The electromagnetic propagation (EMP) measurement frequently acquired with logging-while-drilling (LWD) tools in high-angle wells is sensitive to geometrical effects that can mask the true formation resistivity. Less commonly used, the LWD laterolog measurement is sometimes perceived as providing data too shallow to give a true formation resistivity (Rt). This paper presents modeling and actual examples to demonstrate that the laterolog can provide a superior resistivity measurement for formation evaluation than the does the EMP LWD tool.
We examine the laterolog and EMP resistivities in several high-angle wells crossing carbonate formations in 8.5-in. and 6 1/8-in. hole sizes. In the 8.5-in. sections, producers and water injectors (high and low resistivity ranges) were evaluated. In the 6 1/8-in. sections, one reservoir sandwiched between two very high-resistivity layers and another borehole in a highly fractured reservoir were examined. The laterolog data were corrected for invasion using a 1D inversion of the memory data. Structure-based forward modeling was used to examine and explain the differences between the resistivity methods.
In the first example, the laterolog data showed a clear conductive invasion profile. While the deepest laterolog real-time resistivity data indicated lower resistivity than the EMP resistivity, the true resistivity, Rt, from the 1D inversion matched the EMP resistivity. This result validated both measurements and emphasized that those differences were due to invasion.
In the second example, a reservoir zone was initially drilled with resistivity measurements made only by the EMP tool. The LWD laterolog was run several days later, and the resistivity data were much lower in the relogged section compared with the EMP resistivity. The laterolog 1D inversion was unable to resolve Rt because of the excessively deep invasion that occurred over the course of several days. These two examples demonstrated that when acquired in normal drilling conditions, the laterolog measurements can provide the uninvaded formation resistivity even in the presence of invasion.
A reservoir in another example was sandwiched between resistive layers that caused difficult-to-explain elevated EMP resistivity readings. Structural modeling reproduced the elevated behavior of the EMP data and explained the differences between resistivity measurements. This result showed that the laterolog is better suited to evaluate resistivity in thin reservoirs where there is a high-resistivity contrast to the adjacent layer.
Finally, fractured reservoir examples are presented, which show that both the laterolog and EMP can be affected by the presence of fracture swarms.
The examples presented in this paper demonstrate that in high-angle wells, under normal drilling conditions, invasion-corrected laterolog resistivity is nearer to Rt than is the EMP resistivity. Furthermore, the laterolog measurement provides data that are better input to water saturation calculations.