Abstract
Abstract
Reservoir navigation with LWD resistivity has traditionally relied on matching real time measurements with ideal logs. Reservoir navigation engineers initially build one or more resistivity models including all expected resistivity boundaries such as oil-water contact, reservoir to cap rock interface, faults and unconformities. Then, during drilling, they direct the well and update the earth model by matching actual measurements with forward response model data.
Because common LWD resistivity sensors cannot differentiate between an oil-water contact approaching from below and a shale lens approaching from above or from the side, the reservoir navigation engineer fills in the missing information through expertise and local knowledge. In case of complex geology however, such as reservoirs with tilted or rotated fault blocks, multiple fluid contact levels, cross-stratification and shale intrusions, navigation becomes much more challenging and the risk of getting geologically lost is high. In recent years imaging LWD tools were introduced to help reduce the azimuthal uncertainty but they were limited to a few inches in lateral investigation.
A new azimuthally sensitive propagation resistivity tool was recently tested for reservoir navigation and formation imaging in some of the more complex reservoirs of the North Sea. In cases where standard omni directional tool responses would lead to ambiguous interpretations, the azimuthally sensitive tool provided the basis for clear geosteering advice. A new imaging algorithm helped visualize approaching beds much like modern imaging devices, but with a depth of investigation reaching several feet into the formation. At fault crossings, the azimuthally sensitive signal helped recognize the relative movement of the formations on either side of the fault. In other instances where the well was run immediately below the cap rock, deep looking azimuthal propagation anticipated the intersection by several hundred feet. Also, analysis of the detailed deep electrical images brought a more complete understanding of the subsurface.
Introduction
After about two decades of development, LWD resistivity measurements now are one of the most commonly used tools for geosteering, reservoir navigation, and formation evaluation (Clark et al., 1988; Bittar et al., 1991; Meyer et al., 1994). Such measurements provide information about formation resistivity near the bit and approaching beds. Comparison of real-time measurements with pre-calculated tool responses allows the reservoir navigation engineer to determine the bit position relative to the geological target.
Although the conventional LWD resistivity measurement allows discrimination of a conductive approaching bed from a resistive one, the measurement does not distinguish the direction of approach. For instance, a decreasing apparent resistivity value indicates an approaching conductive formation. The conductive formation can occur above (e.g., a shale roof) or below (e.g., an oil-water contact) the tool. To avoid penetrating the conductive formation, the direction of the conductive formation must be known. Reservoir navigation in a deepwater sand channel may require similar directional information to remain in the channel.
LWD measurements with azimuth sensitivity include azimuthal gamma-ray, neutron, density, and resistivity imaging tools (Bonner et al., 1994; Greiss et al., 2003; Ritter et al., 2004). These measurements are extremely useful for reservoir navigation and formation evaluation. However, they all have relatively shallow (less than a foot or so) depths of investigation. Some of them are no more than a couple of inches deep. Only until very recently has a deep directional measurement technique been reported (Li et al., 2005).