Search

Thinly laminated pay zones are often anisotropic. The horizontal resistivity is dominated by relatively conductive shale layers, and the vertical resistivity is dominated by relatively resistive hydrocarbon strata.When the relative inclination is low, conventional induction and Logging-WhilWhile-Drilling (LWD) propagation resistivity tools only sense the horizontal resistivity and triaxial wireline measurements can be used to estimate the vertical resistivity. However, the relative inclination is often high enough to enable accurate determination of the vertical resistivity from the LWD propagation resistivity data and estimates of the structural dips. The vertical resistivity values can then be used to improve identification and quantification of productive intervals. Cases discussed in this paper demonstrate the modeling software for determining these anisotropy parameters, even in thinly bedded sequences. In practice, the results can be determined rapidly enough to affect completion decisions. A data processing procedure involving anisotropy analysis, thin-bed modeling, and shaly-sand analysis is described in the context of examples wells which penetrate thinly laminated anisotropic beds.After the modeling exercise, it was evident that the vertical resistivity was significantly higher than indicated on any of the raw resistivity curves. Two shaly sand analyses were performed, the first using the raw resistivity curves, and the second using the modeled resistivity values. A comparison of the results from both shaly sand analyses showed that significantly more hydrocarbons were identified after the modeling was done.

Summary Several recently published studies discuss the concept of inductive resistivity-logging devices with oblique transmitting and/or receiving coils. Both wireline induction and logging-while-drilling (LWD) propagation resistivity-tool concepts have been considered. Directional resistivity measurements and improved anisotropy measurements are among the benefits promised by this type of device. Analyses based on point-magnetic dipole antennas were used to illustrate these potential benefits. The effects of a metallic mandrel, borehole, and invasion were not considered because of the absence of a suitable forward model. This paper characterizes mandrel, borehole, and invasion effects for a variety of candidate tilt-coil devices with antenna array parameters similar to those of the previous studies. The characterization is based on calculations from a new forward model that includes tilted transmitting and receiving coils of finite diameter embedded in a concentric cylindrical structure. Important details of the forward model used in the calculations are also provided.