Presented by: Martin G. Lüling, Schlumberger-Doll Research
Traditionally, oil wells were drilled vertically. Only since the late 1980's, drilling and oil production changed toward directional, frequently horizontal holes. This drilling technology permits profitable exploitation even of thin oil-bearing zones: instead of traversing the layer vertically for a few feet, the borehole trajectory leads along the oil-zone for many hundred feet, even a few miles under favorable conditions. The drill bit is steeerd along the true geologic structure by real-time measurements, made while drilling, of the formation electric resistivity.
The electric-resistivity measurements are used to discriminate resistive oil from more conductive water-bearing zones or shales. For Logging While Drilling, the measurement sonde is integrated into the drill collar close to the bit. The measurement sonde transmits a 2-MHz radio wave into the rock and measures the phase shift (wave length) and attenuation (skin effect) of the wave. From these two measurements, two resistivities are derived. These resistivity data are transmitted to the surface where they help the driller to control the direction of the borehole trajectory.
The resistivity measurements can be reproduced by log-simulation modeling. The resistivity data from a nearby reference well, usually a vertical well, serve to generate a formation model, namely a layer-cake resistivity profile. Seismic surveys extend the profile from the reference well across the entire reservoir. Into this reservoir model, a directional drilling trajectory is planned. Part of the planning will be a predictive computation of the resistivity measurement along this trajectory through the layer-cake formation. The driller has these modeled data available and compares them to the real-time data acquired while drilling. Several examples from actual field data illustrate the success of the ``GeoSteering" procedure.
Many oil zones consist of fine laminates, such as alternating shales and sands which cannot be individually resolved. They induce an effective anisotropy in the electric resistivity with a low resistivity parallel to the laminates and a much higher resistivity perpendicular to the laminates. In vertical wells with horizontally laminated formations, the resistivity measurements only measure the parallel, or ``horizontal resistivity". However, in directional wells, the resistivity measurements become sensitive to the perpendicular, or ``vertical resistivity". The 2-MHz resistivity measurements can even discriminate the two resistivity components and thus quantify the resistivity-anisotropy tensor. The log-simulation modeling successfully reproduces observed anisotropy effects.
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