Navigating and Imaging in Complex Geology With Azimuthal Propagation Resistivity While Drilling

Abstract

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractReservoir 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.

Calculation of Tilted Coil Voltage in Cylindrically Multilayered Medium for Well-Logging Applications

“…Substituting (9) into (1), and take the parity of the integrand into account, thus leading to (10) for the voltage on a tilted receiver coil relate to a co-axial transmitter coil.…”

“…Comparing the expression (17) and (10), they have the same expression in homogeneous medium if we exchange their transmitter and receiver. For cylindrically concentric multilayered medium, each harmonic components of EM field excited by tilted transmitter coil is uncoupled for their reflection and transmission [36].…”

“…In geosteering and reservoir navigation applications, logging-while-drilling (LWD) azimuthal resistivity tools equipped with tilted transmitter and/or receiver coils are used to measure and deliver the directional information associated with formation boundary. Two typical measurement ways of azimuthal tools are employed: a co-axial transmitter coil and a tilted receiver coil, or a tilted transmitter coil and a co-axial receiver coil [9], [10].…”

Some aspects of the directional resistivity physics

“…Directional resistivity has evolved into an enabling technology for geosteering and reservoir navigation (Li et al, 2005;Bell et al, 2006;Wang et al, 2006;Christiaansen et al, 2007;Bittar et al, 2007). LWD resistivity tools equipped with directional antennas provide deep-reading information for proactive well placement.…”

A study on a deep-looking resistivity tool for geosteering

“…Inspired by the azimuthal sensitivity of wireline triaxial induction tools, cross-component measurements have been introduced to LWD tools (Li et al, 2005;Bell et al, 2006). Using transverse or tilted coils as receivers, bed boundaries from above and below can be differentiated by opposite signs of the received signal.…”