Although high-deviation wells are common nowadays, formation evaluation in such an environment still faces challenges because the logs behave differently in highly deviated wells than in vertical ones. The well deviation affects response characteristics of resistivity logs significantly, although it may also influence density and other logs. In a highly deviated well, one in general cannot directly use the measured physical values in petrophysical analysis without first compensating for the deviation-effect.
To correct the deviation-effect, one normally needs to know the apparent dip angle between borehole and formation beds first. The apparent dip angle is usually derived from an image log. However, in many situations, an image log may not be available due to various reasons, yet it is critical in interpreting the logs as well as in building a reliable reservoir model. Recognizing the fact that the higher the apparent dip the larger is the deviation-effect helps the extraction of the apparent dip while also correcting for the deviation effect on the logs. Through modeling the resistivity tool responses, it is found to be feasible to derive the apparent dip from the logs such as LWD propagation resistivity and/or array induction tools for moderate to high apparent dip value and at the same time to correct the deviation-effect or to derive the true formation resistivity.
Visualization of the geometric relationship between the wellbore and bedding is another important component in deviated-well formation evaluation. An effective and user friendly software tool is necessary to properly understand and account for the deviation-effect. It integrates well trajectory information and/or, if any, image logs to derive plausible formation model and its true-resistivity value. It enables one to understand the unique response characteristics and the proper appraisal of the reservoir geometry. Examples are provided to illustrate the importance of deriving apparent dip and the correction of deviation-effect on resistivity logs.
Both wireline and LWD tool responses are affected by borehole deviation. The "hidden" assumption that the borehole is perpendicular to the bedding used by the focusing algorithm of wireline array induction is often violated in a deviated well, resulting in anomalous log value and curve order. For example, the papers by Barber and Howard (1989) and Barber and Shray (2001) pointed out that induction logs are affected by deviation effect. Over the years, people developed less dip-sensitive ways of focusing the induction logs (e.g. Xiao and Zhou, 1998; Barber and Minerbo, 2002) although they are not routinely used.
For LWD resistivity logs, the well-known "horns" are observed when an LWD propagation resistivity tool crosses formation bed-boundaries in a high-deviation well (Wu and Barnett, 1991). Although the "horn" is useful for the purpose of qualitative geosteering, signaling that an LWD resistivity tool approaches and crosses a bed-boundary, it may compromise quantitative petrophysical analysis if not accounted for first. Some non-observable phenomena in vertical wells, such as anisotropy, become significant in high deviation wells and need to be properly addressed too.