates computed from these measurements result in more accurate reservoir models and reserves estimates, especially for formations with laminations, anisotropy, or faults.

Tool Description

The tool is typically conveyed into the borehole on wireline, but can be deployed on drillpipe, on coiled tubing, or by means of a downhole tractor for highly deviated wells. It contains multiple triaxial arrays, each consisting of colocated X, Y, and Z coils that collectively measure various depths into the formation. Rv and Rh are calculated at each triaxial spacing. An electrode sleeve with short-single-axis and colocated triaxial receivers can be used to fully characterize and manage borehole signals in air-filled boreholes and oil-based and water-based muds.

In addition to resistivity, formation dip and azimuth are available for structural interpretation. Standard array-induction-imaging-tool measurements are also delivered for correlation with legacy field logs. The tool can be used with most openhole services. A caliper and general-purpose inclinometry tool are also required in the tool string.

Tough Anisotropy Issues Can Be Resolved

Laminated sand-shale models with anisotropic shales have been discussed extensively. However, there has never existed a clear procedure to determine shale anisotropy. Today, knowledge of formation anisotropy plays an increasingly important role in deciding where to land and place lateral-completion sections of oil and gas wells, and how to design stimulation treatments.