High-angle and horizontal (HA/HZ wells are commonly drilled and often the logging suite is selected primarily to aid geosteering the well to a specific target. In many situations, these wells penetrate zones or portions of the reservoir for which no vertical appraisal wells are available. In these situations, the logs acquired will be used for routine formation evaluation of net-to-gross, porosity, and possibly water saturation. Comparison of log responses in a given formation from vertical wells to responses in HA/HZ wells indicates that significant differences can exist. For example, recent studies show that porosity values can be off as much as 6 p.u., water saturation uncertainty can exceed 50%, and true stratigraphic thickness can be off by 200-300% because of uncertainty in apparent dip in highly deviated wells. Thus, routine formation evaluation approaches for vertical wells may not be adequate for accurate characterization of formations and fluids in HA/HZ wells. Many causes for different responses exist, but most relate to the fact that many logging tools were designed for vertical wells with near-horizontal strata. In extremely high-angle to horizontal wellbores, our interpretation schemes must be modified to account for 1 geometry of beds relative to the borehole, 2 circumferential direction of the measurement, 3 formation anisotropic effects on resistivity, acoustic, and nuclear responses, 4 unusual invasion profiles, and 5 gravity causing eccentering of tools and cuttings bed accumulation on the bottom of the wellbore. Although borehole image logs may be the best approach for characterizing relative bed dip in near-horizontal wellbores, issues still exist in obtaining sufficiently accurate dips when wellbore deviation exceeds 80°. Cuttings beds can have a significant effect on both density and nuclear log responses. Since logging of these wellbores is challenging, it is critical to establish a protocol for distinguishing poor log responses from "unusual" log response just due to geometry, anisotropy, and other borehole effects. In November 2004, an SPWLA Topical Conference was held in Taos, New Mexico to address formation evaluation challenges in HA/HZ wellbores. A survey of participants at the end of the conference indicated that the standard qualitative use of logs in HA/HZ wells for geosteering would not be sufficient for many future situations. In many locations vertical assessment wells cannot be drilled and it is critical to evaluate formations using extended reach wells (e.g., drilling from onshore to reach reservoirs offshore. A general consensus was that our current interpretation schemes developed for near-vertical wells often are not adequate for HA/HZ wells. Addressing these challenges will require collaboration between the operating companies and service companies. Operators need to be willing to share examples of the interpretation problems and difficulties currently encountered in evaluating field data. Service companies need to be willing to share current limitations of tool responses and provide additional aids in interpretation (e.g., tool response functions for modeling and chart books to assist in interpreting HA/HZ wells. Both operators and service companies agreed that some new tool designs are required to obtain quantitative characterization from HA/HZ wells.

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