North Sea Horizontal Well Project OilBased Mud vs KCl Mud
Log ex. 6 displays a section of the pilot hole drilled in conjunction with a horizontal well in the North Sea. The pilot well, drilled with an 8.5 inch diameter bit and at an inclination of 60 degrees through the target formation, utilized a KCL/polymer mud system. At the circulating temperature of 77o C, Rm was 0.05 ohm-m and Rmf was 0.036 ohm-m. Formation water resistivity was 0.075 ohm-m at 77o C.
The phase shift-derived resistivity curves exhibit a slightly erratic character in the xx58 to xx61 meter interval, due to the combination of thin resistive coal streaks, irregular hole size, and the very conductive mud. The attenuation measurements are similarly affected, but to a lesser degree.
A whole core was cut from xx77 to x142 meters, which included an oil/water transition zone from xx96 to x104 meters. The long formation exposure time, due to the two-day coring operation, created the potential for deep invasion in the hydrocarbon-bearing interval from xx61 to x104 meters. The effect of the very conductive invading mud filtrate on the eight Resistivity curves is apparent in Log ex. 6. The attenuation-derived resistivity curves plotted in Track III exhibit much less separation than do the phase shift-derived resistivity curves plotted in Track II, because of the apparent deeper depths of investigation of the attenuation measurements in this environment. On the other hand, the vertical resolutions of the phase shift resistivity measurements are superior to their attenuation counterparts.
Some values of Rxo, Di, and Rt, calculated using a step-profile model of invasion and the phase resistivity data as inputs, are annotated on Log ex. 6. From xx65 to x125 meters, invasion diameters vary from 16 inches to 43 inches. The computed values of Rt are consistent with the medium and deep attenuation resistivity values in zones where these two measurements agree. When the computed value of Di is less than approximately 30 inches, the deep phase resistivity measurement agrees with the modeled value of Rt. When Di exceeds 30 inches, the computed Rt value is slightly greater than the deep phase resistivity value.
Log example 6 Phase vs. Attenuation Resistivities in North Sea Pilot Hole
Figure 7 illustrates a section of the horizontal well through the hydrocarbon zone shown in Figure 4. The attenuation resistivity data are not presented because they add minimal additional information in this particular environment. In contrast to the pilot hole, this horizontal borehole was drilled with an oil-based mud. The coal interval at ><221 to ><227 meters is more easily identified because borehole effects are much less pronounced in oil-based mud than in the very conductive mud used in the pilot hole.
The resistivity peak at ><256.5 meters is a "polarization horn." This phenomenon is caused by a discontinuity in the propagating electrical field as the tool crosses the boundary between beds having different resistivities. The size of the polarization horn depends on the contrast between the resistivities of the adjacent beds and the relative dip angle between the borehole and the formation. The higher the relative dip angle and greater the resistivity contrast, the larger the "horn."4 Also, note how the magnitude of this horn diminishes with decreasing transmitter-receiver spacing for the four phase resistivity curves plotted.
The different depths of investigation of the Resistivity tool are apparent in the varying degrees of anticipation of the formation top at ><256.5 meters. The deepest reading curve plotted in Figure 7, D-RES, "sees" the approaching bed first, followed by the M-RES, S-RES, and X-RES curves respectively.
Reference:
4. Anderson, B., Bonner, S., Luling, M.G., and Rosthal, R.: "Response of 2-MHz LWD Resistivity and Wireline Induction Tools in Dipping Beds and Laminated Formations," paper A presented at the SPWLA 31st Annual Logging Symposium, Lafayette, Louisiana, U.S.A., June 24-27, 1990.
Log example 7 North Sea Horizontal Well Phase Resistivities in Oil-Based Mud
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