Other effects complicating the borehole environment include asymmetric invasion profiles, the presence of cuttings beds and drilling mud segregation (Passey et al., 2005). The difficulty begins with the fact that LWD data acquisition strategies in horizontal wells are often designed for geosteering purposes and can be unsuitable for petrophysical evaluation. Even when full LWD logging suites are available in HaHz wells it is often difficult to apply vertical well petrophysical interpretation techniques because of bed boundary effects and proximity to uncrossed layers, local layering or resistivity anisotropy, and polarisation horns on the resistivity measurements (Griffiths et al., 2012). Note the extreme vertical exaggeration: 50 ft TVDSS (true vertical depth sub-sea) vs.
Geological section of example well A showing the target layers and faulting. Modern technologies allow standard petrophysical measurements to be acquired under these conditions, such as coil tubing and drill pipe assisted logging, and the development of LWD tools and assemblies which have become the primary means of acquiring logging data in HaHz wells. A well is said to be horizontal if it deviates more than 80° from vertical (Passey et al., 2005). In HaHz wells it is often difficult to apply the traditional petrophysical interpretation techniques used in vertical wells, particularly the resistivity logs, due to geometric effects on the data. The practice has been proven to significantly increase production even the first commercial horizontal well delivered twenty times the expected production in a vertical heavy oil well (Bosio, 1986). Modern drilling technology, geosteering, and logging while drilling (LWD) tools allow high angle and horizontal (HaHz) wells to be routinely undertaken and the economic development of unconventional reservoirs, such as gas and oil shales and tight sandstones, would not be possible without the extensive use of horizontal wells.ĭirectional drilling presents a number of formation evaluation challenges, however. Increasing numbers of wells are drilled horizontally or at high angles for a number of reasons – to improve reservoir exposure, avoid or target particular features, and in relief-well scenarios.
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