Teruhiko Hagiwara

Macroscopic anisotropy approach to analysis of thinly laminated sand/shale sequences : Sensitivity analysis of sand resistivity estimate and environmental corrections

Thinly laminated sand/shale sequences are often mistaken as low-resistivity wet sands in spite of their high hydrocarbon saturations, because conventional electric logs and induction logs read the low average resistivity of the formations. Imaging tools and other high-resolution logging tools may be needed to identify such laminated formations when cores are not available. Such formations appear as homogeneous and anisotropic formations to conventional resistivity tools when the typical lamina thickness is much finer than the tools vertical resolution. The anisotropic formation resistivity is determined by the sand- and shale-laminae resistivities, and the sand fraction (or sand/shale ratio). Conversely, the sand-laminae resistivity can be estimated from the anisotropic resistivity data if the shale-laminae resistivity, the sand/shale ratio, and the relative dip (deviation) angle are all known. We call this approach to laminated-sand analysis the macroscopic anisotropy approach. When multiple resistivity values are measured, as in the case of using LWD 2-MHz resistivity devices in highly deviated boreholes, the sand-laminae resistivity and the sand/shale ratio can be estimated simultaneously from the anisotropic resistivity measurements. We examine the sensitivity of sand-laminae resistivity determination to each parameter and review how these parameters are estimated. The two most sensitive parameters are the shale-laminae resistivity and the sand/shale ratio, especially at smaller dip angles. When the total thickness of such a laminated sequence is not thick and/or sand laminae are invaded with mud formation evaluation becomes complicated, because the logging tools cannot read the macroscopic resistivity correctly. To estimate the correct macroscopic resistivity, special shoulder-bed (or thin-bed) corrections and/or invasion corrections must be applied. We examine special shoulder-bed corrections for anisotropic formations at higher dip angles. In homogeneous anisotropic formations, significant borehole effect is observed for laterologs and spherically focused logs (SFLs), but not for induction logs. We found that difference between induction-log resistivity and laterolog- or DFL/SFL-log resistivity is logarithmically proportional to the formation anisotropy, but is almost independent of borehole size. The difference may be used to estimate the formation anisotropy. Shale formations are often anisotropic. The effect of shale anisotropy is examined and a method is provided to correct such shale anisotropy in laminated-sand analysis.

Induction Log Shoulder-Bed Corrections to Anisotropic Formations and the Effect of Shale Anisotropy in Thinly Laminated Sand/Shale Sequences

Induction log responses to layered, dipping, and anisotropic formations are examined analytically. The analytical model is especially helpful in understanding induction log responses to thinly laminated binary formations, such as sand/shale sequences, that exhibit macroscopically anisotropic resistivity. Two applications of the analytical model are discussed. In one application we examine special induction log shoulder-bed corrections for use when thin anisotropic beds are encountered. It is known that thinly laminated sand/shale sequences act as macroscopically anisotropic formations. Hydrocarbon-bearing formations also act as macroscopically anisotropic formations when they consist of alternating layers of different grain-size distributions. When such formations are thick, induction logs accurately read the macroscopic conductivity, from which the hydrocarbon saturation in the formations can be computed. When the laminated formations are not thick, proper shoulder-bed corrections (or thin-bed corrections) should be applied to obtain the true macroscopic formation conductivity and to estimate the hydrocarbon saturation more accurately. The analytical model is used to calculate the thin-bed effect and to evaluate the shoulder-bed corrections. We will show that the formation resistivity and hence the hydrocarbon saturation are greatly overestimated when the anisotropy effect is not accounted for and conventional shoulder-bed corrections are applied to the log responses from such laminated formations In another application, we examine the effect of shale anisotropy in thinly laminated sand/shale sequences. It is known that the macroscopic conductivity of a laminated formation is determined uniquely by the sand and shale laminae conductivity and the sand/shale ratio. Conversely, the sand-laminae conductivity is estimated from the macroscopic formation conductivity, and the hydrocarbon saturation in the sand laminae is computed from the sand conductivity. The shale-laminae conductivity itself may be anisotropic in such laminated sequences. How do induction logs respond to such laminated formations when shale laminae are anisotropic? How accurate are the estimates of the sand-laminae resistivity and of the hydrocarbon saturation in these sand laminae? To answer these questions we used the analytical model to examine the effect of shale lamina anisotropy on induction log responses to thinly laminated formations. We learned that the macroscopic formation resistivity and hence the sand-lamina resistivity can be greatly overestimated if the shale anisotropy is not accounted for in interpreting induction log data from laminated formations. On the other hand, the estimate of the net-to-gross ratio is insensitive to shale anisotropy except for low sand-laminae resistivity (Rsd/Rsh < 5).