Beyond Water Saturation Induction Logs As Problem Solving Tools Available to Purchase

In the traditional log analysis workflow, array induction data received from the field are processed to yield true formation resistivity, which is then used to yield an estimate of water saturation. The wireline and LWD service providers offer a range of techniques for estimating formation resistivity that work well in the majority of borehole and geological environments. We present three case histories where determining precise water saturation was not the highest priority, where we have worked with raw data, developed new techniques, and applied recently introduced modeling software to address problems other than determining water saturation. In the first example high skin values observed during well tests were thought to indicate formation damage. A new technique for determining invasion diameter from differences between conductivity signals has been shown to be simple and reliable. The technique has also been applied to data from wells drilled with conductive mud. In these cases, a significant part of the total signal can be due to the borehole environment, which introduces a degree of uncertainty in the computed curves that increases with mud conductivity. We quantify this by considering the relative magnitudes of signals from formation and borehole. In the second example we consider logs run in oil based mud through massive, highly resistive beds. Conventional wisdom is that high resistivity readings are unreliable. We demonstrate that, subject to appropriate processing, these logs can be trusted to at least 1000 ohm-meters. This has helped characterize an oil reservoir with fresh formation water by more accurately representing a resistivity signature thought to be associated with a change in formation wettability. The third example is a horizontal well where first impressions were that the reservoir was unexpectedly thin. A range of forward modeling techniques ranging from one to three dimensional m were used to test various akernative structural explanations of the log response observed around the mid-point of the well path through the reservoir, and at its exit point. We concluded that the log response indicated a that the well crossed a highly resistive fault plane with a throw corresponding to the apparently missing section. Alternative earth models based on a thin reservoir were found to be inconsistent with the observed log response.