A quantitative comparative analysis of some bipolar and unipolar resistivity arrays is made in terms of perturbation effects due to buried lateral inhomogeneities. The Schlumberger, the collinear dipole‐dipole, the pole‐dipole, and the unipole electrode configurations of approximately the same dimensions are employed. A “focusing effect” in the vertical component of the current density distribution is demonstrated in the unipole configuration. Characteristic diagrams illustrating the configuration responses to lateral inhomogeneities located at different depths and under various overburden layer conductivities indicate that, for comparable geometric spreads, the unipole and the pole‐dipole configurations are significantly more effective.

Much more accurate formation evaluation is now made possible by new generation propagation resistivity tools that make more measurements than previous systems. The additional measurements provide new information from the borehole and the near borehole environment as well as the formation itself. They also provide the capability to identify and differentiate between environmental effects. An new, advanced processing method combines many of the measurements collected by the tool then identifies and corrects for environmental effects. This includes a new method for dielectric constant correction. The method also calculates horizontal (Rh) and vertical resistivity (Rv) for high angle wells where anisotropic effects are present. A global solution for Rh and Rv is derived using a minimum of four measurements. This eliminates uncertainty where multiple solutions of Rh and Rv are possible. After corrections are made for environmental effects the method then generates four resolution-matched resistivity curves with fixed depths of investigation (radii) at 10", 20", 35", and 60". This format facilitates invasion interpretation, particularly where both resistive and conductive invasion are occurring proximal to one another. Field comparisons to wireline array resistivity measurements demonstrate the robustness of the method under a variety of formation resistivity environments and clearly shows where interpretation methodology is improved. P. 431

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.

A new propagation resistivity tool has been designed and built with a measure point only 15 feet from the bit. Measurements of near-bit inclination and gamma ray are also provided. All of these measurements are in a package that includes a mud motor. The primary purpose of this tool is to provide a means for staying in a target formation during horizontal drilling. The propagation resistivity measurement of this reservoir navigation tool has the ability to see radially much deeper than focused resistivity devices. The radial direction is the direction of interest in horizontal drilling because the distance between adjacent beds is in the radial direction (relative to the tool). Devices which attempt to see beyond the end of the bit have limitations in a horizontal environment because the distance to the adjacent bed is very large in the direction of the borehole. In addition to the standard 2 MHz frequency, this new propagation resistivity tool has a second frequency of 400 kHz which has a much greater depth of investigation than the 2 MHz frequency. Measuring amplitude and phase at both frequencies produces 4 different depths of investigation at the same measure point. Modeling has shown that adjacent conductive beds can be detected as much as 12 feet away with the 2 MHz frequency and about 32 feet away at the lower frequency. These detection distances refer to the true vertical distance from the borehole to the adjacent bed, but the borehole may not intersect this bed for several hundred feet in horizontal drilling because the borehole is nearly parallel to the bed boundary. In addition, new state-of-the-art electronics and the use of a second transmitter have greatly improved the precision of the resistivity measurements. The result of these improvements is better accuracy in highly resistive formations. Therefore, this tool is able to provide both geosteering and a propagation resistivity measurement as accurate as the best wireline induction tools.

The Grane field causes significant challenges with respect to reservoir drainage and wellbore placement. The true resistivity profiles from offset wells are reflecting an irregular oil water transition zone in the field, likely to be caused by subtle facies variations and/or local variations in the oil water contact (OWC). In addition, horizontal wells penetrated many shales.Baker Hughes INTEQ has in collaboration with Hydro developed an extra deep resistivity service called DeepTrak™ to navigate at a distance of up to 12 meters from a resistivity contrast boundary. This service has been used in several wells.The tools as well as the surface software components provided full service capability and were used successfully to geosteer along the OWC at the required distance. In addition, the DeepTrak service was capable of detecting shale injectites, thus giving valuable insight for reservoir characterization and geological model update.By use of field data, it is demonstrated how the DeepTrak service can be used for accurate wellbore placement.